Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia

Treball realitzat per: Oriol Barbosa i Llauet Dirigit per: Agustí Pérez Foguet Grau en: Bachelor’s degree in Civil Engineering Barcelona, Juny de 2022 Departament d’Enginyeria Civil i Ambiental (DECA) Secció de Matemàtica Aplicada i Estadística (MAE) TREBALL FINAL DE GRAU Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 1 2 “Un riu que vulgui viure amb nosaltres per tota la vida i per vida d’hereus perquè la tragèdia del nostre poble és que el riu, un cop descobert, no dura” Viatge al poble dels rius sense aigua - Mercè Rodoreda 3 i Agraïments Hom pot comunicar-se i fer ciència en mil i una llengües, però només pot pensar i estimar en una, la seva pròpia. Jo penso, parlo, escric i estimo en català, i vull que aquest treball sigui un tribut a la gent de casa, als amics, la família i els professors que m’han acompanyat durant aquest viatge. Perquè aquest treball no és només el resultat acadèmic de quatre anys a l’Escola de Camins, aquest treball és, sobretot, l’amor per la professió i l’esforç per millorar les condicions de vida d’aquells que mai han tingut les oportunitats que hem tingut a casa nostra. En primer lloc, vull agrair a la meva família tot el suport durant aquests anys. Jaume, Maria Àngels i Roger, gràcies per estar al meu costat en les bones i en les dolentes, i per fer-me valorar des de petit tot allò que m’ha fet falta durant el grau; sacrifici, passió, empenta, seny, força i treball. Als amics que m’han acompanyat aquests quatre anys. Xavi, Mireia, Jesús, Laia, Nicolás i Clàudia. Hem crescut junts i hem compartit alguns dels millors moments de les nostres vides, només tinc paraules d’agraïment per haver-ho fet amb vosaltres. Sé que, tot i que aquest any els nostres camins es separen, no serà un adeu, més aviat un a reveure. Als professors, amics i coneguts que m’han ajudat a arribar fins aquest punt. A la colla d’amics de Balaguer, d’Esterri d’Àneu i als bons amics que he fet d’Erasmus, que ja poden convalidar l’assignatura d’hidrologia després de tot el que han aguantat. A en Ricard Giné, el meu tutor a SIWI, per introduir-me al món de l’aigua i donar-me una perspectiva amplia i humana d’aquest bé tan necessari al qual no tothom té accés. I a en Rainer González i en Fasika Diro de la IOM, per col·laborar en aquest treball i aportar la consistència tècnica i la perspectiva de camp que hi feia falta. Finalment, vull agrair al professor Agustí Pérez Foguet tot el suport que m’ha donat no només en aquest treball final, sinó també des del dia que ens vam conèixer. L’Agus ha sigut el millor mentor que podia tenir dins l’Escola i sé que part del que sigui en un futur dins d’aquest camp, serà gràcies a tot el que he après amb ell i a les oportunitats que m’ha brindat. Ha sigut, és i serà sempre un plaer debatre sobre aigua, cooperació i desenvolupament amb ell. ii iii Abstract The tropical wet and dry climate has characteristically two sharp seasons, a rainy one and a dry one, that cause both floods and droughts and complicate the access to water and sanitation in Africa, South America and Southeast Asia. We are confident that climate change will worsen the hydrologic situation in tropical regions, increasing extreme events and reducing water availability. Furthermore, sustained demographic growth and its associated water and food demand will threaten social, political and economic stability, increasing conflicts and displacements related to water scarcity. The water stress assessment using standard water metrics (not including seasonality) shows a bias between the indices’ good performance and the current alarming situation in tropical regions. Even though these countries receive dreamlike amounts of water, its imbalanced and concentrated rainfall (and its associated floods) creates a problem where stakeholders must build water storage facilities above their economic capacity to overcome the dry season. Lack of water storage is one of the most concerning water issues in tropical countries. Results show that Economic water scarcity is predominant in all the analysed regions, even beyond physical water scarcity. It is, by now, the most limiting factor for the development and universal access to water and sanitation. In this context, Water Resource Management (WRM) methods appear as a set of feasible and sustainable solutions to enhance water conservation. Storing water from the wet season to use in the dry one is the primary goal of WRM in tropical countries because it helps mitigate floods and droughts simultaneously. Detention, retention, recharge and reuse techniques and appropriate land management are crucial to developing healthy hydrologic conditions in anthropogenically impacted basins, ensuring environmental protection and water for human activities. The case study in Lake Tana (Ethiopia) became the perfect occasion to contextualise and quantify the hydrologic changes that WRM interventions could provide to a waterstressed basin. Susceptible to demographic and climate changes, the application of terracing and the construction of sand dams in Infraz’s basin shows promising results toward water stress reduction. The analysis highlights increasing water storage and peak water flow reduction, resulting in mitigating floods and droughts. The case study supports the hypothesis that WRM is a potent tool that can simultaneously help solve water storage and security concerns while mitigating flood hazards on their way. iv v Resum El clima tropical de sabana es caracteritza per dues estacions marcades, una de plujosa i una altra seca, causant inundacions i sequeres que compliquen l'accés a l'aigua i el sanejament a Àfrica, Sud Amèrica i el Sud-est Asiàtic. Sabem del cert que el canvi climàtic empitjorarà les condicions hidrològiques en les regions tropicals, augmentant els episodis meteorològics extrems i reduint l'aigua disponible. A més a més, el creixement demogràfic i per conseqüència l'augment en la demanda d'aigua i menjar, faran perillar l'estabilitat social, política i econòmica, causant un increment en migracions degudes a l'escassetat d'aigua. L'anàlisi de l'estrès hídric amb els índexs usuals (que no inclouen l'estacionalitat) ens indiquen un clar biaix entre els bons resultats dels índexs i l'alarmant situació actual de les regions tropicals. Encara i rebre quantitats vigoroses d'aigua, la concentració de pluges (i inundacions derivades) creen un problema on els beneficiaris han de construir infraestructura d'emmagatzemament per sobre de les seves possibilitats econòmiques per sobrevenir l'època seca. La falta d'emmagatzematge d'aigua és un dels temes més preocupants en el context tropical. Els resultats ens mostren com l'escassetat d'aigua és predominantment econòmica en les regions estudiades. És, actualment, el factor més limitant del desenvolupament i l'accés a l'aigua i el sanejament. En aquest marc, la gestió dels recursos d'aigua (WRM) es presenta com un conjunt de solucions viables i sostenibles que ens permetin conservar l'aigua. Un dels principals objectius de WRM és emmagatzemar l'aigua durant l'estació humida i subministrar-la durant l'estació seca, contribuint així a la mitigació d'inundacions i sequeres simultàniament. Les tècniques de detenció, retenció, recarrega o reutilització, conjuntament amb una adequada gestió del sòl, són claus per aconseguir condicions hidrològiques òptimes en conques que presenten impacte antropogènic, assegurant la protecció ambiental i la disponibilitat d'aigua per activitats humanes. El cas pràctic al llac Tana (Etiòpia) ha esdevingut una ocasió perfecta per contextualitzar i quantificar els canvis hidrològics en una conca amb estrès hídric un cop introduïm mesures de WRM. Sota l'impacte demogràfic i de canvi climàtic, l'aplicació d'agricultura en feixes i la construcció de preses de sorra a la conca d'Infraz mostra resultats prometedors. Destaquem l'increment d'emmagatzematge d'aigua així com la reducció del cabal màxim, resultant en la mitigació d'inundacions i sequeres. El cas pràctic dona suport a la hipòtesi que la gestió dels recursos hídrics és una eina molt potent per resoldre el problema de l'emmagatzematge d'aigua i la inseguretat hídrica, així com per mitigar els desastres naturals esmentats. xii Figure 37. Water withdrawals from a well in the middle of Kenia. (Reuters)............40 Figure 38. Malawi railway line damaged with flooding events in 2015. (EFE agency). ...................................................................................................................41 Figure 39. South Asia political map and Köppen classification map. (McMahon, T. A., 2007) ....................................................................................................42 Figure 40. Container terminal in Vietnam. (Asian Development Bank) ....................44 Figure 41. Air pollution episode in Mumbay. (Getty Images)....................................44 Figure 42. Overexploited well in India. (EFE agency) ...............................................46 Figure 43. Annual mean temperature change relative to 1850-1900. (IPCC A, 2021)... ...................................................................................................................47 Figure 44. Annual mean precipitation change relative to 1850-1900. (IPCC A, 2021) . ...................................................................................................................48 Figure 45. Annual mean total colum soil moisture change. (IPCC A, 2021) .............48 Figure 46. Köppen classification map 2016 vs 2070. (Beck, H. E. et al., 2018) ........48 Figure 47. World population projection. (United Nations, 2019)...............................50 Figure 48. South Africa’s water crisis. (www.nationalgeographic.com)....................53 Figure 49. New displacements by conflict and disasters 2020. (iDMC, 2021). .........53 Figure 50. People forced to migrate in the Sahel. (UNHCR) .....................................54 Figure 51. Grand Ethiopian Renaissance Dam. (Reuters, 2019) ................................55 Figure 52. Mossul dam captured by ISIS (AFP).........................................................56 Figure 53. Mumbay water mafia distributing water from a tank truck. (EFE agency) ... ...................................................................................................................57 Figure 54. WRM scheme. (Oriol Barbosa i Llauet)....................................................58 Figure 55. Flow through a detention weir (N.K., 2014) .............................................60 Figure 56. Gully plugs in Ethiopia (Global Climate change Alliance, 2020) .............61 Figure 57. Cross section of a stream with check dams. (FAO watershed management field manual, 1986) ....................................................................................61 Figure 58. Terracing scheme. (Deng, C., et al., 2021) ................................................62 Figure 59. Terracing in Vietnam. (Getty Images).......................................................62 Figure 60. Creation of fanya chini in rural Tanzania. (Lead Foundation) ..................63 Figure 61. Detention pond in the outskirts of an urban area. (Getty Images) .............64 Figure 62. Rain garden in Virginia, USA. (VCAP) ....................................................64 Figure 63. Rain garden scheme. (Pinterest) ................................................................65 Figure 64. Bioswale construction. (Wikipedia) ..........................................................65 Figure 65. Rainfall harvesting tanks in a school in Kenya. (www.waterislifekenya.com).....................................................................67 Figure 66. Fog harvesting nets installed in Peru. (Frontiers) ......................................68 Figure 67. Water collection in the great lakes region. Photo by S. Morrison. ............69 Figure 68. Victoria falls. (Getty Images) ....................................................................70 Figure 69. Katse Arch Dam, Lesotho. (SkyPixels) .....................................................70 Figure 70. Nexus Project in California. (New Atlas)..................................................71 Figure 71. Sand dam construction and operation process. (Kamel, A., 2016) ...........72 Figure 72. Sand dam in northern Kenia. (KenGen Foundation) .................................73 Figure 73. Fukuzato underground dam, Japan. (Wikipedia).......................................73 Figure 74. Cross section of a standard percolation tank with a check dam. www.chaitanyaproducts.com/) ..................................................................75 Figure 75. Percolation tank in rajasthan, India. ..........................................................75 Figure 76. Flooding pools for infiltration (www.earthdate.org) .................................76 Figure 77. Ditch and furrow method. (Pinterest) ........................................................76 Figure 78. Injection well. (https://www.austintexas.gov/) ..........................................77 xiii Figure 79. Vertical recharge shaft. (Harmeson, R. H., 1963) .....................................78 Figure 80. Horizontal recharge shaft. (Harmeson, R. H., 1963) .................................79 Figure 81. Induced recharge diagram. ........................................................................79 Figure 82. Different vertical well options. (https://thereaderwiki.com) .....................81 Figure 83. Children pumping water from a well. (UNICEF)......................................81 Figure 84. Qunat scheme. (Pinterest) ..........................................................................82 Figure 85. Horizontal well scheme. (Houben, G. J. et al., 2022)................................83 Figure 86. Desalination plant in Egypt. (constructionreviewonline.com/) .................83 Figure 87. Sot de Ferrer water catchment in Sagunt, Pais valencià. (www.acequiamayordesagunto.com) ........................................................84 Figure 88. Greywater tank used in backyard agriculture. (Pinterest) .........................85 Figure 89. Constructed wetlands in Verdú, Catalunya. (Samsó, R.., 2014) ...............86 Figure 90. Ethiopia administrative map, with zones and regions. (Wikipedia) ..........88 Figure 91. Panoramic view of Addis Ababa (Reuters) ...............................................89 Figure 92. Elevation, temperature and precipitation in Ethiopia. (Yimam, K.A. et al., 2021) ..........................................................................................................90 Figure 93. Köpper classification map for Ethiopia (1980-2016). (Beck et at., 2018) 90 Figure 94. Köpper classification map for Ethiopia (2071-2100). (Beck et at., 2018) 91 Figure 95. Lake tana administrative zones. (Shimelis, S. et al., 2013) .......................92 Figure 96. Fishermen at Lake Tana. (Pinterest) ..........................................................93 Figure 97. Water balance in Lake Tana. (Duan, Z., 2018) ........................................94 Figure 98. Evolution of water balance parameters. (Duan. Z., 2018) .........................94 Figure 99. Geology classification of Lake Tana. (Kindie, A. et al., 2019) .................95 Figure 100. Soil classification of Lake Tana. (Kindie, A. et al., 2019).....................96 Figure 101. Land use of Lake Tana. (Kindie, A. et al., 2019)...................................97 Figure 102. Guzara Castle, Infraz. Photo by Evan Williams. ...................................98 Figure 103. Infraz’s bain Hyetograph (2015-2021). (Oriol Barbosa i Llauet) ........100 Figure 104. Infraz’s basin evapotranspiration (2015-2021). (Oriol Barbosa i Llauet) . ..............................................................................................................100 Figure 105. DEM of Infraz’s basin over an orthophoto with modified band colour. (Oriol Barbosa i Llauet) .......................................................................101 Figure 106. Demographic projection of Infraz’s sub-basin (2015-2060). (Oriol Barbosa i Llauet) ..................................................................................103 Figure 107. Water demand projection of Infraz’s sub-basin (2015-2060). (Oriol Barbosa I Llauet)..................................................................................104 Figure 108. Infraz’s sub-basin delineated from water accommulation network. (Oriol Barbosa i Llauet) .......................................................................106 Figure 109. Infraz’s sub-basin map. (Oriol Barbosa i Llauet) ................................107 Figure 110. Longest possible river path derived from Google Earth. (Oriol Barbosa i Llauet) ..................................................................................................107 Figure 111. Soil classification map. (Oriol Barbosa i Llauet).................................108 Figure 112. Slope classification map. (Oriol Barbosa i Llauet) ..............................109 Figure 113. HEC-HMS Infraz’s sub-basin model. (Oriol Barbosa i Llauet) ..........111 Figure 114. HEC-HMS results for precipitation-runoff-infiltration. (Oriol Barbosa i Llauet) ..................................................................................................113 Figure 115. Results for precipitation-discharge-withdrawals (2015-2021). (Oriol Barbosa i Llauet) ..................................................................................114 Figure 116. Results for groundwater storage evolution (2015-2021). (Oriol Barbosa i Llauet) ................................................................................................115 xiv Figure 117. Hydrologic parameter’s projection (2015-2060). (Oriol Barbosa i Llauet) ..................................................................................................116 Figure 118. Groundwater stoage projection (2015-2060). (Oriol Barbosa i Llauet)..... ..............................................................................................................117 Figure 119. HEC-HMS Infraz’s sub-basin model under modified conditions........121 Figure 120. Results for precipitation-discharge-withdrawals (2015-2021) under modified conditions (Oriol Barbosa i Llauet) ......................................123 Figure 121. Results for groundwater storage evolution (2015-2021) under modified contions. (Oriol Barbosa i Llauet)........................................................123 Figure 122. Groundwater storage evolution comparison. (Oriol Barbosa i Llauet)124 Figure 123. Hydrologic parameters’ projection (2015-2060) under modified conditions. (Oriol Barbosa i Llauet) ....................................................124 Figure 124. Groundwater storage projection (2015-2060). Infraz’s modified basin. (Oriol Barbosa i Llauet) .......................................................................126 xv Index of charts Table 1. Falkenmark Index............................................................................................7 Table 2. Region I - General characteristics .................................................................20 Table 3. Region I - Water characteristics ....................................................................22 Table 4. Region II - General characteristics ................................................................24 Table 5. Region II - Water characteristics...................................................................26 Table 6. Region III - General characteristics ..............................................................29 Table 7. Region III - Water characteristics .................................................................31 Table 8. Region IV - General characteristics ..............................................................35 Table 9. Region IV - Water characteriscs ...................................................................38 Table 10. Region V - General characteristics ............................................................43 Table 11. Region V - Water characteristics ...............................................................45 Table 12. Population projection (2015-2060)..........................................................103 Table 13. Water demand projection (2015-2060)....................................................105 Table 14. Hydrologic parameters’ projection (2015-2060) .....................................116 Table 15. Groundwater level projection (2015-2060) .............................................117 Table 16. Summary of sub-basin characteristics .....................................................121 Table 17. Hydrologic parameters’ projection (2015-2060) .....................................125 Table 18. Groundwater level projection (2015-2060) .............................................126 xvi Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 1 PART I - INTRODUCTION 1. Motivation & background This final thesis started with a tweet. Xavier Aldekoa, a journalist specialising in Africa, uploaded a photo taken by Peter Canton in the South Sudanese province of Jonglei in the 2021 flood season. The picture shows a child and his mom during a flood. I kept that photo in my retina. I have always been interested and aware of social topics despite my passion was maths, physics and their application in engineering fields. That same night I replied to the tweet. “South Sudan, a country punished every year by severe floods. Photos like this one confirm my vocation. I’ve chosen the correct studies.” Water issues have interested me since I knew that I would be a Civil Engineer, and at that point, in the 3rd year of my degree, I knew that hydrology would be my speciality some years later. With my intrinsic curiosity, I started looking up some more information. One article led to another five, which opened tabs with more information, graphics, maps, etc. I started developing a high curiosity about South Sudan and floods because they also affected refugees, a topic I was already interested in. But then I realised there was no water, just clayey soil and savanna vegetation in all the previous videos about South Sudan, war and refugees I had seen. Water scarcity and hunger. I researched its climate, and to my surprise, wet season with devastating floods and a dry season with poor water and food availability. It had an explanation, tropical wet & dry (savanna) climate. Some months later, I already had an idea for the final thesis. How can we use the exceeding water from floods to have acute hydric stress for the rest of the year? How can we mitigate it from an engineering and managerial point of view? I guess that at this point, the thesis was born. Figure 1. Mom and child in flooding in Jongelei, South Sudan. Photo by Peter Canton. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 2 2. Objectives, structure & methodology The thesis aims to mitigate floods and droughts in tropical wet and dry climate regions using Water Resource Management techniques. The first objective is to understand the different theoretical factors characterizing water hazards, like the particularities of the climate, the hydrological cycle, and the social and economic context. The second objective aimed to apply the previous knowledge to a real engineering problem in a complicated developing region like Lake Tana. Thirdly, working along very different project stages to get a comprehensive view of engineering, ranging from investigation to modelling and design. Theoretical and practical frames characterize the thesis structure. The thesis begins with an analytic aim, studying the problems caused by floods and droughts in tropical countries and why water-related hazards punish these regions. It is an extensive scope, where we can compile data and map the situation of the affected areas to get a general understanding of the topic. Afterwards, we introduce the term “mitigation”, directly associated with the solution proposal. Here we start studying the different available solutions, both from management and infrastructure point of view, inside the WRM frame. Then, in the practical part, we apply the theoretical knowledge on climate, water metrics and WRM to a case study in Lake Tana, Ethiopia. We focused on a sub-basin with tremendous demographic and climate pressure, inducing it into a water-scarce situation. Regarding the employed methodology in this thesis, the theoretical part was elaborated through a formal literature review of scientific papers and reports from various organizations during the first two weeks of April 2022, following two more weeks of synthesis, drafts and final writing. All the documents and information are cited in the bibliography, but it’s important to highlight some organizations that facilitated the research. Regarding climate, hydrology and meteorology, WMO and IPCC reports were beneficial, as well as the World Bank and WFP report for the economic impact. Regarding local data in the affected countries, the UN agencies working in the WASH cluster and other organizations helped report detailed data. This data is extracted from UNHCR, UNICEF, OCHA, ICRC, WFP, IOM, etc. their fieldwork is worthy from all perspectives. In the case study, we followed the principle of working every time with open-source data and free software. The intention was to reach a result with potential implementation everywhere, without economic limitations apart from the hardware. All the obtained data comes from open access portals like NASA POWER and EarthExplorer, and the hydrologic processing of the basin was performed with QGIS and HEC-HMS software, which helped us delineate the watershed and simulate the precipitation-runoff-infiltration data. With the available results, we suggested terracing and sand dam construction to reconduct the water-stress tendency of the basin. We reran the model with the introduced modifications to evaluate the groundwater level and water availability results, which proved to be satisfactory. The whole thesis ends with a set of conclusions extracted from the theoretical part and validated -partly- by the case study. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 3 Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 4 PART II - THEORIC FRAME 1. Tropical wet & dry climate Tropical wet & dry climate, or Tropical savanna climate, is a type of tropical climate defined in the Köppen classification as Aw or As. It extends near the equator, between 5º to 25º latitude, and is characterized by two very pronounced seasons, the dry and the wet one, warm to hot temperatures throughout the year (average temp. between 20º to 30º from winter to summer), concentrated rainfall in the wet season, that can vary between 900mm and 1800mm approximately. These three characteristics make this climate different to the other two tropical ones (Physical Geography: A landscape Appreciation,2022). Comparing it to the Tropical rainforest climate, we can see how this last one has balanced seasons, almost indistinct, with more moderate temperatures and a higher amount of rainfall. Let's compare it with the Tropical monsoon climate. The differences are shorter because we have seasonality and high temperatures in both, but the precipitations in our scoped environment are lower. This means that in a Tropical savanna climate, the dry season can become severe, and drought conditions are prevalent, affecting land use, water availability and food security. To distinguish between monsoon and savanna climate, we must quantify the rainfall records using the following method. We will have a tropical savanna climate if the rainfall in the driest month fulfils both requisites (and tropical monsoon if the second one is not fulfilled): 𝑅𝑑𝑟𝑦<60𝑚𝑚 𝑅𝑑𝑟𝑦 <100−(𝑇𝑜𝑡𝑎𝑙 𝐴𝑛𝑛𝑢𝑎𝑙 𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛 (𝑚𝑚) 25 ) Figure 2. Tropical climates acording to Köppen climate classification. (Beck et al., 2018) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 5 Figure 3. Tropical climates characteristics. (Physical Geography: A landscape appreciation, 2022) Even though we have already shown how the climate is classified concerning the others, before talking specifically about the hydrological conditions and putting names to the affected areas, it is our interest to sub-classify this climate more in detail. We will consider three scenarios: wetter to drier, from more water availability to less. The threshold characterising a dry month is 60 mm of rain. - Lengthy wet and short dry seasons: This subdivision is characterised by seven or more wet months and a similar pattern to some monsoon climates. However, it receives less water annually than a Monsoon region. Good water availability, with sporadic floods and droughts. - Equally long wet and dry season: In this case, we find approximately six dry months and six wet months during which most of the rainfall is concentrated. The water availability is sufficient, and systematic risk of droughts and floods. - Lengthy dry season and short wet season: This is the most vulnerable of all the subclassifications, with seven or more dry months and a short rainy season. It can result from regions generally arid but with super intense rainy episodes in the wet season or areas with a few drops of rain in the wet season to escape from semi-arid classification. Water availability is meagre, the risk of droughts is exceptionally high, and floods can be expected if the wet season is intense. Figure 4. Change of seasons in Ethiopia. (Wordpress – Tales from the big country) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 12 Figure 11. Hazard exposure. (Carrao et al., 2016). Figure 12. Risk level. (Carrao et al., 2016). Figure 13. Hazard vulnerability. (Carrao et al., 2016). Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 13 Figure 14. Hazardous index. (Carrao et al., 2016). We can see again how most tropical countries become drought risk-prone (and consequently water-scarce) mainly due to vulnerability associated with the economic situation. The hazardousness is generally low, apart from India, but the exposure is considerable in some tropical locations. If we consider the seasonality, we can start figuring that the actual situation is worse than it initially thought when analysing the water budget annually with the Falkenmark and Water Stress Indices. On the other hand, floods are climatic hazards that occur when a large water body overflows beyond the defined normal limits, affecting primarily the zones where human settlements are established, occasioning property damages and even injuries and deaths. Figure 15. Population exposed to significant flood risk. (Rentscheler, J., Salhab, M.., 2020) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 14 Figure 16. Population exposed to significant flood risk and living in poverty. (Rentsheller, J., Salhab, M.., 2020) It is estimated that 50% of humans living on the coastline assume the risk of flooding. People living in floodplains are also widely affected. Floods are catalogued as the most dangerous hazards nowadays by the United Nations and affect more than 30 million people each year, with the prevision of increasing the affected people as the climate change advances, causing billions of dollars each year. We will explain the particular flood scenarios in each region later, but, by now, a map is worth a thousand words to get introduced to the countries and zones with higher flood risk. When analysing the flooding situation, we find a different reality from droughts. Floods are enormously present in tropical regions, much more than droughts, and the top 10 regions with affected populations are all classified as tropical wet and dry climates. These last maps justify the aim of the thesis ultimately. Why it is essential to mitigate floods in tropical countries is crucial for the proper development of the poorest regions in the world. In the Water Resources Management part, we will see why we have to take advantage of floods as a water resource that is predominantly not harvested. 1.1.4 The key: storage & water conservation In this part, we start to aim for the solution to the problem without explicitly talking about Water Resources Management. We have seen that tropical wet and dry countries have abundant water but are unequally distributed over time and with a high evaporation ratio. The solution to the problem goes through one fundamental concept: water conservation. Water conservation is the set of policies, strategies and activities that allow us to increase the sustainability of water consumption and usage, helping to reach the standards of water quality and availability for the targeted population, as well as enhancing the healthy conservation of the watershed resources. Water conservation in tropical wet and dry countries aims principally at water harvesting and storage. The goal is to collect the flood waters and retain them for the maximum possible time so that when water scarcity problems arise in the dry season, food, water and energy insecurity becomes less intense. In the end, we are talking about two mitigations connected by the same concept. The Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 15 problem that we are facing is, as we have seen, mainly economic. The climatic conditions are harsh but could be worst; the struggling situation is financial. When rich countries can overcome water stress through connected networks, storage infrastructure, desalination, and water technology investments, they clearly show the role that infrastructure and wealth play compared to developing regions. Tropical countries need to work and invest in vulnerability reduction, which means water infrastructure to mitigate floods and water infrastructure to store water to reduce drought risk. Understanding this part is key to designing accurate management plans and enhancing sustainable water resource utilization. 1.1.5 The importance of groundwater and its pollution Groundwater resources play a vital role in the hydrologic cycle (especially in tropical wet and dry regions), provide daily water supply for at least 50% of the human population, and represent 43% of the irrigation consumption. Groundwater accounts for two-thirds of the total available freshwater worldwide. It is well known that storing water underground has benefits like water quality improvement, a decrease in evapotranspiration or reduction of mortality rates due to the prevention of illness caused by water or mosquitos living in it. But groundwater currently faces one big problem that can make it an unreliable source for many regions: contamination. Groundwater contamination occurs when pollutant substances percolate and meet the aquifer, polluting the groundwater. Primary groundwater pollutants come from agricultural, mining and industrial activities and wastewater that hasn’t been treated. So, water reuse and treatment are crucial to tackling water pollution. But we can’t only rely upon domestic reuse and treatment. There is a need to aim for agricultural policies to cut off excessive chemical use in farming. Currently, in terms of hygiene and sanitation, there is still 10% of the population with no other choice than open defecation. Still, 17% of the population has access to unimproved or limited sanitation facilities like latrines. Big cities worldwide are already improving sanitation conditions and wastewater treatment while also improving water access with potabilization plants and monitoring of water quality. But in rural or illegal settlements, the situation is far more complicated. In rural communities, latrines and wells are close to each other. Latrines are almost at ground level, so if the groundwater table is lower than the latrine, the faecal substances flow into the well, probably the only water source available if the community is isolated. In illegal settlements like slumps, we don’t just have this problem, but also the quantity of faecal mass is enormous in comparison. Slums are, by definition, dense and crowded, and the situation can worsen when more rural population emigrates to the cities without money or shelter. As we can see, the gap between the urban and rural world is vast, and we can attribute it to economic and demographic causes. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 16 Figure 17. Child near a polluted water stream in South Africa. (Reuters) As we said, contamination can be partially attributed to using fertilisers and pesticides in agricultural activities like nitrogen-based ones, which are one the primary sources of groundwater contamination on a broad scale. Agricultural contamination is the main target of US and European Union water pollution policies. It is also remarkable the amount of pollution generated by industrial activities, not just from the chemical leakage events we all heard about but also because they are the leading waste producers in the world. Part of the waste ends up in landfills where poor treatment or supervision is guaranteed, contaminating the immediate groundwater and worsening the overall situation. Another sector that we must mention as a source of groundwater contamination is mining. Mining is an intensive and contaminant activity, but with the correct professionals and investment, we can overcome some environmental problems. The big deal here is illegal mining. Extended in the poorest countries, it is a sector moving insane quantities of money that can end up in all kinds of other activities, like blood diamonds that end up financing insurgencies in war zones. Colombia, Congo, Sierra Leone and a long list of developing countries rich in natural resources suffer the consequences of illegal mining. The problem here is that without professionals, water pollution (not just groundwater but also streams) is common and affects whole regions, with little capability to stop them because of the dangers it can cause locals. So, when analysing the WRM methods, we must pay special attention to wastewater treatment. The food sector must also take action in policies that aim to decrease nitrogenbased fertilisers, and the industry must improve quality standards and stop illegal waste dumping. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 17 1.1.6 The role of geology & land management In this last part about the hydrology of the tropical savanna climate, we must also focus on land management and geology because they are both clues when characterising the watershed and determining the hydrologic behaviour of the basin. Geology will determine lots of things. From land use to aquifer productivity, geology is crucial to understanding the interaction between water and soil. We cannot provide a common geological factor for tropical climates because we cannot associate climatic regions with geology. Geology will characterise parameters like infiltration rate, availability of vegetation and agriculture that the soil can host, the nutrient richness, the river streamflow and its slope, the land tendency to erosion, the groundwater flow, available depth and consequently, the aquifer productivity. Also, geological properties and terrain shape suffer alteration processes boosted by wind, rainfall and snow or by anthropogenic and climatic changes. So, it is crucial to understand that geology shapes the basin and partially determines its water budget, so how we deal with the geology will influence the water availability and agriculture of the region, leading to two main topics; land management and WRM. Figure 18. Representation of the global lithological map database GLiM. (Hartmann & Moosdorf, 2012) Land management is the set of policies, strategies and activities that promote a sustainable and optimal use or development of the land and its derived activities, from urban to rural areas. It is an interdisciplinary field including; agriculture and reforestation techniques, water resource management, ecosystems management, etc. Land management and water resource management are alleys to obtain healthier basins, quality water and also floods and droughts mitigation, enhancing the principle of water conservation and consequently bringing healthier water budgets with less risk and stress. Land management policies regarding flood risk must target land erosion. We can achieve Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 18 this by reducing water flow velocity with techniques like reforestation and certain agricultural activities. Vegetation helps retain and store water, and at the same time, the runoff velocity through it is slower than in eroded lands. So, by stopping the floods, we also give more storage facilities and reduce future drought risk. We can also point out that evaporation is slower when the land is covered with vegetation, retaining more water. Also, land management techniques regarding agriculture should reduce the dependency on fertilizers, reducing the pollution of groundwater while keeping food supply security. Figure 19. Terracing in Rwanda, the land of a thousand hills. (EFE) We must study which WRM methods will help us accomplish the goals of land management, groundwater storage and water treatment. They will help us eliminate pollutants, shape lands for flood mitigation and agricultural purposes, and help solve all the derived water problems in tropical savanna regions. Good land management practices and policies and WRM methods will be crucial to entire healing basins. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 19 1.2 Current Situation Now that we have already defined how to classify the climate using the monthly precipitation, it is time to focus on the regions where the rainfall and climatic characteristics currently apply. We will study the hydrological characteristics and how they interact with the demography, the socio-economic factors, and the impact of floods and droughts. If we look at the map in Figure 1, there are five extensive main affected regions. Central America and the Caribbean, the central region of South America, sub-Saharan Africa between the tropics (excluding the tropical rainforest region in the Democratic Republic of Congo), the great lakes region and Madagascar, and southeastern Asia. Suppose we take a general overlook, excluding Australia, Miami and some particular areas in the Indochinese peninsula with high development. In that case, we will find, generally, developing countries, some on their way towards growth and others currently failing states. To this extent, as part of the project, we analyse the social and economic context of the tropical wet and dry climate regions and the hydrologic characteristics, including the floods and droughts and their impact. 1.2.1 Central America & the Caribbean Central America and the Caribbean are tropical regions around the Gulf of Mexico. It hosted ancient civilizations like the Mayans and the Azteca before being colonized by Spain from 1492ac on. Latin America and the Caribbean are the second most disasterprone regions in the world, affected by 1205 recorded disasters between 2000 and 2019 (OCHA, 2020). It is remarkable that, apart from the targeted disasters, hurricanes, earthquakes, and volcanic activities are all around. Figure 20. Central America and the Caribbean political map. (WordPress) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 20 Figure 21. Köppen classification map of central America. (McMahon, T. A., 2007) Table 2. Region I - General characteristics REGION I Countries Demography Economy & facilities Population in millions (2020) Urban population in millions (2020) Rural population in millions (2020) $GDP per capita & GDP growth (2018) Electricity access (2019) Internet access (2019) Belize 0,4 0,18 (46%) 0,21 (54%) 5001$ (3%) 93% 51% Costa Rica 5,09 4,11 (81%) 0,98 (19%) 12485$ (3%) 100% 81% Cuba 11,33 8,74 (77%) 2,58 (23%) 8824$ (2%) 100% 68% Dominican Rep 10,85 8,95 (83%) 1,89 (17%) 8051$ (7%) 100% 76% El Salvador 6,49 4,76 (73%) 1,72 (27%) 4053$ (2%) 100% 50% Guatemala 16,86 8,74 (52%) 8,12 (48%) 4478$ (3%) 96% 44% Haiti 11,4 6,51 (57%) 4,89 (43%) 1479$ (2%) 45% 33% Honduras 9,9 5,78 (58%) 4,12 (42%) 2493$ (4%) 93% 42% Mexico 128,93 104,09 (81%) 24,84 (19%) 9687$ (2%) 100% 70% Nicaragua 6,62 3,91 (59%) 2,72 (41%) 2015$ (-3%) 88% 45% Panama 4,31 2,95 (68%) 1,36 (32%) 15545$ (4%) 96% 64% Puerto Rico 3,19 2,99 (94%) 0,21 (6%) 31605$ (-4%) 100% 78% Venezuela 28,44 25,1 (88%) 3,33 (12%) 0$ (0%) 100% 62% Nowadays, the region is under development, experiencing, in general, a highly economical investment trend. Let's look at the GDP per capita. We can see a two-paced region, where countries like Mexico, Cuba, Costa Rica, Panama, and Puerto Rico have relatively strong economies compared to Haiti, Nicaragua and Honduras. Access to electricity is almost universal in all countries. Still, we don't have details on how good it is, and the internet access is growing fast but is only available for 40-70% of the population. New infrastructure is built across the regions with the help of the World Bank and BID (Banco Iberoamericano de Desarrollo), amongst other contributors. Logistic Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 21 corridors are made, including roads, railways, ports and airports that connect the region from north to south, aiming for trade with the USA, and from east to west, connecting the Atlantic to the Pacific Ocean. Urban areas are getting remodelled, new public transport facilities are getting introduced (mainly in big cities), and countries like Panama or Mexico are introducing themselves to the renewable energy market. These countries seem to have begun to escape from the economic poor, although time and continuous improvement are yet required. Figure 22. Panama canal. (Banco Interamericano de Desarrollo) But the social problems are still alive. Violence, gangs, inequality, urban/rural bias or poverty are still noticeable. There is a substantial migratory movement across the region, where people from all over the world try to get to USA & Canada through a narrow area, the Darién Bottleneck. So, the zone is prone to illegal activities, like people smuggling, drug dealing, weapon trafficking, illegal prostitution, etc. Figure 23. Imprisoned men from “Mara Salvatrucha” in El Salvador. (Agencia EFE en América) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 28 1.2.3 Western & Central Africa Western and Central Africa are two adjacent African regions located between the Sahel and the north of Angola in latitude and between the Atlantic coast and the African Great lakes in longitude. They host 550M Africans. Most of the countries we will analyse border the Gulf of Guinea, although we can find five inners, Burkina Faso, Mali, Central African Republic, Chad and South Sudan. The region is predominantly francophone, and the link with France is still persistent, although China and Russia are gaining influence this last decade. In this region, we will find some of the poorest countries and hope in some of the most promising African economies. Politically, we can categorise the Sahel region as the most unstable region in the world. If we add the Democratic Republic of Congo and the Central African Republic to the equation, we find ourselves in a territory that can blow up at any time. Figure 29. West & Central Africa political map and Köppen classification map. (McMahon, T. A., 2007) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 29 Table 6. Region III - General characteristics REGION III Countries Demography Economy & facilities Population in millions (2020) Urban population in millions (2020) Rural population in millions (2020) $GDP per capita & GDP growth (2018) Electricity access (2019) Internet access (2019) Angola 32,87 21,96 (67%) 10,9 (33%) 3290$ (-2%) 46% 36% Benin 12,12 5,87 (48%) 6,25 (52%) 1242$ (7%) 40% 29% Burkina Faso 20,9 6,4 (31%) 14,51 (69%) 805$ (7%) 18% 18% Cameroon 26,55 15,28 (58%) 11,27 (42%) 1585$ (4%) 63% 34% C. African Rep. 4,83 2,04 (42%) 2,79 (58%) 476$ (4%) 14% 10% Chad 16,43 3,86 (24%) 12,56 (76%) 726$ (2%) 8% 10% Congo, Rep. 5,52 3,74 (68%) 1,78 (32%) 2607$ (-5%) 48% 9% Cote d'Ivoire 26,38 13,64 (52%) 12,74 (48%) 2314$ (7%) 69% 36% DRC 89,56 40,87 (46%) 48,69 (54%) 561$ (6%) 19% 13% Eq. Guinea 1,4 1,03 (73%) 0,38 (27%) 10006$ (-6%) 67% 26% Gabon 2,23 2,01 (90%) 0,22 (10%) 7959$ (1%) 91% 61% Gambia, The 2,42 1,51 (63%) 0,9 (37%) 733$ (7%) 60% 51% Ghana 31,07 17,82 (57%) 13,25 (43%) 2261$ (6%) 84% 53% Guinea 13,13 4,84 (37%) 8,29 (63%) 955$ (6%) 42% 23% Guinea-Bissau 1,97 0,87 (44%) 1,1 (56%) 803$ (1%) 31% 28% Mali 20,25 8,89 (44%) 11,36 (56%) 895$ (5%) 48% 26% Nigeria 206,14 107,11 (52%) 99,03 (48%) 2028$ (2%) 55% 34% Senegal 16,74 8,06 (48%) 8,69 (52%) 1458$ (6%) 70% 40% South Sudan 11,19 2,26 (20%) 8,93 (80%) 1123$ (-3%) 7% 7% Togo 8,28 3,54 (43%) 4,74 (57%) 902$ (5%) 52% 19% The region has a very dispersed economic development or even economic worsening. From these countries, we must point out Nigeria as the leading nation in Africa regarding economic development. The 200M people and continuous growth and investment in the urban areas make Nigeria the most attractive economic pole in the region. Politically, it is remarkable the situation in Ghana is a real example of stability in a region prone to all kinds of political problems. Also, Gabon and Angola are good examples of countries under development and already scaping the harsh reality of poverty and crime in the region. If we look explicitly at the table, we can see how the mentioned countries have higher GDP per capita, higher electricity and internet access. Equatorial Guinea's high GDP/capita is due to the petrol industry but is not representative of the everyday life standards in the country, which are much lower. The following step is a conglomerate of less wealthy countries that show relatively stable political situations or countries as rich as the first ones but with some threats to stability. This group will find developing economies like Benin, Togo, Cote d'Ivoire, Gambia and Guinea Bissau. Also, with a very different case, we find Senegal, a promising economy that performs as the first-mentioned country, comparable to Ghana, but threatened by climate change, inner political problems and non-healthy neighbours who like "coups d’état". On the other hand, even if the GDP increase has been constant through these years in some of these countries, we will find hell on earth. Sahel countries, DRC and CAR. They Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 30 accumulate all the possible problems that can coincide in a country (or even more). The Sahel has presenced six putsches in 5 countries in less than three years, from 2019 to 2022. We find Guinea, Mali, Burkina Faso and Chad as the analysed countries. The region is also uncontrolled in the north of Nigeria and Cameroon (this last one is holding a civil war, too), and jihadism is present day by day, killing and kidnapping thousands of people every year, advancing into more and more territories. As we will develop later, water and food security are threatened, healthcare systems are practically inexistent, and access to basic facilities or education is scarce. This ends up creating an unstructured society. Moreover, UNICEF has pointed out that 2,3M children are unprotected, and a part of them do force labour due to poverty or tradition. Illegal mining, petrol extraction, child smuggling, forced marriage, guns and drug dealing are all around. This list of constant threats makes the Sahel an awful place to live, and 2,5M people have migrated from it this decade to seek better living conditions. Figure 30. Armed civilians in northern Nigeria. (Reuters) In this last paragraph, we want to highlight the importance of infrastructure for the regions' development. Here, we must say that the area is trying its best with Chinese investors. Africa is building ports, trans-African roads, railways, and electrifying the countries, and there is also high investment in telecommunications. Although threatened by many problems, Africa is receiving monetary injections to launch this region's economy and productivity. They are called to substitute China and India as the new factories of the world once these two countries are in total development. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 31 Figure 31. Africa’s major infratructure development plan. (www.one.org) Table 7. Region III - Water characteristics REGION III Countries Demography Water Population in millions (2020) Yearly average precipitation in mm (2017) Renewable freshwater (m3) per capita per year (Falkermark) (2018) Access to basic drinking water (2020) Access to basic sanitation (2020) Mortality due to water per 100k people (2016) Drought risk Index (1-5) (Statista, 2015) Angola 32,87 1010 4964 (No stress) 57% 52% 48,8 2,60 Benin 12,12 1039 922 (Scarcity) 65% 17% 59,7 1,20 Burkina Faso 20,90 748 651 (Scarcity) 47% 22% 49,6 3,40 Cameroon 26,55 1604 11113 (No stress) 66% 45% 45,2 2,24 CAR 4,83 1343 30679 (No stress) 37% 14% 82,1 1,20 Chad 16,43 322 999 (Scarcity) 46% 12% 101,0 3,16 Congo, Rep. 5,52 1646 43438 (No stress) 74% 20% 38,7 1,20 Cote d'Ivoire 26,38 1348 3144 (No stress) 71% 35% 47,2 1,40 DRC 89,56 1543 11057 (No stress) 46% 15% 59,8 1,80 Eq. Guinea 1,40 2156 20602 (No stress) 22,3 2,44 Gabon 2,23 1831 79426 (No stress) 85% 50% 20,6 1,60 Gambia, The 2,42 836 1355 (Stress) 81% 47% 29,7 2,32 Ghana 31,07 1187 1040 (Stress) 86% 24% 18,8 1,40 Guinea 13,13 1651 18728 (No stress) 64% 30% 44,6 1,32 Guinea- Bissau 1,97 1577 8752 (No stress) 59% 18% 35,3 1,84 Mali 20,25 282 3241 (No stress) 83% 45% 70,7 3,04 Nigeria 206,14 1150 1158 (Stress) 78% 43% 68,6 1,20 Senegal 16,74 686 1673 (Stress) 85% 57% 23,9 4,00 South Sudan 11,19 900 2383 (No stress) 41% 16% 63,3 2,52 Togo 8,28 1168 1494 (Stress) 69% 19% 41,6 2,04 Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 32 In this part of the project, we will analyse the essential characteristics of water in the region. Starting with the precipitation, we must consider that the Köppen climate is a discreet classification. Still, the precipitation maps are continuous, so we must expect Equatorial countries to be safe regarding water availability, with supply and scarcity problems related to economic difficulties. In there, we find a flood-prone zone. On the other side, we will discover territories that border the arid climates of the Sahel, and there we will see both floods and droughts that can be very severe. Even in these drought-prone zones, we can't ignore floods, and we still hold the previously mentioned idea that the zones with drought are more vulnerable than flood-prone because, in the first case, they cohabitate in the wet season. Secondly and very remarkable is that most countries share two or three climates simultaneously so that the average may lie to us. There are scarce zones around the Sahel where the main focus must be on the drought risk index. We will find healthy countries like Cameroon, CAR, Congo, Gabon, Equatorial Guinea, Guinea or Guinea-Bissau in hydrological terms (UNDRR & CIMA, 2019). On the other hand, we will pay special attention to countries like Senegal, Mali, Burkina Faso, Benin, Nigeria, Chad and South Sudan. Some of these countries have water insecurity due to climatic conditions, both most of them also add up a crucial factor, the demography. The region is growing with no expectancy of flattening the curve in the following years. On the contrary, the horizon seems far away by now. Regarding the drought risk index, we must point out that four countries overscore among the rest, Burkina Faso (3,40), Chad (3,16), Mali (3,04) and Senegal (4,0), the most drought-prone country in the zone. Now it's time to talk about the water access and facilities. This time, different from the previous two cases in America, the data we obtained regarding drinking water and sanitation facilities is essential. In this case, we can see how none of these countries has an appropriate level of critical drinking water because the better performing is 15% away from encompassing the service universally. In sanitation, the best data is found in Senegal, where 60% of the people meet basic sanitation facilities. If we took the best of the data and multiplied it by the 550M people living, still more than 75M people would lack basic drinking water facilities, and 220M would lack basic sanitation. So, we can see that the problem is enormous and out of scale. And it is intensified in the most undeveloped places like South Sudan, DRC, Chad, CAR and Burkina Faso, where basic drinking facilities are only available for one out of two or three people. These countries have basic sanitation only available for the 15% of the population or less. Let's look at the directly associated mortality due to water. We will also find the highest ratios in the world, with a unique alarming situation in Chad, with 1010 deaths per million people per year. But, to focus on the good too, we must say that the coverage has been increasing continuously during the 21st century, even though the rate per cent of the covered population is still low and has a lot of way until reaching acceptable levels (UNICEF, 2017). Regarding floods and droughts in the region, we have already said that flood is generally predominant due to outstanding precipitation records. But we must also consider the numerous droughts and harsh water availability conditions in the Sahel. Regarding this second problem, more focus on the boundaries of the climate when meeting the desertic and arid zones, also considering the most unstable zone politically. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 33 Regarding droughts, we can start analysing the western part of the region, the one connecting the Sahel and the Gulf of Guinea. These countries have around 10% of their population impacted by droughts yearly. For example, almost 4M people in Ghana, more than 2M people in Cote d'Ivoire, almost 2 M in Cameroon and 0,5M people in Mali, but the tendency is that the droughts will affect 30% of the people living in these countries (Tsegai, D. et al., 2022). Knowing that the countries of the region we took as a sample follow the 10% rule, we could estimate an affected number of 30M people from Senegal to Nigeria. The countries facing the Atlantic seem to suffer the most, as we will see in future climate change predictions (GFDRR, 2019). Then, let's move to the countries that face the Gulf of Guinea westward. We find a slightly different scenario for droughts in Equatorial Guinea, Gabon, Congo, etc., where the affected population is around 1% to 3%. Also, the losses are lower in comparison. The zone is more flood-prone and the precipitations higher. If we continue down, we find Angola, partially tropical and semi-arid, so here we go again with the previous rates. 2M people have been affected annually, representing the 7,5% of the population, with projections of 8M people affected being the 10% of the population. Regarding the losses, $134 million are attributed now, but after 2050, the losses could be more significant than $840 million, turning Angola into one of the most affected regions (UNDRR & CIMA, 2019). Regarding the central African countries like Chad, South Sudan, the Central African Republic, and the Democratic Republic of the Congo, the obtention of good data has not been easy at all, and the records vary a lot between regions. CAR and western DR Congo have similar conditions to Gabon, Equatoria Guinea or Congo due to the same rainfall patterns, temperatures and vegetation. As far we go to the east or the south, the precipitations decrease, and we have transition climates to those found in Rwanda, Uganda, Zambia and Angola, so the evolution of the affected people ranges from 2% to 10% of the regions. On the other hand, Chad is an arid country, affected yearly by droughts, where about 2 to 5M suffer from food insecurity annually, and where the main water lake has gone dry by 90% in the last century (OCHA, 2020). The same happens in South Sudan, another drought spot in the region. The whole country falls into maximum food insecurity levels in the dry season. Figure 32. A dry river in Mali showing the importance of water. (UNHCR) Flooding is the most common hazard in the region. The equatorial part is so rainy that floods are intrinsic, but also because the dry lands in the Sahel and Angola cannot absorb water, prolonging the flooding situation when the wet season comes, inducing the massive population displacements. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 34 Floods in the western region affect 500k Malians on an annual average, as well as 40k people in Ghana and 40k in Cote d’Ivoire, with associated economic losses of $260M, $100M and $70 million, respectively. Extracting data from the last year, 145k Nigerians were affected, as well as 100k Congolese and 34k people in Burkina Faso. Floods in Cameroon affect around 120k with losses of nearly $100 million, whilst Gabon has 6k impacted and $50 million in losses, and Equatorial Guinea has 2k affected and $20 million in losses. If we keep going down, in Angola, we start to find severe effects again, with 25k affected annually on average and $93 million casualties (UNDRR & CIMA, 2019). Figure 33. Floods in South Sudan, 2017. (Reuters) We can appreciate how the impact in semi-arid zones is higher in floods and droughts. The coastal zones and the equatorial strip have lower indices of affectation in both droughts and floods, revealing to us that places with less water are more prone to all kinds of hazards. The direct economic repercussion is very high in regions with poor development, limiting the possibilities or pace to escape poverty in the following decades as stated by the Sustainable Development Goals. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 35 1.2.4 The African Great Lakes & Madagascar The eastern and southeastern region of Africa is one of the most amazing places in the whole world. Considered humankind's birthplace, it has its origin in the rift valley that extends from Ethiopia to Mozambique, creating the Great African lakes along its way. The region hosts about 420M people, and it is also catalogued as one of the world's poorest regions, together with the neighbouring central and western Africa. Politically though, it is generally a more stable zone but still has severe problems, as we will see. It is again remarkable the presence of Chinese investment in all kinds of infrastructural projects in the region, from roads to railways and harbours. Figure 34. East Africa political map and Köppen classification map. (McMahon, T. A., 2007) Table 8. Region IV - General characteristics REGION IV Countries Demography Economy & facilities Population in millions (2020) Urban population in millions (2020) Rural population in millions (2020) $GDP per capita & GDP growth (2018) Electricity access (2019) Internet access (2019) Burundi 11,89 1,63 (14%) 10,26 (86%) 239$ (2%) 11% 5% Ethiopia 114,96 24,94 (22%) 90,02 (78%) 772$ (7%) 48% 25% Kenya 53,77 15,05 (28%) 38,72 (72%) 1794$ (6%) 70% 23% Madagascar 27,69 10,67 (39%) 17,02 (61%) 524$ (3%) 27% 15% Malawi 19,13 3,33 (17%) 15,8 (83%) 545$ (4%) 11% 16% Mozambique 31,26 11,59 (37%) 19,67 (63%) 503$ (3%) 30% 15% Rwanda 12,95 2,26 (17%) 10,69 (83%) 784$ (9%) 38% 26% Tanzania 59,73 21,04 (35%) 38,69 (65%) 1043$ (5%) 38% 20% Uganda 45,74 11,41 (25%) 34,33 (75%) 771$ (6%) 41% 20% Zambia 18,38 8,2 (45%) 10,18 (55%) 1516$ (4%) 43% 19% By economic and development indices, Eastern and Southeastern Africa are the poorest and most undeveloped regions in the world. The population, mostly rural, have poor living standards and a GDP per capita lower than 2000$ annually with despair growing rates that don’t matter deafer that much from the world average. In most cases, it doesn’t Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 36 even reach 1000$, with average countries having between 500$ and 800$ per capita. Burundi is remarkable because 239$ is the poorest country in the world, and this data is validated with other indices that situate the county in the same position. The situation is improving globally, and all the countries have improved economically. In terms of access to essential services and facilities, the truth is that the atmosphere is fragile to relax and think that these countries will continue growing and developing without any problem. As in Central and Western Africa, we can also find a set of countries with promising futures. Kenia, Ethiopia and Tanzania are the leading economies in the region. Kenia is the wealthiest country in the area and hosts also the most diversified economy, with a significant tourism sector devoted to nature and animals sightseeing and an intensified presence of tech companies in the capital Nairobi. There is also a substantial economic pole around Mombasa, where the most significant part of the country and the petroleum refinery are established. A new railway is being built across Rwanda, Uganda and Kenia that will strengthen the potential of the harbour (paid for by the Chinese). The gross of exportations is due to the production of agricultural products like cereals, tea, coffee and sugar. Figure 35. New Kenian railway line between Nairobi and Mombasa. (EFE agency) Tanzania is a vital economy, but it is far behind the Kenyan one. Industrialization is lower in this case, and most people work in agriculture. Even just 4% of the land is cultivable. There's also an important fishing industry around Lake Victoria. The tourism sector has constantly been growing, and now it has become one of the largest sources of income. Tanzania also has tons of natural resources like gold and diamonds that are exported together with agricultural goods like coffee, tea and cotton. The importance of Dar-es-Salaam is crucial for the development of the region, showing the immense significance of infrastructures like its harbour again. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 37 Lastly, Ethiopia is both a vital economy currently and a promising one for the whole of Africa. Its GDP per capita has increased by 200% in 10 years, and the political determination to make Ethiopia the leading country in the Horn of Africa is accelerating the region's development. Ethiopia was one of the poorest countries. It is scaping from it with a mixture of foreign investment, liberalization of the economy and public expense and control in clue sectors like telecommunications, airways and energy. Ethiopia still has a long way to develop. Still, its 120M people create a labour network that, with the right policies, will make this country one of the most powerful economies in Africa. The treaty with Eritrea has led to freight importation and exportation through its ports. But currently, Ethiopia has an open war against the separatists of Tigray, blocking some of the logistic operations in the region. The region has also found in Rwanda an up-and-coming economy. Rwanda, the land of 1000 hills, is an African country rich in multiple mineral resources that have liberalized the economy, and it is currently attracting many companies to the country. Even though we must question some political aspects, the truth is that Paul Kagame and its government have been able to situate Rwanda as a pole of investment and development in the middle of Africa. They have duplicated the GDP/capita in 12 years, with 8% sustained economic growth per year, and reduced poverty levels from 80% to 55% of the population. The electricity and internet connections have increased constantly, duplicating the household connections in five years. Also, tourism is becoming popular, and the sector perceives noticeable incomes. The imports from the country are based on agricultural products and mining resources like tungsten and pewter. Since Rwanda is an inner country, the connection with other countries by railway is crucial to export their goods. This is why they are investing in a railway line to connect Kigali with Kampala, Nairobi and Mombasa, and a railway connecting Dodoma and Dar-es-Salam. The rest of the countries have more modest economies. We can differentiate between three positive evolving countries and three of them that are stuck or even retroceding. Despite being poor, Uganda, Malawi and Zambia are taking reasonable steps toward development. Zambia has shown political stability, which is crucial to attracting investment and very rare in Africa. Uganda has a lot of potential due to fertile lands and mineral resources. Malawi is starting to tackle the corruption, the nightmare of the country, showing a better evolution and more confidence in the investment. On the contrary, Burundi, Mozambique and Madagascar are very fragile and now show little signs of changing their trends. Burundi is a failed state. Mozambique is suffering terrorist incursions in the northern part and struggles to escape poverty, losing 30% of GDP per capita in just six years. Meanwhile, Madagascar suffers from multiple hazards, as we will show later. Famine and crop failure are so common that urgent humanitarian assistance was needed after four years of drought, leaving us with pictures like the following. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 44 Figure 40. Container terminal in Vietnam. (Asian Development Bank) We have to talk about the problems in the region too. We can distinguish two undeniable concerns, the environmental and the political. We can still find several political issues in the area because we have problems between India and Pakistan, India and China, and Bangladesh and Myanmar due to the exodus of Rohingya, an ethnicity persecuted by the Myanmar military. Also, Myanmar suffered a coup in 2021, and Thailand is recovering from another coup in 2014, holding the first elections in 2019. Also, Laos is a particular state that has adapted to capitalism but without any political changes, and it has a similar regime to North Korea without the same military level. If we look at Sri Lanka, the country has been immersed in several riots this 2022, and the crisis above them seems to be unstoppable. Vietnam and Thailand are the region's stabilisers and show the path the rest of the countries should take.Environmentally, climate change is striking the region, and many problems will arise in this nation. Droughts, crop harvesting decrease, overpopulation, and air and water pollution are critically affecting the region. Although the area consolidation is one of the most fast-developing and the scenario is promising, they can't stop keeping an eye on the problems that strike them every day. Figure 41. Air pollution episode in Mumbay. (Getty Images) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 45 Table 11. Region V - Water characteristics REGION V Countries Demography Water Population in millions (2020) Yearly average precipitatio n in mm (2017) Renewable freshwater (m3) per capita per year (Falkermark) (2018) Access to basic drinking water (2020) Access to basic sanitation (2020) Mortality due to water per 100k people (2016) Drought risk Index (1-5) (Statista, 2015) Bangladesh 164,69 2666 658 (Scarcity) 98% 54% 11,9 3,00 Cambodia 16,72 1904 7533 (No stress) 71% 69% 6,5 2,88 India 1380,00 1083 1080 (Stress) 90% 71% 18,6 3,44 Lao PDR 7,28 1834 27384 (No stress) 85% 79% 11,3 2,00 Myanmar 54,41 2091 18789 (No stress) 84% 74% 12,6 1,40 Philippines 109,58 2348 4554 (No stress) 94% 82% 4,2 2,60 Sri Lanka 21,92 1712 2462 (No stress) 92% 94% 1,2 2,44 Thailand 69,80 1622 3244 (No stress) 100% 99% 3,5 3,24 Vietnam 97,34 1821 3799 (No stress) 97% 89% 1,6 2,40 In this part, we will analyse the region's actual precipitation, water resources and facilities. We can see how the countries have a clear pattern of tropical rainfall, above 1000mm, and some above 2000mm, due to the coexistence of rainforest zones in the country. Just India is water-stressed, and Bangladesh is highly water-scarce. The rest perform enormously well in the Falkenmark Index, which clearly shows that this zone is more prone to floods than droughts. But, looking at the drought risk index, we can see that the values must not be underrated. Four countries perform above 3 in the ranking, which is precisely tiny. Bangladesh and India, both the most water-stressed, are also suffering droughts. We must remember that this zone is amongst the most vulnerable and exposed. And Thailand, a province in the top 3 of world flood-prone zones, has a score above three too. The lowest value is found in Myanmar, with a 1,4. Regarding the drinking water and sanitation basic facilities, we find values close to universal coverage in the region's most developed countries, like Vietnam, Thailand and Australia again. The fact that Bangladesh has a 98% coverage is, by far, the most shocking data in this column. In India, the access is still at 90%. Let's look at the UNICEF report on water access. We can see a considerable improvement in basic drinking facilities in all the zone, from 81% to 93% in 17 years and also a reduction of unimproved drinking water facilities by 10%. As always, we have to take into consideration the growing population of the region. The sanitation coverage performs better than all the other regions, and the basic facilities are satisfied in Thailand and Sri Lanka, while Vietnam is on its way. The country with the most lack of basic sanitation infrastructure in Bangladesh, with a poor 54%. Southern Asia has experimented with the most significant growth in sanitation coverage and is an excellent example of focusing the policies on basic infrastructure implementation. It is, in fact, the region with the best infrastructure coverage while economically is not the richest, lying between Africa and America. We can find a big problem when considering the mortality rates because we might widely spread sanitation. However, the region is still the most polluted in the world by residual plastics, chemicals and agricultural products, so the water pollution is incredibly high and consequently the mortality. India, the biggest country in the world in a couple of years, Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 46 has a tremendous 186 deaths per million people yearly. Considering a population of 1380M, 256.680 people die directly from water conditions annually. The pollution in India is vast, and we have already discussed how bad the situation is in Asia, not just with plastics and microplastics, but also with oil spilling, contamination in the harbours, sulfates and nitrates in the rivers, etc. Also, the rest of the region performs extremely severely in terms of deaths due to pollution, and it is even worse when comparing it with the acceptable levels of sanitation services that other areas don't have. When it comes to floods and droughts, the following data arises. Flooding is the most frequently occurring disaster in South Asia, causing half the total damages in the last 40 years (World Bank, 2012). The yearly average affected people by floods in the western region is; 7,5M in Bangladesh, 20M in India and 248k in Sri Lanka, causing an average of 2000 deaths per year. Drought numbers are also considerable; 641k in Bangladesh,25M in India and 56k for Sri Lanka. Even though the economic impact of droughts is high, we can't compare it to floods, which compute approximately 70% of the potential financial loss (2035MEUR just in India) (World Bank, 2012). Figure 42. Overexploited well in India. (EFE agency) In the Indochinese peninsula, the numbers are not that different. Floods are the most common natural hazard and cause enormous socio-economic causes. Cambodia, Vietnam and Thailand are in the top three, with 33, 28 and 22% of zones under high flood risk, respectively. These three countries have an annual average of affected people of 250k, 750k and 1M, respectively. And they have significant drought records, with 250k in Cambodia, 200k in Vietnam and 1M in Thailand. It is also remarkable that Myanmar doesn’t have drought records and that the Philippines has 250k affected people both by droughts and floods, being one of the most affected countries. In terms of economic losses, floods annually cause 340M dollars, and droughts about 50M. (ASEAN Disaster Risk Management Initiative, 2010). Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 47 1.3 Future Scenarios Once we have analysed the different regions from both social, economic and hydrologic aspects, it is time to focus on how these conditions will change due to climate change and demography. We will first focus on the impact of climate change to understand the trends in precipitation, evapotranspiration, soil moisture, climatic zone expansion or decrease, etc. And afterwards, with the new water budgets, we will be able to introduce the demographic variable and contemplate the changes in water availability. These changes are the ones that will help us understand the social and economic impact that we may see. 1.3.1 Affected regions Climate change is the set of weather pattern changes that occur on the Earth and can be due to multiple causes like natural or anthropogenic ones. When we refer to climate change nowadays, we focus on the anthropogenic changes that are affected our planet during this last century and will affect the mid-term and long-term future of humankind as a response to the massive quantities of carbon dioxide emissions from the industrial revolution. We have been using materials that emit carbon dioxide when burned to use its energy. As a consequence, we introduced carbon dioxide that was underground into the atmosphere, changing its energy balance due to radiation absorption of these emitted gases. These energy changes modify the temperature of the Earth, the water cycle, the current intensity in the ocean, and hence, the climatic zones on the planet. The IPCC reports four different RCP (Representative Concentration Pathway) scenarios of average temperature rise, between 1,0º and 2,0º between 2046-2065 and between 1,0º and 3,7º between 2081-2100. Figure 43. Annual mean temperature change relative to 1850-1900. (IPCC A, 2021) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 48 Figure 44. Annual mean precipitation change relative to 1850-1900. (IPCC A, 2021) Figure 45. Annual mean total colum soil moisture change. (IPCC A, 2021) The increase in temperatures and its derived effects on precipitation and evaporation will shape the new Köppen Climate map, with more rainfall and a loss of precipitation moisture. In the following photos, we can see these changes graphically. Figure 46. Köppen classification map 2016 vs 2070. (Beck, H. E. et al., 2018) The four maps above can already explain the changes that originated in our study regions. Central America and the Caribbean will start losing tropical rainforests due to monsoon or savannah tropical climate characteristics. Although the temperature increase in the area is not the most significant among the studied zones, the loss of precipitation will be noticeable, between 10-40% depending on the RCP. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 49 Regarding Latin America, the trends are even worse than in central America. The temperature rise will be even higher, especially in the Amazonian, all the region will lose between 10-20% of precipitation, and the soil moisture will decrease drastically. The rainforest climate will decline, and the savannah will be predominant, displacing subtropical climates in Argentina (Liu, C., & Allan, R. P., 2013). In Africa, the climate will significantly change in the southern part, which will become dryer. The tropical rainforest seems stable, but the tropical savannah will extend between the tropics, displacing the subtropical climates of Zambia to the south, which will disappear due to the advance of the desertic and semi-desertic environments of South Africa and Namibia to the north. The Sahel strip will be similar in shape, but it will retrocede some kilometres to the south even if the precipitation and moisture increase. The situations will worsen in Mali, Burkina Faso and Nigeria, which will become drier. Senegal will fall out of the list of tropical savannah climates, hosting just arid and semiarid environments. The eastern part of Africa is supposed to lose desertic terrain that will become semi-desertic. This would be great news for countries like Tanzania or Kenia, which are very stressed. These changes in the climate occur due to substantial changes in rainfall and moisture in the whole central African strip. Madagascar, Malawi and Mozambique are the only analysed countries in East Africa where precipitation and humidity will decrease (Liu, C., & Allan, R. P., 2013). The last region, the southern and south-eastern areas of Asia, will have two different situations. On the one hand, India will predominantly have a tropical savannah climate, and the rains and soil moisture will increase. On the other hand, the Indochinese peninsula will lose rain (less or equal to 10%) and water, but they will still keep the same climate. 1.3.2 Hydrology & water budget: updating the situation We have seen how the world must prepare itself for significant climate changes and their impact on the hydrologic cycle. But we cannot predict the future of water resource access and availability without interaction with people; as in water resources, both the human and hydrological systems cohabit and interact. We can summarize the impact on water resources with two main factors: more people (human system) and less renewable water (hydrologic system). Regarding the first factor, it is known that the human population will continue to grow. The majority of predictions show that we will reach the 10.000M people. The UN predictions show an 80% confidence that the population in 2100 will be between 10-12 billion, and the chances of flattening the curve by then are minuscule compared with the scenarios with moderate but still growth. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 50 Figure 47. World population projection. (United Nations, 2019) This leads us to the simple conclusion: if we have the same water resources in 80 years, there will be less water per capita. To be precise, in 2018, with a population of 7592M people, we had 5658m3 per capita per year of renewable freshwater. So, in 2100, with the current demographic predictions, we would have between 3579m3 and 4296m3 per capita per year of renewable freshwater, approximately 35% less. We are getting closer to the limit of 1700m3, and we haven't even considered polluted water, climate change, seasonality or an increase in water consumption. This last one is crucial since developed countries consume ten times more water than undeveloped, and most of the countries are growing, so the required supply will be higher and higher. If we applied this reduction to our studied countries, we would have three to five countries with absolute scarcity instead of one and 14 water-scarce countries instead of 8. And we are just talking about one modification in the most conservative index to analyse water availability in a rainfallprone region. But, if we want to explore the actual water per capita in the future, we must also talk about the hydrologic system. Predicting the available freshwater resources for 2050-2100 is difficult to quantify. Still, qualitatively, we can describe how this future will be and the implications it will have in the Falkenmark Index and posteriorly in other indices. The warmer air temperature in the atmosphere is accelerating the water cycle with 1-3% more rain per each extra Celsius degree of temperature, which implies, in some of the studied regions like Africa and India, but also in other parts of the world, an increase in rainfall records. In other places like Central and South America, or the Indochinese peninsula, the rain will decrease as a result of multiply variables. This change doesn't come alone, though; it doesn't mean that we will have more renewable freshwater because warmer temperatures will also increase evaporation. Extreme events will also be more common, so renewable freshwater resources are about to decrease (Allan, R. P. et al., 2022). Thus, we must expect the water per capita to reduce even more since the denominator of the equation decreases. Entering into the details of future water hazards, the acceleration of the cycle is causing more extreme weather events like intense rainfalls, cyclones or lack of rain and snow. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 51 The IPCC AR6 reports that with an increase of 1.5ºC, droughts will divide their frequency by 2, the rainiest day will see 1,5 times more rain, and tropical cyclones will increase by 10%. The same results for a 4ºC increase are droughts of the same intensity with four times lower frequency, the rainiest day receiving three times its original precipitation and a 30% increase in tropical cyclones (IPCC, 2021). We can see then how floods and droughts will become more frequent and intense, as we have seen in the reports of the affected tropical regions. Their social and economic impact will be huge, especially in countries where development is still an issue and where future political and economic prospects aren't good. We will have to care primarily about food insecurity until the food production product and its supply chains are more robust than now. We should remember that floodwater represents 20% of the accessible freshwater that we cannot collect by now, so, with more flood water than ever and severe floods more frequent than expected, we must add some resources that we previously had access to the list of inaccessible ones. We insist on this point and must aim at techniques able to collect more floodwater. Also, the destructive repercussion of floods in soil and vegetation doesn't help retain water, and more land management is needed after this episode to obtain the same results. This means that floods constitute a challenge for collection, storage and conveyance structures, and their economic effects will multiply if we cannot mitigate them and take advantage of all this water for the dry season by improving the storage and conservation. Regarding droughts, the other main topic that we analysed, we have also seen that they will be more severe both in time and intensity, with the reduction of soil moisture and increased land temperature in many tropical wet and dry regions. Droughts are worsening the water crisis in places where, due to relatively low time or intensity, they enjoyed suitable climate conditions to keep up with an average life until now. We have to consider also the already drought-prone places where, by general rule, drought will be even more common, where entire regions are getting drier and see threatened their economic and food system. Droughts are one of the most widespread catastrophes, and their slow but constant impact keeps undermining the situation until we can do nothing else. It's essential to have emergency supply plans prepared for crop failure, but states also need to invest in drought resilience, which is the first thing they can offer to the population. The droughtrelated indicators like WSI will increase almost everywhere due to both droughts and an increase in population. It's time to see (again) massive droughts and famine. Regarding the Water Poverty Index, even if all the regions are supposed to hold economic development, adverse climatic conditions like the ones we described could make their escape from poverty more complex and implement basic drinking and sanitation facilities universally. Increasing GDP expense in mitigation and aid after catastrophes can weaken economies yet be undermined. One of the biggest problems is that the water crisis could even help turn down stable governments during prolonged droughts or food crises, returning to economic positions where the country cannot sustain and manage basic infrastructure (failed states). The truth is that economy is one of the most unpredictable things. All countries come across cycles of prosperity and austerity, so we cannot give statements of fact about what will happen next. Still, climate change and particularly lack of water are political and economic concerns of the first order worldwide. And, again, the situation and circumstance are more remarkable in the most disfavoured places. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 52 So, in summary, the general picture indicates that renewable freshwater resources will decrease worldwide because the rain might be heavier. Still, it will also evaporate proportionally (both variables are correlated), while this extra intensity will cost human lives and economic losses, and we are not taking profit from the floodwater. At the same time, the lack of rain for extended periods as well as the loss of moisture and the high temperatures of the soil will induce droughts that will damage both the food production and the soil conditions, killing vegetation and biodiversity and inducing more water runoff due to soil conditions so, again, less water percolating and being storage. Since the crops and people still need water, groundwater extraction during more prolonged drought will cause environmental problems. Also, the aquifers can run out of water or, at least, have significant water level drops that are difficult to recover. Hence, less water will be available again, and the problems will continue in this retro alimentation chain, without groundwater, more droughts, worst soils, worst floods and less storage. Less storage means the least groundwater, and the cycle keeps spinning as we look at it. This is why, in the next part of the thesis, we try to answer the following question: What happens when you open the tap and no water is running? What happens when the well is dry? Until which point can people remain in a place with little water? It is time to introduce what, at least from my point of view, is the most challenging topic I read and talked about in the crisis - the way from water shortages and supply cuts to migration and conflict. 1.3.3 The world we leave to our children: from supply cuts during water shortage to migration & conflict We have already analyzed the problems regarding the new water future that we will have to face, with more people, less renewable water, more extreme events and economic difficulties to meet the climate change crisis. But these problems seem to be political problems that only the governments can solve, high-calibre national plans involving engineers, economists and lawyers. Both legislation and execution of big projects regarding water will indeed be conducted by the different competent authorities to readdress the situation, but what will happen when the governments and communities cannot cope with the water crisis? What happens when taps and wells run dry? The general answer is that we face different levels of impact on people. The first response is supply cuts during water shortages. Water shortages represent the difficulty of obtaining freshwater resources, and supply cuts result (or sometimes solution) in facing drought and shortage situations. Supply cuts have different severity, and most of us have lived under this condition at some point in our life (summer of 2007 in Catalonia, for example). Supply cuts imply a change in our daily habits, but they are made to ensure fair access to water. On the contrary, they are unintentional, but we can still cope with the situation. They can also affect agriculture, but the severity of the problem starts being more serious, inducing migrations in the worst case. Deliberate water supply cuts can target activities or water structures like swimming pools, public fountains, golf courts or car washing, and target daily water consumption, limiting hours of use or pressure in the system. They are standard water policies when the reservoir's lack of rain and water starts to be alarming. We have seen them around Europe (Spain, France, Germany, Italy), Cape Town and Durban in South Africa, where the concept of "Day- Zero" arose because there was no water available, and they supplied people with tank Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 53 trucks and illegal connections. In Cape Town and Durban, there is an extensive list of places where people take turns to access water, like Caracas in Venezuela or Sao Paulo in Brazil. With increasing population and less freshwater, we must expect water supply cuts to be normalized almost everywhere. Figure 48. South Africa’s water crisis. (www.nationalgeographic.com) The next level is that we will find a different place if there is no water to live in. This migration is a hot topic related to climate change (climate refugees). This doesn't only imply a lack of water access but also implies displacements associated with floods, cyclones, sea-level rise and more severe droughts that are not limited to some water supply cuts. Let's take a look at Groundswell reports. The impact of climate change could increase the number of climatic refugees to 216M per year, and the Internal Displaced People (IDP) could rise to 85,7M people, with the most significant percentage representing the poorest people with fewer resources (Clement et al., 2021). Nowadays, we are limited to just 10% of these values, and our climate refugees oscillate between 20- 30M per year (Michelle Yonetani et al., 2015). Figure 49. New displacements by conflict and disasters 2020. (iDMC, 2021). Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 60 2.1 Detention methods Detention methods are the first barrier against floods that we can provide. Their main goal is to reduce the peak flow to prevent the water level from rising excessively, causing damage to the environment, agricultural lands and urban facilities. Detention methods control the water velocity (slowing it down from the headwater) or by momentarily storing the difference between the inflow and outflow of a detention structure. Also, they can use absorption methods to decrease the water run-off, especially in urban environments, draining waters into rain gardens using high-drainage pavements, for example, following the concept of “sponge cities”. All these methods prevent soil erosion and use land management, and we can even improve the situation of the surrounding area to strengthen communities against floods. If we look at a hydrograph, a detention method will show smaller values on pick flow. Still, longer discharges after the event resulted from intermittent storage and slowed down. This particularity also helps us collect water from the points where velocity characteristics are more optimal, targeting the collecting and conserving flood water (considered non-accessible) in posterior stages and consequently establishing a connection between detention methods and retention or recharge ones. Even though the structures and methods work combined, the separation between them can be blurry. We could settle the difference between detention and retention with the following reasoning: storage can be a consequence of detention (especially when using nature-based solutions that will store water) but is not the primary target. The magnitude of storage is minuscule compared to reservoirs, sand dams, etc. Figure 55. Flow through a detention weir (N.K., 2014) 2.1.1 Velocity control methods Check dams & Gully plugs Check dams and gully plugs are small structures built across rivers with little water flow, streams, gullies, ditches or channels to reduce water velocity and erosion downstream. They interrupt water flow and decrease the channel gradient, helping groundwater infiltration, delaying pick runoff and increasing the baseflow. They also contribute to water quality by trapping garbage. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 61 These structures are typically used in small streams with basins smaller than 0,04 km2. They are short in height, ranging from 0,5m to 2m (3m at most, and it’s not usual), and their span depends on the gully, stream or riverbed profile. They can be built of diverse materials, from wood logs to rocks, sandbags, gabions, masonry or concrete, giving certain flexibility to relocate or remove the structures. Figure 56. Gully plugs in Ethiopia (Global Climate change Alliance, 2020) They are usually built in several positions of the stream, following a 3% rule gradient, as we can see in Figure 57. They shouldn’t be placed in streams where big solid masses tend to detach because they can destroy the structure and themselves can serve as water retention obstacles. The check dams or Nala bunds are then classified as surface recharge methods if used for recharge as the main characteristic. Figure 57. Cross section of a stream with check dams. (FAO watershed management field manual, 1986) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 62 Terracing Terracing is an ancient farming and flood protection technique in Southeast Asia and the Andes region. It consists of the re-shaping of hills using terraces. They were thought to retain more water for agricultural activities in high-sloped terrains once the gradient was flatted. But, as we previously explained, the decreasing water velocity is highly correlated to flood protection and the introduction of vegetation to our system. Moreover, soil erosion is prevented and conserved. Terracing results are a good flood mitigation tool and an excellent method to improve farming results and enhance biodiversity preservation. The terraces can be flat or little graded, and they are typically built on slopes between 15% and 25%. They are retained by stone walls (called terracing walls) on some occasions to prevent the soil from moving due to active forces, and their height is typically less than 2m. They have proven to decrease water runoff and soil erosion by 50% in regions like China (Qi, W. et al., 2021) and Morocco (Meliho, M. et al., 2021). The disadvantage is that terraces are extensive earthworks and require adequate management and building. In case of abandonment, the results can be worse than in natural hills (Deng, C. et al., 2021). Common failure causes are unsustainable floods that create alternative streams across the terrain, terracing collapse, landslides, water circulation interruption or progressive loss of vegetation. Figure 58. Terracing scheme. (Deng, C., et al., 2021) Figure 59. Terracing in Vietnam. (Getty Images) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 63 Fanya Chini & Fanya Juu trenching Fanya Chini and Fanya Juu are trenching techniques used in farming areas with low to middle slopes (<15%) to prevent soil erosion and promote water infiltration, helping in flood mitigation. They consist of the excavation of a little trench, upper or lower, depending on the method, that helps retain water in the trench but also in the soil, keeping it moist. The advantages and disadvantages are very similar to terracing because, in fact, they are very similar techniques, but the main difference is in trenching instead of terracing, working with flatter slopes and easier to manage, but the difference is blurry. Figure 60. Creation of fanya chini in rural Tanzania. (Lead Foundation) Detention or retarding ponds Detention or retarding ponds are medium water collectors’ facilities that get a water input from adjacent water bodies to absorb part of the runoff and release it posteriorly, in much less volume per unit of time. The difference between the intake (that allows large water flow) and the outlet (with a limited discharge) is momently stored in the pond. Detention ponds are catalogued as one of the best flood management practices when stormwater events become very intense. The basic ponds are designed to empty in the first 6 to 12 hours after the storm, but new structures like extended detention basins can store the water between 24 and 48 hours. As slow the output flow rate becomes, fewer suspended solids are discharged, improving water quality. Economically, detention ponds are not expensive and particularly useful in dry places where water storage on ponds is not feasible but helps mitigate occasional floods. The disadvantage is the attraction of mosquitoes and their related diseases like malaria, which kills 600.000 people per year. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 64 Figure 61. Detention pond in the outskirts of an urban area. (Getty Images) 2.1.2 Absorption methods Rain garden (bioretention) Rain gardens (or Bioretention facilities) are nature-based spaces designed to absorb the urban runoff from storms, either temporally or permanently, in the ground. They store more water resources than in conventional gardens or urban facilities thanks to adequate vegetation and bio-retentive soils. Rain gardens appear as feasible, sustainable and lowcost water detention, storage and infiltration methods (Ghofrani, Z. et al., 2021). They are also an excellent alternative to starting water treatment since they can treat the polluted runoff. Even though it is categorized as a detention method, it highlights the wide range of options it offers when combined with storage facilities or sustainable urban drainage systems (SuDS). Quantitatively, rain gardens reduce stormwater up to 80 under regular rainfall events and should be able to drain more than 100-125 mm per hour in initial conditions. Figure 62. Rain garden in Virginia, USA. (VCAP) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 65 The rain garden comprises three layers; vegetation, soil and conveying or storage facilities. The first layer, vegetation, slow down the water velocity. The absorption of water is done with wetland vegetation and small trees. The second layer is the soil, where the priority is building a very porous soil able to infiltrate as much water as possible. This layer is usually protected with gravel, a geotextile coat and filtering protection towards the third component, the structural one. The last object, and the first to be placed, is the storing facility or the drainage system. Since we must limit water infiltration to prevent structural damage with swelling and shrinking of the soil, the most efficient action is to place a collector that conveys into tanks or balance lakes outside the city that can act as recharge ponds. Figure 63. Rain garden scheme. (Pinterest) Figure 64. Bioswale construction. (Wikipedia) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 66 2.2 Retention methods Water retention methods consist in the collection, control and storage of water resources in both surface and subsurface. Due to the storing capacity, surface retention methods allow to control the water flow in the watersheds where infrastructure is built. In fact, dam engineering and reservoir management are a whole subject, and a key point to work of the water-food-energy nexus, but out of scope in this thesis. But what we can learn from reservoir management is that these huge water masses constantly contribute to flood control and mitigation, and ensure water security during the dry season. Settlements where dams are present upstream have certainty of better protection against floods, and they need les financial investment to access the water resources. On the contrary, isolated settlements far from rivers, lakes or reservoirs, or located in short basins where there is no margin for storage, will rely more in small water harvesting from rainfall or groundwater withdrawal and will need to pay more attention to floods and how to detain them. This problem is common for example in the hilly villages near lake Malawi, which suffer flash floods during rainy season. 2.2.1 Rainwater harvesting Rainfall harvesting Rainfall harvesting consists of the installation of structural facilities able to collect the rainfall water runoff generated on the roofs of buildings, houses, industrial structures, or in the street to store it for domestic, agricultural or industrial use. Rainfall is stored in household tanks (for domestic use) or standard tanks, usually more extensive and made to collect the water from the houses and convey it to the central storing point. The rainfall that generates runoff in the streets is harvested, too, ideally through detention methods that we previously detailed, like bio-retentive gardens or high-draining pavements. The critical aspect of rainwater harvesting, particularly from rooftops, is to obtain substantial amounts of water with very cost-effective and simple modifications to urban facilities and to avoid flooding in streets due to excessive runoff. The quantity of water harvested depends on three things; precipitation, area of the elements (rooftop, street or basin) and geometry. As an example, if we harvested rain from a school’s flat rooftop of 1000m2, considering a typical precipitation record from a tropical country (i.e. 1200mm) and a loss coefficient of 20%, we could potentially harvest 960m3, or what it’s the same, or 960.000 L of fresh water per year. This simplification shows us how much water can be extracted just from rooftops, which gets intensified in cities where the infiltration rate is practically zero and many surfaces are available for harvesting. Standard-sized plastic collectors for households can store 1000L to 10000L of water. We place them in the garden or on the rooftop under the supervision of a professional. Conversely, municipal collectors are more expensive, probably built with stainless steel or concrete, and require individualized design. Still, the storage capacity can go from a battery of small tanks of 10m3 to elevated tanks of 80m3 or ground tanks of 2000m3 or more, depending on the type and configuration. If placed at a higher elevation than the Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 67 house/building, the tanks are suitable for keeping the desired water pressure while using the facilities and can be recharged by the municipal network when needed. Figure 65. Rainfall harvesting tanks in a school in Kenya. (www.waterislifekenya.com) Fog harvesting Fog harvesting is a passive water collection method based on the installation of fog collectors (flat rectangular nets of nylon supported by a simple frame) that retain the water particles from the fog. Then, they redirect them to the base of the collector, where the water is sent to the storing facilities and ready to be delivered for domestic use. It is a straightforward, efficient, cost-effective and relatively new method that can work in places with the right atmospheric conditions, like in coastal and mountainous areas of 400 to 1200m. Ideal locations are characterized by being in an open position with relatively high elevation exposed to wind flow. This system has been successfully implemented in the Andes, from Chile to Peru. Another main advantage of this system is that we can install more fog collectors if the demand grows or uninstall them if we don’t need them anymore because the design is simple and light. The main disadvantage is that fog collectors have to be near the settlements because long hydraulic conduction works could make it economically unsustainable, considering that the structure is cheap to help access water to isolated and rural communities. Regarding technical details, fog collectors have plenty of sizes, but typical ones are smaller than 50m2, costing 1000€ to 1500€. Each squared meter of installation can harvest 3L to 20L per day, so a conventional fog collector (40m2) could supply approximately 150L and 750L per day. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 68 Figure 66. Fog harvesting nets installed in Peru. (Frontiers) 2.2.2 Surface water storage methods Storage in rivers, lakes and reservoirs Rivers and lakes are the primary natural water surface resources. Their easy access made these waterbodies the most used source of drinking water for centuries. Afterwards, when cities started to grow, and the human impact was noticeable everywhere, the rivers, lakes, and wetlands began to absorb the waste that humans produced, and in some areas, these water bodies have high levels of pollution, from plastic to wastewater, antibiotics, sulphur, etc. The current situation in Asia is critical, with 8 out of 10 most polluted rivers and the top 3 ruled by Yangtze, Ganges and Huan He. Nowadays, rivers are still reliable water sources, but the potabilization treatment is expensive where the water is more polluted. Returning to the main topic, the demand associated with human consumption and agriculture grew when cities were growing, and rivers and lakes became even more critical. Is that high our dependence on river water resources that, by convenience, humans have created artificial lakes (reservoirs) from the 3000BCE? This practice has intensified in the last 200 years when the global population has exponentially increased. We have multiplied by six the irrigated area from the 1900s (2% of the world's land), and the total human consumption is now 600% larger than in 1900, with an additional increase prevision of 30-50% until 2050. So, we have built over 30.000 dams (>15m in height) able to store 7000km3, equivalent to the 20% of the water runoff in the world, decreasing water discharge by 2.1% (Zhou, T. et al., 2015). The data, as mentioned earlier, shows the great importance of this type of structure that has the mission of storing large amounts of fresh water and regulating rivers' discharge, hence, being a flood risk management facility. They can also use them to produce Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 69 hydroelectric energy. So, now that we know the relationship between lakes, rivers and reservoirs that are the central water bodies of any watershed, we must analyze how this network works and the advantages and disadvantages of each component. Lakes are natural water accumulations in basins. Endorheic lakes are water accumulations in a basin with no natural outflow, and their level varies as a function of precipitation, seepage and evaporation. Most endorheic lakes are salty because the lake cannot outflow the salty sediments. Still, the freshwater ones constitute a challenge for nearby communities since the water conditions can quickly change physically and chemically. Exoreic lakes, on the other side, discharge the water excess using a river or a stream that keeps a constant water level (unless the level drops more than this position), constituting reliable water sources and necessary economic poles. We have to look at the population density of the great African lakes to understand how important they are. The disadvantage with lakes is that the same attraction to human settlements enhances an interaction that, in some cases, pollutes the lakes and the neighbouring environment, causing additional water treatment problems, in most cases almost inexistent. Figure 67. Water collection in the great lakes region. Photo by S. Morrison. The following figure to be analysed are rivers. They also constitute a reliable water source for communities and farmers. They built most cities beside them because of the continuous water supply. As we previously said, most rivers are polluted not just because of wastewater but also because of agricultural activities, where all kinds of chemicals and waste end up in the river. But the problems continue because rivers are losing discharge globally, giving fewer sediments to the deltas, worsening the coastal protection, and making water less available. Of course, climate change is not helping at all. Evaporation levels are higher as temperature rises, contributing to more discharge reduction. And we cannot forget that rivers, if not regulated by reservoirs, experience the effects of increased runoff due to floods. This dramatically impacts communities living near the river, especially in floodplains. So, as we can see, rivers are so crucial to us for drinking water and agriculture. Still, they have both the negative disadvantages attributed to humans and the nature-based problems. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 76 vertical percolation rate. The disadvantages are that we need significant and flat surfaces with good geological characteristics (like alluvial basins) because when using thin water layers, we need them to percolate fast. Otherwise, the evaporation losses will be substantial. Its construction requires a channel distribution system for a fast and equitable distribution along the zone because we aim to cause this vertical percolation rate. Figure 76. Flooding pools for infiltration (www.earthdate.org) Furrow or ditch method The ditch and furrow method shape an irregular terrain with ditches and furrows closely spaced, increasing the water contact area and consequently the water percolation and recharge. Trenches should be flat-bottomed with a width of 0,3m to 1,8m. There must be a gentle slope to keep low velocity and avoid sediment deposition. We must build a collection ditch too to conduct water excess again to the main channel where we extracted water. The main advantage of this technology is its good efficiency. Still, the high costs and detailing supervision and maintenance make it sustainable in places where qualified individuals can work on it. The configuration of ditch patterns varies and follows threestar patterns; the lateral, dendritic and the contour (Michelson, A. T., 1934). Figure 77. Ditch and furrow method. (Pinterest) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 77 2.3.2 Sub-surface methods Injection method Injection methods consist of using structures like tube wells, connector wells or shafts (usually coupled with pumps) aiming to increase groundwater storage in confined aquifers using pressurised treated surface water. The goal of this technique is to inject water into depleted confined aquifers. To do so, we usually use pumps that reach higher pressures than the water in the confined aquifer, and hence we introduce it on forces trying to store as much water as possible. It’s a valuable technique to raise water to fractured hard rocks with low permeability. We can also take advantage of dual-purpose injection wells, where we can obtain water pressure by accumulating a large water column from an unconfined aquifer. If the water column has more pressure than the water in the confined aquifer, by building a connection well between the two aquifers, water is transferred from the unconfined to the confined. Injection wells are problem solvers in a current scenario where communities worldwide lose access to water (even in deep confined aquifers) because of the ungauged overexploitation. Injection wells can help restore the confined aquifers or the unconfined with negligible permeability. The only disadvantage is that the cost is higher than other recharge methods (but no other options for confined aquifers) and that we have to take care of the water quality before injecting it, an unusual parameter when talking about groundwater recharge. Figure 78. Injection well. (https://www.austintexas.gov/) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 78 Recharge shafts Vertical and horizontal shafts are two different sub-surface recharge methods that consist of introducing surface water into a shaft, temporally or permanently, so that it infiltrates into the aquifer strata along all the shaft boundaries. For example, it promotes infiltration in deeper layers where permeability can be more favourable, like sand layers below clay ground layers. Some advantages are; less land requirement than percolation tanks, almost no evaporation losses and recharge rates comparable to percolation tanks. Vertical recharge shafts are built to avoid the clay layer and soils with negligible vertical permeability. The depth can vary but is usually 10m to 15m, depending on the geological characteristics of the terrain. The usual infiltration for 2 to 3-meter shaft diameter with inverted filter (from top to bottom – fine sand to coarse gravel) is 7-14 L/s, resulting in 2,3 to 7 L/m/s. We can also couple the shaft with an injection well at the bottom to reach confined aquifers, adding 3 to 15 more meters of depth and obtaining infiltration rates over 7 L/m/s. Figure 79. Vertical recharge shaft. (Harmeson, R. H., 1963) Horizontal recharge shafts are similar to vertical in function and infiltration characteristics. Still, the geometry is different here, with an equal width but a shorter depth and a more extended excavation. It is suitable for sandy areas within 3m of depth, and the continuation of the same sand strata until meeting the groundwater is necessary for a good performance. They can also incorporate little injection wells to increase recharge and connect faster with the groundwater table. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 79 Figure 80. Horizontal recharge shaft. (Harmeson, R. H., 1963) 2.3.3 Induced recharge Induced recharge (called River Bank Filtration when referring to the water quality treatment) is the only indirect recharge method that appears in this part and consists of the water pumping from an aquifer that is hydraulically connected with a surface water mass. When the pumping creates a depression cone with groundwater level in the well below the water mass bed, the gradient becomes negative, and the water flows from the river to the aquifer. This technique is controversial in the recharge classification because no water surplus is being generated theoretically. But the truth is that it is a handy method to obtain clean drinking water from the stream without adequate treatment, and we are recharging the aquifer with water that can come indirectly from rain. Also, recharging the aquifer from a transient flow source means that by groundwater table level control, we are also controlling the flow of the river or reservoir, which helps to avoid floods, and this water being pumped can be stored or injected posteriorly to the ground. It is an excellent technique with a lot of potential combinations. The pumping can require the construction of collector wells, with diameters between 4 and 8m, or, if the aquifer is shallow, we may need horizontal shafts, which we already discussed. Figure 81. Induced recharge diagram. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 80 2.4 Extraction methods The extraction methods consist of withdrawing water resources from storage facilities, groundwater or natural watercourses, either temporally or permanently. This practice is differentiated from water harvesting because, in this last one, our aim is water storage, and extraction is the next step: reaching the stored water for human consumption, agriculture or industrial activities. In the following part, we will see the different infrastructures required to ensure water supply in settlements using extraction and conveyance. Still, before entering the detailed structural features, we must look at a crucial point. When building extraction facilities, we are altering the equilibrium of the water resources because we introduce a sink factor in the watershed. This means we must be aware of the problems that extraction can cause. A widespread cause is aquifer drying. Worldwide, household and farmers’ wells are ungauged and can run dry quickly if the demand is exceeded. Technology and policies must be introduced to avoid this condition since drying is effortless compared to restoring an aquifer. On the other side, we have the effect extraction causes on the groundwater gradient. If wells are overexploited, we can end up reducing water discharge or nearby streams (like in implicit recharge, but without no intention or planning) or causing saline intrusion if the wells are near the coast, pushing inland the freshwater resources. Out of our climatic region scope, but serves as a good example, the case of the Israel-Palestine-Gaza Strip is not just fascinating from the Water Governance point of view. Still, it is also an excellent example of how a territory of 2M people with no surface water had to rely on groundwater sources until saline intrusion was present. Now the majority of the wells have run dry. The pumping can require the construction of collector wells, with diameters between 4 and 8m, or, if the aquifer is shallow, we may need horizontal shafts, which we already discussed. Vertical wells Wells are groundwater extraction facilities obtained by digging or drilling circular holes that go deep into the ground until the groundwater table level is reached. Then, the energy difference between the well and its surrounding groundwater creates a flow inward, and the well fills up, so they can pump out the available water to fulfil the population's demand. Wells can also act as injection devices, but the gradient would take the opposite sign, as previously explained in the recharge methods. Wells are installed all over the world. Considering that groundwater constitutes 50% of all the daily drinking water in the world, it makes sense to find them in various configurations along the regions. Wells are the principal water source for rural communities across the globe and places where surface water is challenging to reach or treat. To pump water from the well, many rural communities or developing countries use mechanical pumps, but electromechanical pumps must be used if there is a need to reach deep aquifers. Compared to surface water, groundwater has excellent drinking standards unless it is contaminated by industrial or farming activities or particular geological outcrops of water-polluting minerals. Among the wide variety of wells that we can find, the horizontal ones are the most common. Dug wells are not deep and have an excellent diameter, ready to obtain water from free surface aquifers. Deep wells, like gravel-packed or open-hole ones, can reach confined aquifers or even fractured rocks, taking advantage of a shallower but deeper structure. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 81 Figure 82. Different vertical well options. (https://thereaderwiki.com) Figure 83. Children pumping water from a well. (UNICEF) Horizontal wells Horizontal wells are groundwater collection and extraction facilities placed horizontally across a soil layer or various layers, allowing the water to flow into the facility and get extracted by pumps. The utilization of horizontal wells is very ancient, and the first ones, the qanats, were found in ancient Mesopotamia. Qanats were old water transporting systems created to extract water from the aquifer through a horizontal perforation and convey water underground until the gentle slope of the tunnel meets the surface, and then water is distributed. They were mainly constructed in Iraq and Iran, but we can also find them in other regions like India, Indonesia or the Andes. Qanats were built by crews of Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 82 3-4 “muqannis” (qualified and well-paid workers), and the work started from the alluvial plain where the water was about to be provided, towards the shaft where a control well was placed. Separation shafts were rated every 20 to 35m. The qanat gradient is between 1:1000 and 1:1500 to avoid supercritical flow. Most qanats are less than 5km long, but 70km long tunnels have been found in Kerman (Iran). The vertical shafts take a range between 20m and 200m usually. Their construction time could last several years, considering that 20 horizontal meters are the maximum speed per day. The well-known advantages of a system like this are that it can easily transport freshwater through the ground preventing it from evaporation in dry and hot climates, a characteristic affecting tropical countries too during the dry season. But the disadvantages are the tedious work in excavation (considering modern construction standards) and the economical cost compared to good vertical drilling. Their discharge is not controlled, with registered qanats that went dry when populations and pumping increased and the water table required to work dropped. Figure 84. Qunat scheme. (Pinterest) Qanats are structures with many advantages but archaic in many other aspects. It would be logical to focus on the technological improvements we could apply to them, and here is where we find the “modern qanats”, the horizontal wells. To overcome the main disadvantages, new types of horizontal wells work using similar characteristics, taking advantage of the flat display of collectors to increase the contact area and the water inflow. Horizontal wells take advantage of their display in the most porous layers of the soil. They can be very efficient compared to vertical wells when the permeable layers represent a tiny percentage of the well height or shallow aquifers. The distribution of the horizontal collectors also helps to distribute the drawdown evenly near the well or to approach further away from water bodies, optimizing the water obtention. Additionally, the pumping cost per unit of volume is lower than in vertical wells (Houben, G. J. et al., 2022). But, as disadvantages, we also have to consider that the investment is still high and requires professional workers. In case of failure, it is still challenging to repair, and few companies know how to build them. The most used models are the radial collector wells (RCW) and horizontal directionally drilled wells (HDDW). Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 83 Figure 85. Horizontal well scheme. (Houben, G. J. et al., 2022) Desalination Desalination is an artificial process of water extraction from seawater, where the saline and mineral content is removed from the samples. As a result, drinking water and residual brine (water with high salt concentration). Desalination is a relatively new industrial field, and it proves efficient in supplying fresh water in case there is a need for extra supply. Figure 86. Desalination plant in Egypt. (constructionreviewonline.com/) Even though the benefits of getting drinking water from seawater (which constitutes the 97% of the water) are noticeable, some disadvantages still need to be overcome to implement this rainfall-independent technology around the coasts. Desalination is an Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 84 energy-intensive practice only cost-effective in arid zones with high population density, like UAE and Saudi Arabia, or in places with few alternatives to this practice, like California. How to deal with the brine is also the other main disadvantage and a huge environmental problem because most desalination plants send back the brine to the ocean, decreasing the oxygen levels in the water and causing trouble to species unable to adapt to sudden salinity changes. Regarding the different methods, we can differentiate between two extensive methods, thermal and membrane methods. Thermal methods consist of the obtention of water steam thanks to solar irradiation in a water pond. This steam condenses, and we are left with drinking water in a collector and high salt-concentrated water in the pond (that constantly exchanges with new seawater due to salinity gradient). On the other side, membrane methods consist of the mechanical application of pressure on water, forcing it to go through a membrane where salt is kept, and only freshwater gets to the collector. There are other methods like crystallization, but they are not that common now. Desalination methods need from 3 to 25 kWh to obtain a cubic meter of drinking water, with reverse osmosis ahead in cost-effectiveness (100L for 0,05 to 0,1€). Surface water catchments (in dams or weirs) Surface water catchments are structural or mechanical facilities designed to extract or derivate water resources from rivers or reservoirs to channels. There are two main methods, gravitational ones and pumping ones. Gravitation methods use the own geometry of the retention structure to collect water. We could use lateral or frontal catchments, weir catchments with collector channels and bottom catchments with or without the Coanda effect (Erazo, S., 2017). Pumping is required when the catchment is lower than our outflow pipe, and we can use catchment towers for the variable water level in dams, floating catchments that redirect water to a tunnel, well catchments, etc. Figure 87. Sot de Ferrer water catchment in Sagunt, Pais valencià. (www.acequiamayordesagunto.com) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 85 2.5 Reuse methods Water reuse methods are activities and structures designed to enhance water conservation by taking profit from used water resources before returning them to the watercourses, providing alternative solutions to existing water supplies. Reuse methods are considered a crucial part of the circular economy, and new technologies are meant to maximize the reuse, using the residues from it too. Actual policies aim to reuse 80% of wastewater for irrigation in developed countries (UN Habitat & WHO, 2021). The most positive characteristic of water reuse is not only the water conservation but the implicit water treatment. Currently, and as stated previously in the groundwater chapter, the lack of sanitation due to open defecation and the groundwater contamination due to agricultural pollutants are severe issues regarding public health and water security. Reuse implies treatment, and treatment prevents (all together with appropriate policies) water pollution. We will explore methods consisting of the conveyance of used water that will be used in other activities like gardening, helping reduce the water budget. We will also see different ways that require intensive wastewater treatment. But, again, the most important thing to consider is that treatment will always be beneficial for the ecosystem even if we don’t reuse this water instantaneously because we may use this water again downstream of the river. If the water is polluted from upstream, the infiltrated water is polluted, the river bed is contaminated, and the cost of removing pollution to obtain potable water downstream will be difficult and costly. Grey water reuse Greywater reuse consists of collecting greywaters (gently used water containing traces of food, hair or household cleaning products) for gardening, farming, cleaning or industrial use. It’s a common practice when the system of greywater and blackwater are split, so, at the moment, they are not very implemented in tropical countries where sometimes we have none. It helps reduce the water budget since the same water for domestic consumption is used for irrigation or industrial purposes. On a local scale, it is a system that helps with gardening or cleaning private or shared zones, but the implementation is not cheap compared to regular sanitation facilities. Greywater reuse is beneficial since the same components polluting rivers are nutritious for plants. It is long-term beneficial and, once installed, straightforward to sustain, but on the other side, we cannot store greywater for more than 24 hours, and we must minimize the contact with it. Figure 88. Greywater tank used in backyard agriculture. (Pinterest) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 92 1.2 Lake Tana basin Lake Tana is an exoreic lake located in the Amhara region, in the Ethiopian highlands, the country's northwestern part. It has a depth of 15m and a surface of 3200 km2, storing an average of 5000 hm3. Declared as a biosphere reserve in 2015 with its basin, this lake is the largest in Ethiopia and is fed by three main rivers; Gilgel Abay, Reb and Gumara, providing an outflowing river at the basin catchment (Bahir Dar city). This river is one of the most famous ones because the outlet of Lake Tana is the birthplace of the Blue Nile River. Due to the high seasonality difference in precipitation and evaporation, the lake has level fluctuations of 2-2,5m, with pick flow in September-October after the rainy season and lowest levels in April-June coinciding with the end of the dry station. When the water level is high, most of the plain terrains adjacent to the lake are flooded, and the swamps connect to the lake. Figure 95. Lake tana administrative zones. (Shimelis, S. et al., 2013) According to UNESCO, the lake basin has an area of 15114 km2 and hosts more than 2 million people. The vast majority of the inhabitants work in agriculture (subsistence farming principally), becoming the backbone of the economy. Other activities such as fishing or tourism-related vary and rely on the state of the lake, the rainy season, etc., constituting an unstable employment pole. Tourism has been increasing for the last twenty years due to cultural attractions like the churches inside the lake and tourism devoted to nature because lake tana hosts incredible biodiversity in its wetlands. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 93 Figure 96. Fishermen at Lake Tana. (Pinterest) 1.2.1 Hydrology in Lake Tana basin The lake Tana Basin is rich in water resources. Still, the available land and water resources are not utilised effectively to improve the livelihood and socio-economic conditions of the inhabitants (McCartney, M. et al., 2010). Depending on the analysed zone, the basin perceives rainfall of 800-2350mm. The current average value is 1635mm (Duan, Z., 2018). The driest points are located in the north, whilst the wettest area is found in the most mountainous region in the south, where the height is more than 4000m. The potential evaporation measured at Bahir Dar station is about 4-5mm/day depending on the season, resulting in approximately 1400-1700mm/year of potential evaporation. But the actual (measured) evapotranspiration will represent 65% of the water from the basin instead of 100% because, during the dry season, the potential values are not met. This high proportion is mainly because of the evaporation in the lake, which is constant, and it does account almost for 100% of the precipitation that the water body area receives. But, since the water body is 20% of the basin, the inland evaporation must be around 55%, which is a more feasible measure and the one that we will take to calculate the water budget in our basin later. More precisely, if we look at the following picture and the report from the Zheng Duan in 2018, we will see how the evaporation value is about 500- 700mm/year, numbers closer to what we approximated. This report shows the importance of soil moisture change (groundwater table) and the deep percolation that help recharge the aquifer. The total estimated runoff was 564mm. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 94 Figure 97. Water balance in Lake Tana. (Duan, Z., 2018) Figure 98. Evolution of water balance parameters. (Duan. Z., 2018) Regarding the use of these water resources, here is where we start to see the basin's problems. The first problem is that 2M people (mainly rural) live in the bay, completely disconnected from the others in little settlements. It is not just that they have less development and investment in water facilities because the country is on its way to action (because at some point, they will eventually do). Still, the isolation makes them more vulnerable and expensive for the administration in many ways. Supplying Bahir Dar, with 200.000 living together beside the lake that contains 50% of the freshwater in Ethiopia, is relatively cheap. But supplying settlements of 20 people that cannot pay or contribute to the construction with their taxes because their annual wage is lower than 2000€ means that other projects will have priority. Also, the cost per capita of pipelines or the cost per capita of storage facilities is higher, and not just that, but the financial and technical requirements are also more considerable due to isolation. One thing must be remarked on this part, even though it's a little bit of the technical scope, we must be critical to understanding why there's this significant bias between urban and rural. The human right to water and sanitation is alienable, and we must pursue it in all ways. Suppose we put ourselves in the administration's shoes. In that case, the economic Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 95 impact of supplying and treating wastewater in big cities may not only help in achieving faster the development goals, but its economic impact may be necessary to implement projects in the rural context afterwards. A sewer system in a 200.000 inhabitants city makes a difference. A sewer system in a village of 20 houses doesn't (and to achieve the same, and the cost is higher). That's why it's vital to take balanced steps in both directions aiming at the HRWS and not just at economic feasibility. As a second example of why things are not working as they should, we have irrigation, and why are they using poorly the water potential for agricultural purposes. Still today, the agriculture in the region is underdeveloped, and the use of extensive lands gives little food for what they need. This region is maybe not the one that will suffer the most from drought and dry season but having that little food means that extreme weather events like massive stormwater and floods can become a nightmare for their food security. Some irrigation programs have been developed, and they are still under construction. Still, the reality is that by now, the water management in the basin is impoverished for its vast potential, and it is affecting land management too, where deforestation in the zone is alarming (McCartney, M. et al., 2010), (Kebede,S., 2016). 1.2.2 Geology and land use in the Lake Tana basin Figure 99. Geology classification of Lake Tana. (Kindie, A. et al., 2019) Regarding the geology of the lake, we must consider that it originated due to volcanic activity, stopping the river flow and creating a 15m depressed basin where the lake started to fill. So, generally, the basin hosts a variety of basaltic (volcanic) rocks. If we take a closer look at the lake's boundaries, we will find sedimentary rocks. In the outlet of the three main rivers, we find alluvium formations and marsh soils in the floodplains near the lake. We can also find colluviums in the northeastern part of the basin and lacustrine deposits on both sides of the lake following the NW-SE direction. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 96 Figure 100. Soil classification of Lake Tana. (Kindie, A. et al., 2019) Regarding the soil classification, We can find a disparity between them, but luvisols, fluvisols and vertisols are widespread, particularly in the surrounding lake. Luvisols are generally fertile and used in agriculture but have a subsurface zone of clay content, making the soil less permeable. Fluvisols, on the other side, are young alluvial deposits occurring in river sediment zones and lacustrine deposits, as well as in tidal marshes, and this is the reason why we can find them, again, aligned with the three prominent river locations. It is common to grow dryland crops in fluvisols if water control is available, and the soil is far more permeable than fluvisols (FAO, 2015). Vertisols (commonly known as black soil) are soil formations with high content of expansive clay minerals. The constant shrinking and swelling constantly mix the material that is formed above high basaltic rocks in humid climates subject to droughts and floods, and this is why they dominate a great part of this basin. The vegetation found in these soils is mainly grassland or savanna because the soil makes it almost impossible for trees growing. When irrigation is available, its low permeability allows for the cultivation of rice, cotton or wheat, but in general terms, vertisoil is not a grateful soil neither for humans, animals, infrastructure building or agriculture (FAO, 2015). We can also find some leptosols in the northern part. Leptosols are shallow soil layers over hard rocks, particularly common in mountainous areas like the Ethiopian highlands, and very low appealing for rainfed agriculture. Since water percolates very easy due to gravel content and high permeability, these soils are highly erodible, and drying is common too (FAO, 2015). So, generally overviewing, we have low permeability soils or very shallow permeable layers, so in any case, the aquifer productivity is low. We will see soil use in the next picture. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 97 Figure 101. Land use of Lake Tana. (Kindie, A. et al., 2019) The basin is dominantly cultivated. As we have explained before, agriculture is the main economic activity in the region and is characterized by auto consumption. The remaining zones are moderately cultivated, and we can also find some grasslands and one big shrubland zone. As far as we can see, there is a direct relation between less agriculture intensity and chromic luvisols. Fluvisols is where agriculture is developed the most. As a general summary of this part of the analysis, as well as in the previous hydrological part, the lake tana has enormous potential for irrigation projects and agriculture. Still, the overpopulation, deforestation and poor land management practices are considerable problems for the basin. The drought and flood potential are higher as they are not improving in any of the areas because populations still grow (until finding a limit and starting migrations), precipitations do not increase a lot, but evaporation does. Land management allows this hazard proliferation conditions that, altogether, generate less retention (storage) and more runoff. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 98 1.3 Infraz Since we wanted to perform a study in the Lake Tana basin, we had to search for a subbasin where we could work out a hydrologic analysis and water budget, conducting assessment and intervention proposals after the study. And we came across Infraz. Figure 102. Guzara Castle, Infraz. Photo by Evan Williams. Infraz is a village in Central Gondar Woreda, located beside the road between Bahir Dar and Gondar, the region's two main cities. The city existed at least from the 14th century on. It hosted an essential market with great activity, and it was more critical than Gondar until they moved the capital to this second village. The village had 10.000 inhabitants in 2007, and other inhabitants spread around the administrative boundaries (wereda), called Gondar Zuria. The road worked in 2005 helped provide electricity to the city; nowadays, it is a consolidated fish marketplace. Despite looking for reliable sources, we couldn't find much information about the city and its people, so, in the next part of the thesis, we will need to adjust the quantitative parameters that define the city and its associated basin. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 99 2. Hydrologic analysis of Infraz’s sub-basin Our goal in this part will be to obtain a hydrologic analysis and water budget of the Infraz’s sub-basin and then propose solutions to help solve the water scarcity and flooding problems of this sub-basin population suffers. We will use QGIS to delineate the basin and analyse the land and topography. We will also use HEC-HMS to determine runoff, infiltration and natural storage. Also, to perform the water budget and define the total runoff, we will need withdrawal data to use water demand projections. All these results must come from input data, so that we will focus on pre-processing input data in this first part of the project. 2.1 Input data for the model We will base our work on five primary data. A digital elevation model (DEM), an evapotranspiration dataset and a Landsat8 orthophoto set, and both downloaded from EarthExplorer, a precipitation dataset (2015-2021) from NASA POWER, and the population metrics from the national census report of Ethiopia. We will also help ourselves with using Google Earth for profile delineations. Using these same tools, we can download data like humidity, temperature, wind velocity, surface moisture, root zone moisture, and potential evapotranspiration that help calibrate the model at more advanced academic and professional levels. Just as a brief digress, the power of open data nowadays is enormous, and we have to enhance it, as we said in the introduction of the thesis. Apart from the input data, it would be desirable to have data to validate the model, such as discharge records in the river. The problem is that most African basins are ungauged and rely entirely on data from meteorologic stations and remote sensing instead of water gauging station data. Additionally, the analysed river is non-perennial, and water gauging stations are very uncommon since we found ourselves in a poor country. So, we will have to calibrate our model with information from previous studies and analyse it to be aware of the existing uncertainties and the challenge it represents to have only theoretical data, with the bias it can produce. 2.1.1 Precipitation data The downloaded data from NASA POWER is processed in an Excel sheet. The time step selected is measured in days. Consequently, the image of the time function is the daily precipitation in millimetres (with a precision of 2 decimals). The hyetograph extends from 1st January 2015 to 31st December 2021, covering the 2015-2021 season. And we found it appropriate to put significant marks every six months because they give us an intuition of the wet and dry seasons and their variability. The yearly average rainfall in this period has been 1345mm, but we can see a difference between the typical precipitation pattern and the 2018-2019 seasons. In these years, the averages were 984mm and 1060mm, respectively. We will introduce the precipitation input values in HEC-HMS in the following steps. We know from the IPCC report that rain will be more common in the zone, up to 10% more in 2050, so, to simulate future water resources, we will project the linear increase by interpolation. Expected rain in 2050 will be 1480mm, and 1672mm in 2100. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 100 Figure 103. Infraz’s bain Hyetograph (2015-2021). (Oriol Barbosa i Llauet) 2.1.2 Evapotranspiration The evapotranspiration data obtained from EarthExplorer is measured in steps equal to one day and gives us an image of the daily evaporation in mm. The graph extends from the 1st of January 2015 to the 31st of December 2021, covering the 2015-2021 season, like the precipitation record. The evapotranspiration average in this season is about 400mm per year. Evapotranspiration is physically constrained during the dry months when less water is available to evaporate. When potential evapotranspiration is higher due to high temperatures and radiation, it is energy-constrained since both soil, plants and water bodies cannot evaporate out of their capacity. It means that actual evapotranspiration is equal to the potential one. Figure 104. Infraz’s basin evapotranspiration (2015-2021). (Oriol Barbosa i Llauet) The IPCC reports that evaporation will be higher, so as rainfall, but in this case, it could increase up to 15%. In 2050 and 2100, we could have evapotranspiration values of about 486mm and 647mm, respectively. 0 20 40 60 80 100 120 140 160 Daily precipitation (mm) Time (days) Hyetograph 2015-2021 16 18 20 22 24 26 28 30 0 0,5 1 1,5 2 2,5 3 Temperature (ºC) Evapotranspiration (mm) Time (days) Evapotranspiration and temperature (2015-2021) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 101 2.1.3 DEM and Orthophoto The DEM and the orthophoto were obtained from the same open data source EarthExplorer. The data comes from the Landsat8 satellite, which gives us a resolution of 30m x 30m in the DEM and an excellent orthophoto resolution. The DEM attributes a height to every cell of the domain and helps delineate the basin, identify the sinks, the water accumulation and the river streams. The orthophoto, with its multiple bands, will help us obtain the NDVI to characterise the land use in the basin. Figure 105. DEM of Infraz’s basin over an orthophoto with modified band colour. (Oriol Barbosa i Llauet) 2.1.4 Population projection and associated water demand We previously stated that Infraz was a village with 10.000 inhabitants in 2007, with other inhabitants spread all around the administrative boundaries of Gondar Zuria. We also said the information since this last report is poor. If we want to calculate the current and future population, we must set up some rules to get a projection. Ethiopia has grown at a rate of 2-3% since 2007. Current UN predictions state it will continue to do until 2060. In 2007, the population was 80,7M, and now it is 120,8M. Since the demographic growth curve is not changing, we will apply the same difference to calculate how many people are currently living in the studied region and how many people are projected to live there until 2060. Taking into account that Gondar Zuria (Wereda) had 191.394 people in the 2007 census, we can estimate the population as a geometric series; 𝑟= √𝑃𝐺𝑍2022 𝑃𝐺𝑍2007 2022−2007 =√120,8 80,7 15 =1,02726 𝑃𝐺𝑍𝑛 =𝑃𝐺𝑍2007 ·𝑟(𝑛−2007)=1919.394·(1,02726)(𝑛−2007) Where; 𝑟 is the population growth rate Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 108 2.2.2 Land use classification using NDVI obtained from the orthophoto To obtain this map, we will need the mentioned orthophotos. We got them from the portal EarthExplorer, which is attributed to the Landsat8 satellite. Combining bands 4 and 5 with the following formula, we obtain the Normalised Difference Vegetation Index (NDVI), one of the most used indexes to differentiate land use. 𝑁𝐷𝑉𝐼=𝐵𝑎𝑛𝑑 5 − 𝐵𝑎𝑛𝑑 4 𝐵𝑎𝑛𝑑 5 + 𝐵𝑎𝑛𝑑 4 Once the raster is available, and after cutting it according to the basin boundaries, we classify it according to the thresholds we define, comparing the ESRI satellite imagery with the raster layer until the forest and cultivated lands coincide. In our case, the thresholds were 0.05 and 0.11, which divided the classification into clouds, cultivated land and forest, respectively. Converting the raster pixels into shapefile points, we can extract a table with the number of pixels belonging to each category. We found that forest occupies 30% of the sub-basin whilst cultivated land represents 70%. Figure 111. Soil classification map. (Oriol Barbosa i Llauet) If we want to define now the curve number, we also need to recap the geological information of the previous part. We must know that the sub-basin has Eutric Leptosols predominantly, with a Termaber basalt layer below. Leptsols are composed of loam, sands and gravel, which are acceptably porous and can infiltrate water at reasonable rates. Still, at the same time, they are shallow soils that can’t hold much water inside because the basaltic layer is close to the surface. This is accentuated in eroded zones where the Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 109 soil layer can be just 10-30cm, but, in other zones, it may be 1m to 2,5m deep. Altogether, the basaltic layer forms fractured aquifers that fastly infiltrate water through fractures but are not productive due to low storage capacity. This means that when rains come, the infiltration to the ground will be good at the beginning, but it will rapidly stop due to soil saturation, increasing even more, the runoff. Therefore, our soil can be classified as “HSG – Group D” with high runoff potential (Belete B, 2012). This classification will have a high impact when deciding on the WRM interventions. 𝐶𝑁=𝐹%·𝐶𝑁𝐹+𝐶%·(𝐶𝑁𝐶+𝐶𝑁𝑃 2)=0,3·82+0,7·(87+89 2)=86,2 Where; 𝐹% is forest coverage 𝐶𝑁𝐹 is “fair - orchid forest” curve number 𝐶% is cultivated land coverage 𝐶𝑁𝐶 is “poor – contoured row crop with crop residue cover” curve number 𝐶𝑁𝑃 is “poor – pasture” curve number Since curve numbers range from 30 to 100 being this last number the corresponding with highest runoff, and a resulting value of 86.2, the stated conditions are validated. 2.2.3 Slope Classification map and runoff coefficient To calculate the runoff coefficient, we elaborated a slope map using the function available at QGIS. It gives us a general intuition of runoff in events. It is a powerful geometric analysis tool that shows us where springs could be located, the steepest stream pathways, the most mountainous regions of the basin, etc. Figure 112. Slope classification map. (Oriol Barbosa i Llauet) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 110 To compute the runoff coefficient, we will use the multiplication of matrices derived from the Runoff Coefficient table and land use weights and slope profile weights. 1,45% of the land has a slope smaller than 2%, 3.87% has a slope between 2% and 6%, and the 94,68% remaining has slopes greater than 6%. At the same time, to do this computation, we need to get the data from the NDVI, the weight of forest, pasture and cultivable land, that we agreed on 30%,35% and 35%, respectively. The computation follows; 𝐶= ∑ 𝐶𝑖𝑗·𝑙𝑤𝑖·𝑠𝑤𝑗=(𝐶11 𝐶12 𝐶13 𝐶21 𝐶22 𝐶23 𝐶31 𝐶32 𝐶33)·(𝑙𝑤1 𝑙𝑤2 𝑙𝑤3)·(𝑠𝑤1𝑠𝑤2𝑠𝑤3) 3 𝑖=1 𝑗=1 = (0,12 0,30 0,18 0,16 0,40 0,23 0,30 0,50 0,31)·(0,30 0,35 0,35)·(1,45 3,87 94,68)=0,367 Where; 𝑖 is the slope column (2%, 2-6%, 6%) 𝑗 is the land row (forest, pasture, cultivated) 𝐶𝑖𝑗 is the runoff coefficient determined by slope column and land row 𝑙𝑤𝑖 is the land area weight. 1= forest, 2= pasture 3= cultivated 𝑠𝑤𝑗 is the slope area weight. 1 is <2% 2 is 2%-6% 3 is >6% Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 111 2.3 Hydrologic basin analysis with HEC-HMS HEC-HMS is a hydrologic modelling software created by the Hydrologic Center within the U.S. Army Corps of Engineers. It simulates precipitation-runoff processes over dendritic drainage basins, like the one we are studying in Infraz. To calculate the photographs, hydrographs and precipitation-runoff graphs that will allow us to determine the water budget of the region, we must introduce the requested input data. The program works with three different phases; component phase, simulation phase and result phase. In the component phase, we are forced to create a Basin model, Time-series model, Meteorologic model and Control specification series. The second phase is just developing and running a folder for the computations. The last stage shows graphs, summary tables and extensive time series with all the data we are looking for. This is why the structure of the following parts will be divided into the model set-up and the results, understanding all the necessary data and posteriorly obtaining the outputs required for the water budget that will follow the HEC-HMS analysis after every computation. 2.3.1 Basin model set up under current conditions To find the desired results, we must set up the conditions that regulate the behaviour of the water-soil interaction inside the basin. We start by creating a basin model composed of the sub-basin, a reach, and a sink (equivalent to the outlet). Figure 113. HEC-HMS Infraz’s sub-basin model. (Oriol Barbosa i Llauet) In the sub-basin, we need to define the basic parameters of the model. We introduce the sub-basin area (83,9km2) and connect it downstream with the reach. In our case, we will simulate the basin without the canopy method or surface method. This means that our model will only take into account infiltration and runoff. Regarding the infiltration, defined by the loss method, we chose the “Initial and constant” method. This means that our model will infiltrate water at 0,2 mm per hour if the rain allows it, a total of 4,8mm Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 112 per day at maximum. Afterwards, when performing the analysis with the obtained data, we will also limit the groundwater storage according to the average soil depth of 1,5m. To get the runoff records, we need a transformation method. In our case, we select the “SCS Unit Hydrograph”, considering a lag time of 54 minutes. The lag time is derived from (Sultan, D. et al., 2020), and the time concentration is found using the USBR method (Perdikaris, J., 2018); 𝑇𝑐=𝜏(0,87·𝐿𝑚𝑎𝑥 2 10000·𝑆𝑎𝑣)0,385=0,79(0,87·𝐿𝑚𝑎𝑥 2 10000·𝑆𝑎𝑣)0,385=1,32ℎ Where; 𝑇𝑐 is the time of concentration 𝜏 is an adjustment factor 𝐿𝑚𝑎𝑥 is the length of the longest stream 𝑆𝑎𝑣 is the average slope We will also complement our model with a “Recession” baseflow method, to get an idea of the permanent flow regime during the wet season. Regarding the reach, we will use the Muskingum routing method without any gain/loss. This means that all the runoff units in the streams end up in the basin outlet. Otherwise, we would need to introduce percolation data or additional groundwater flow that is difficult to calibrate. We calculate the value with the following expressions; 𝐾=0,6·𝑇𝑐=0,6·1,32ℎ·60 𝑚𝑖𝑛 ℎ ⁄=47,5 𝑚𝑖𝑛 ℵ=0,35 The outlet needs to be connected to the downstream, and it has no parameters. It will record the inflow and outflow at the outlet location. Regarding the meteorologic model, time-series data and control specifications, we have less work because no settings are needed. We must ensure that dates coincide and that the imported data is correct. To do so, we must create a precipitation gage inside the Timeseries data layer and import our precipitation data. Then, we link the meteorologic model to the established gauge record. The time steps are in days; this record starts on the 1st of January and finishes on the 31st of December. The control specifications have the same period as the meteorologic model. Our model is ready to run; we will show the results in the next part. 2.3.2 Hydrologic simulation under current conditions Once the software has run the model, we are left with raw results. HEC-HMS provides us results for precipitation, infiltration and runoff, but we don’t have effects on water withdrawals and storage. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 113 Figure 114. HEC-HMS results for precipitation-runoff-infiltration. (Oriol Barbosa i Llauet) We will derive the data we are missing from the data series obtained in HEC-HMS. Since our basin hasn’t been altered with collection, detention or retention infrastructure, this first analysis is quite simple. To derive the withdrawals, we will consider that 30% of total demand is obtained from runoff per day when a runoff in the river discharges more than 1m3/s, limiting the extractions to 30%. 𝑊𝑟𝑢𝑛𝑜𝑓𝑓 ={𝑖𝑓 𝑄>1𝑚3𝑠 ⁄, min {0,3· 𝑄,0,3· 𝐷𝑑𝑎𝑖𝑙𝑦} 𝑖𝑓 𝑄<1𝑚3𝑠 ⁄, 0 Where; 𝑊𝑟𝑢𝑛𝑜𝑓𝑓 is the runoff withdrawal 𝑄 is the river discharge at that day 𝐷𝑑𝑎𝑖𝑙𝑦 is the daily water demand The rest of withdrawals will come from groundwater. 𝑊𝑔𝑟𝑜𝑢𝑛𝑑𝑤𝑎𝑡𝑒𝑟 =𝐷𝑑𝑎𝑖𝑙𝑦−𝑊𝑟𝑢𝑛𝑜𝑓𝑓 Where; 𝑊𝑔𝑟𝑜𝑢𝑛𝑑𝑤𝑎𝑡𝑒𝑟 is the groundwater withdrawal 𝐷𝑑𝑎𝑖𝑙𝑦 is the daily water demand Both withdrawals lead to the final runoff and infiltration results. Consequently, the water balance in day steps looks like this; 𝑃𝑖=𝐸𝑖+(𝑅𝑖−𝑊𝑟𝑖)+(∆𝑆𝑖−𝑊𝑔𝑖)+𝑊𝑡𝑖 Where; 𝑃𝑖 is the daily precipitation in mm 𝐸𝑖 is the daily evapotranspiration in mm Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 114 𝑅𝑖 is the daily runoff in mm ∆𝑆𝑖 is the daily storage in mm 𝑊𝑟𝑖 is the daily runoff withdrawal in mm 𝑊𝑔𝑖 is the daily groundwater withdrawal in mm 𝑊𝑡𝑖 is the daily total withdrawal in mm When performing the water balance for every day during the simulation period, we obtain the following graphics that we will use to analyse the behaviour of the basin both qualitatively and quantitatively. Figure 115. Results for precipitation-discharge-withdrawals (2015-2021). (Oriol Barbosa i Llauet) The graph shows how the lack of rainfall in the dry season creates gaps of approximately six months long where water cannot be collected from the rivers because there is not enough or no water at all. During these six dry months, people need to supply themselves with water from the sub-soil, knowing that, on average, they have only 1,5m under their feet to be used. It is remarkable to check the two vertical axes we used because the scale is disproportionate to visualize when water can be subtracted. But the reality is that only 16,9% of the water is extracted from the river annually, with maximum extractions equivalent to 30% of the total demand we previously defined in the policy. We can identify the first problem. If we have a demand of less than 0,2mm per day, how can it be possible that we are only using rivers to supply that few? We find the answer in the infrastructure. We don't have detention and collection infrastructure, so the water we will collect comes directly from the river. If the water velocity is high (due to the high precipitation records and soil erodibility), it will come with more solid particles than healthy levels require. We shouldn't use this water for drinking unless they have no other choices. But have, they have groundwater so that they may use it for cleaning, cooking or sanitation. Cattle could also benefit from rivers' water. Still, the alluvial soils are occupied by agricultural land due to their fertility, and farmers risk losing their animals in flood events. Hence, the water they drink from the rivers is small compared to the ones they can drink from natural springs. We could achieve agriculture diversity with channels diverting water into the fields, but no infrastructure is seen. Even if it rains sufficiently, we are not optimizing its use due to a lack of collection, channelization and potabilization structures. 0,00 0,02 0,04 0,06 0,08 0,10 0 10 20 30 40 50 Runoff withdrawals (mm) Storage (mm) Time (days) Precipitation-discharge-withdrawals (2015-2021) Streams discharge (mm) Precipitation (mm) Runoff withdrawals (mm) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 115 Figure 116. Results for groundwater storage evolution (2015-2021). (Oriol Barbosa i Llauet) In this second graph, we can analyse the behaviour of groundwater. As we can see, unless we start enhancing runoff withdrawals, the weight of groundwater withdrawals will continue to rise accordingly to the demand, reducing the available storage in the long term. We can see how the rains in 2016 and 2017 raised the groundwater level to the surface when they concentrated 525 mm of rain in the ground with minimums of 350mm. The situation lasted about a month and a half. Posteriorly, the groundwater started to drop again when rainfall disappeared, and we. If we look at the subsequent years, we can see how the maximum storage is about 450mm, and the minimum levels are about 300mm. Averaging between wet and dry years, we could settle a typical maximum of 480mm if we still have more infiltration than evapotranspiration and withdrawals and a minimum value of about 325mm at the beginning that we know that it will drop as the consumption grows. The dry season keeps behaving in this way or even worse. The average water storage linear tendency follows a slightly decreasing function, and we will be able to see this parameter reproduced in the following graphics. As an overall summary, we must say that, at this point, the sub-basin enjoys hydrologic conditions that allow the everyday relationship of people with its water resources, at least for mid to long-term dates. The development in the area would be favourable to improving water and sanitation access, even when inducing higher water demands. Without compromising the situation, there would still be a margin for agricultural and domestic water increase. Still, demography, as we will see, is the crucial factor that destroys all kinds of hope in this sub-basin unless we promote water resources management improvements. 2.3.3 Water budget projection under current conditions Once we have analysed how the water income and outcome play a role in the basin, it is time to explore the yearly water budget and its associated projection of both parameters and groundwater. As we previously stated, precipitations and evapotranspiration will increase up to 10% and 15% until 2050. Also, we have shown how the water demand function will vary with the years and that it will make it more challenging to achieve the desired maximum and sustainable minimum levels. But the question is, what is sustainable? When will the situation be sustainable? We found the answers in the following graphs and tables. y = 0,0075x + C 0 0,1 0,2 0,3 0,4 0,5 0,6 0 100 200 300 400 500 600 Withdrawals (mm) Storage (mm) Time (days) Groundwater storage evolution (2015-2021) Storage level (mm) Groundwater withdrawals (mm) Runoff withdrawals (mm) Linear (Storage level (mm)) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 116 Figure 117. Hydrologic parameter’s projection (2015-2060). (Oriol Barbosa i Llauet) Table 14. Hydrologic parameters’ projection (2015-2060) Year Annual precipitation (mm) Annual evaporation (mm) Annual runoff (mm) Annual infiltration (mm) Annual consumption (mm) Annual storage (mm) 2015 1345,00 398,50 722,51 622,00 39,14 192,26 2020 1364,21 410,02 742,14 622,00 49,96 172,08 2025 1383,43 421,87 761,36 622,00 63,37 149,51 2030 1402,64 434,06 780,57 622,00 79,91 124,08 2035 1421,86 446,60 799,79 622,00 100,25 95,26 2040 1441,07 459,51 819,00 622,00 125,18 62,41 2045 1460,29 472,79 838,21 622,00 155,65 24,76 2050 1479,50 486,45 857,43 622,00 192,77 -18,59 2055 1498,71 500,51 876,64 622,00 237,88 -68,75 2060 1517,93 514,98 895,86 622,00 292,60 -126,98 From a short-term point of view, the situation is sustainable because the difference between infiltration and evaporation and consumption is positive and feasible. Local minimums will increase due to more water consumption during the dry season, but the situation will still allow us to recover the defined threshold that we consider normal. One day, in the mid-term, when the aquifer starts to drain, we will understand that the situation has never been sustainable because sustainability is not defined by things working for a short amount of time but by the ability to keep functioning smoothly during long periods. From the first graphic and table, we must notice how the storage margin is reduced yearly. All this extra storage is not accumulated because the aquifer has a specific capacity, but it is traduced into the out will of springs, stream augmentation or evapotranspiration. The situation will change between 2045 to 2050 when the consumption and evaporation will be larger than the water infiltration, and the groundwater infiltration will drop. It is interesting, then, to check the groundwater table's evolution to understand how groundwater reduction will affect people. -500 0 500 1000 1500 2000 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 Annual records (mm) Time (years) Hydrologic parameters' projection (2015-2060) Annual evaporation (mm) Annual runnoff (mm) Annual infiltration (mm) Annual consumption (mm) Annual storage (mm) Annual precipitation (mm) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 117 Figure 118. Groundwater stoage projection (2015-2060). (Oriol Barbosa i Llauet) Table 15. Groundwater level projection (2015-2060) Year Annual consumption (mm) Annual storage (mm) Maximum storage (mm) Minimum storage (mm) Average (mm) 2015 39,14 192,26 480,00 331,03 405,52 2020 49,96 172,08 480,00 321,01 400,51 2025 63,37 149,51 480,00 309,57 394,78 2030 79,91 124,08 480,00 296,42 388,21 2035 100,25 95,26 480,00 281,23 380,62 2040 125,18 62,41 480,00 263,60 371,80 2045 155,65 24,76 480,00 243,06 361,53 2050 192,77 -18,59 480,00 219,04 349,52 2055 237,88 -68,75 362,92 73,78 218,35 2060 292,60 -126,98 210,68 0,00 105,34 The calculations of maximum and minimum groundwater storage is done under the following equations. 𝑆𝑚𝑖𝑛𝑛=𝑆𝑚𝑎𝑥𝑛−(6 12) ⁄·𝑊𝑔𝑤𝑛−0,4·𝐸𝑣𝑛 𝑆𝑚𝑎𝑥𝑛+1=𝑆𝑚𝑖𝑛𝑛+∆𝑆𝑛−(6 12) ⁄·0,7·𝑊𝑔𝑤𝑛−0,6·𝐸𝑣𝑛 Where; 𝑆𝑚𝑖𝑛𝑛 is the minimum water storage 𝑆𝑚𝑎𝑥𝑛 is the maximum water storage 𝑊𝑔𝑤𝑛 is the groundwater withdrawals ∆𝑆𝑛 is the storage change in the rainy season 𝐸𝑣𝑛 is the evapotranspiration This graphic and table show the harmful effect of the water storage component of the budget. We can also see that the groundwater maximum starts to fall after the budget is negative. It is essential to explain why this happens and which are the consequences. This phenomenon occurs in shallow soils because the amount of infiltrated water is greater 0 50 100 150 200 250 300 350 400 450 500 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 Storage (mm) Time (years) Groundwater table projection (2015-2060) Maximum storage (mm) Minimum storage (mm) Average storage (mm) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 124 475mm. This means that we can sustain the groundwater level with less stress in time. It is also remarkable the reduction of groundwater extractions in the wet season. Before, the curves didn’t intersect, and now every station leads to an exchange of characters. This suggests that our improvement interventions would work considering the same case scenario with an identical time frame. To conclude this part, we present the results of storage and withdrawals in the same graphic to contextualize the changes visually. Figure 122. Groundwater storage evolution comparison. (Oriol Barbosa i Llauet) 2.3.7 Water budget projection under current conditions It is our turn to reproduce the yearly water budget considering population and water demand projections. Figure 123. Hydrologic parameters’ projection (2015-2060) under modified conditions. (Oriol Barbosa i Llauet) 0 100 200 300 400 500 600 Storage (mm) Time (days) Groundwater storage evolution (2015-2021) Unmodified storage Modified storage 0 200 400 600 800 1000 1200 1400 1600 1800 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 Annual records (mm) Time (years) Hydrologic parameters' projection (2015-2060) Annual precipitation (mm) Annual evaporation (mm) Annual runnoff (mm) Annual infiltration (mm) Annual consumption (mm) Annual storage (mm) Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 125 Table 17. Hydrologic parameters’ projection (2015-2060) Year Annual precipitation (mm) Annual evaporation (mm) Annual runoff (mm) Annual infiltration (mm) Annual consumption (mm) Annual storage (mm) 2015 1345,00 398,50 631,27 713,73 39,14 293,70 2020 1364,21 410,02 650,48 713,73 49,96 276,24 2025 1383,43 421,87 669,70 713,73 63,37 257,01 2030 1402,64 434,06 688,91 713,73 79,91 235,72 2035 1421,86 446,60 708,13 713,73 100,25 211,99 2040 1441,07 459,51 727,34 713,73 125,18 185,37 2045 1460,29 472,79 746,55 713,73 155,65 155,34 2050 1479,50 486,45 765,77 713,73 192,77 121,26 2055 1498,71 500,51 784,98 713,73 237,88 82,38 2060 1517,93 514,98 804,20 713,73 292,60 37,83 This graphic and table represent the evolution of hydrologic parameters. As we can see, the precipitation and evaporation values are identical to those in the unmodified basin. This time though, due to higher water infiltration and lower consumption from groundwater, we are left with a softer and more moderate curve of annual average water storage. The infiltration is now almost 100mm more, from 622mm to 713mm, and consequently, we have been able to reduce the runoff by the same difference. Also, if we look at the storage register, we have a surplus during the period of our analysis. However, we still see how it is decreasing, and we may face water shortage problems around 2075, with negative water storage from 2065. This means that our interventions are successful at least for 40 years, but more things will need to be done in the long term. Of course, many things can change from 2075 on and even before, but we cannot predict how these changes will be. The situation won’t be better because growth will still be there, but it is supposed to slow down as the UN predicts, and climate change will continue to make people struggle daily. But also the advances in technology (particularly in water supply and management) and the development of Ethiopia will be an ally to overcome the problems that we will have in the future. We have to get from this graphic that interventions in WRM are favourable for the hydrologic cycle, and we can continue performing them because people’s lives will require it. 0 100 200 300 400 500 600 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 Storage (mm) Time (years) Groundwater storage projection (2015-2060) Maximum storage Minimum storage Average storage Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 126 Figure 124. Groundwater storage projection (2015-2060). Infraz’s modified basin. (Oriol Barbosa i Llauet) Table 18. Groundwater level projection (2015-2060) Year Annual consumption (mm) Annual storage (mm) Maximum storage (mm) Minimum storage (mm) Average storage (mm) 2015 39,14 293,70 525,00 353,86 439,43 2020 49,96 276,24 525,00 346,00 435,50 2025 63,37 257,01 525,00 337,24 431,12 2030 79,91 235,72 525,00 327,40 426,20 2035 100,25 211,99 525,00 316,28 420,64 2040 125,18 185,37 525,00 303,64 414,32 2045 155,65 155,34 525,00 289,19 407,10 2050 192,77 121,26 525,00 272,59 398,79 2055 237,88 82,38 525,00 253,43 389,21 2060 292,60 37,83 525,00 231,23 378,12 The calculations of maximum and minimum groundwater storage is done under the following equations. 𝑆𝑚𝑖𝑛𝑛=𝑆𝑚𝑎𝑥𝑛−(6 12) ⁄·0,6·𝑊𝑔𝑤𝑛−0,4·𝐸𝑣𝑛 𝑆𝑚𝑎𝑥𝑛+1=𝑆𝑚𝑖𝑛𝑛+∆𝑆𝑛−(6 12) ⁄·0,1·𝑊𝑔𝑤𝑛−0,6·𝐸𝑣𝑛 Where; 𝑆𝑚𝑖𝑛𝑛 is the minimum water storage 𝑆𝑚𝑎𝑥𝑛 is the maximum water storage 𝑊𝑔𝑤𝑛 is the groundwater withdrawals ∆𝑆𝑛 is the storage change in the rainy season 𝐸𝑣𝑛 is the evapotranspiration The rapid water infiltration allows the groundwater storage to recover its maximum available physical level. The new infrastructure will enable us to reduce the groundwater dependency and enhance the water security we were looking for. This is the first and most positive news that our interventions make sustainable a budget where the demand has multiplied by 6 in just 50 years, a feasible demographic and demand scenario in African countries. But, the second one, not positive, is the accelerating recession of minimum groundwater storage that would return to water scarcity from 2065. This means that if the situation continued with the same projections, which we already explained are unprovable, and the minimum groundwater level would end up being zero from 2075 to 2080. This is a long-term scenario out of our predictions for multiple reasons. Still, if the situation remains unflexible, we should install more storage facilities to stabilize the minimum groundwater storage curve, the most important one in this context. We can generally conclude that the modifications of the watershed using gully plugs, terracing, retention ponds, sand dams, rainfall harvesting deposits, and reuse deposits positively impact the sustainability of the hydrologic conditions. In a context of exponential population and water demand increase, further efforts will need to be made to preserve these conditions. However, they could naturally come from economic capacity increase due to development and brand technological advances that make water storage facilities even more competitive. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 127 Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 128 PART IV - CONCLUSIONS The thesis aimed to mitigate floods and droughts in tropical wet and dry countries using Water Resource Management (WRM). Through literature review, we have been able to understand the characteristics of the tropical savanna climate and its affected regions, characterised by being in development and still affected by severe social, political and economic problems. The fragile future of water resources suggested that the analysis wouldn’t be complete without considering climate change and demographic changes. They will become situation drivers in the following decades and establish new rules in the relationship between people, water, environment and geography. Although sustainable and equitable water resource distribution should go through cooperation and dialogue activities between countries and stakeholders, the reality is far from reaching conciliation and water conflicts will arise as renewable water resources per capita decrease. The anthropogenic impact on water resources is irrefutable from all the possible academic fields, making humankind responsible for the derived consequences. At the same time, we must search for technical and financial solutions to the challenges that will come. We have confirmed the importance of adopting adequate water metrics that include seasonality to assess the hazardous water situations in the countries correctly. There is no correlation between flood or drought risk indices with usual water metrics like Falkenmark Index or Water Scarcity Index in tropical wet and dry regions. Seasonality is the most conditioning factor in the analysed areas, and all the feasible solutions have a common denominator: storage and water conservation. Storage is intrinsically linked to economic expense even though it surpasses the financial capacity of most stakeholders in developing countries. At the same time, it seems the only reasonable solution when rainfall is abundant but conditioned to a concentrated distribution that creates a lack of water during the dry months. Hence, economic scarcity is predominant in tropical wet and dry countries, particularly in Sub-Saharan Africa. Rainfall without storage is called runoff, and runoff eventually disappears. We can affirm that policies have to decline in favour of storage structures, and technology must emphasise cheap solutions that allow broad access to water for human consumption, agricultural activities and sanitation. At this point, WRM is the discipline that can help solve hazard mitigation and water storage from a technical point of view. We must highlight the need to slow down the water pace both for flood protection and to collect water, mainly because the concentrated rainfall threatens the population. It must become a storage opportunity that ensures water and food security during the dry periods. Putting water into the sub-surface is one of the most relevant techniques since the economic expense is low. It prevents the potential evaporation that surface water storage suffers. Though, in some cases, surface water storage is almost compulsory due to geologic characteristics or groundwater pollution. Activities like illegal mining, abuse of pesticides and poor wastewater treatment can incapacitate groundwater as a source of reliable freshwater and must be considered critical water supply threats. Land management and all the benefits associated with adequate vegetation are the clues to having better water management within our watershed, confirming a strong correlation between water and food through appropriate land parameters. Mitigation of floods and droughts in tropical wet & dry climate regions through Water Resource Management. A case study in Lake Tana, Ethiopia. 129 The case study in Ethiopia has been a powerful tool to contextualise a possible situation and how adequate infrastructure could increase water availability in the mid-term. The implementation of gully plugs, terracing, retention ponds, rainfall harvesting deposits, sand dams and reuse of water deposits have been able to withstand the increase in water demand, multiplied by 6 in just 45 years. We have also seen the economic differences between these methods and how, unfortunately, the most cost-effective ones like terracing, retention ponds and sand dams were unable to supply all the required water by themselves. In this case, rainfall harvesting appears as the most feasible long-term solution for isolated and sparse communities, despite being the most expensive method of the five presented. Its versatility and low technical manufacturing and installation are worth the price. Investment in the water tank storage industry and its price reduction could cause a very positive impact all around the tropical regions. On the other side, sand dams have proven to be an excellent and cheap solution to water scarcity. Still, its usual share of 1000 people per dam is not enough in a crowded region like lake Tana if we want to preserve sediment transport. However, a sand dam adds a water source that has an enormous worth for the benefited communities in a complementary way. 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