SpongeScapes Comic - Floods and Droughts: A Water Balancing Act

With Christopher wittmann Climate change is intensifying weather extremes, with more heavy rainfall and droughts, even in traditionally wet regions like the Netherlands. Known for its engineered water management, the country is now turning to nature-based solutions, including sponge measures. These aim to restore landscapes’ ability to absorb, store, and release water gradually, helping to mitigate both droughts and floods. However, could measures for one extreme worsen the other? Christopher, a PhD student in hydrology, explores this question through hydrological modeling. Credits : SpongeScapes has received funding from the European Union’s Horizon Europe research and innovation programme under Grant Agreement n°101112738 and from the UK Research and Innovation/HM Government. Views and opinions expressed in this publication are those of the author(s) only and do not necessarily reflect those of the European Union or the UK Research and Innovation / HM Government. © 2025 - International Office for Water for the SpongeScapes project. Script, drawing and colouring of the comics: Élisa Bézier and Jean Combette (OiEau) Christopher Wittmann's PhD thesis, supervised by: Prof. J. Wallinga (Wageningen University), Prof. A. Weerts (Wageningen University/Deltares), Dr. A Larssen (Wageningen University), Dr. E. Penning (Deltares) Proofreading : Christopher Wittmann Dr. Ellis Penning (Deltares) Text before script : Chloé Déchelette (OiEau) Floods and Droughts: Welcome to the Chaamse beken catchment! A region in the south of the Netherlands that has historically been very wet... Here, flat land means that water runs off slowly and stays longer, while sandy soils soak up water like a sponge. During heavy rains, the soil quickly becomes saturated. That’s why farmers built drains and ditches - to keep feet and crops dry. Drains made wet land farmable… but with longer droughts, we now realize they actually make things worse: soils dry out faster. In fact, in the region, climate change is already visible with heavier rains and longer droughts. One solution is to remove some drains so water stays in the ground longer - turning fields back into sponges. But here's the question: could it be that in some cases measures aimed at mitigating one extreme (droughts) worsen the other (floods)? To find out, I use hydrological modelling. Models help us understand complex systems. They are scientific tools that bring nuance and realism. In my research, I built a hybrid model that combines flood and drought dynamics. Normally, they’re studied separately because they follow different natural processes and rules. Water is always on the move - in the surface in rivers and lakes, and underground in aquifers. Models help understand better how water circulates on the surface or underground. For example, in summer, many rivers aren’t fed by rain at all… but by groundwater rising into the river. Groundwater moves slowly, so we measure its flow in volume per day. Surface water moves much faster, so we measure their flow in volume per second. And floods can develop very quickly, so high precision is needed. The tricky part of building a hybrid model? Surface water and groundwater run on totally different clocks! They also help understand the interactions: how water can move up from groundwater to rivers, or down into soils, depending on seasons, soil types, and other factors. Despite these challenges, we developed a hybrid model to simulate what would happen in times of heavy floods if drains are removed. 1, Without drains, soils are already saturated, so they can’t absorb floodwater - leading to more runoff and higher river flow. 2, The flow peak increased by 10%. The flow peak is the greatest amount of water moving through the stream during a flood event. An increase by 10% means the flood is more severe 3, Less groundwater flowed into rivers. Without drains, groundwater filled up faster and spilled as surface runoff instead. In the end, the flooded area nearly doubled! Of course, models are simplifications. The interaction between surface water and groundwater is simplified. In real life, water takes many hidden paths. Where are they .. Let’s swim over there. THE SIMULATION REVEALED THREE KEY FINDINGS ... Still, this shows an important challenge: any measure taken to reduce drought impacts can also affect floods, and vice versa. And what if floods and droughts keep getting worse? Are we ready to change land use entirely? For example, do we accept that certain areas are flooded more frequently? And how do we make those land use changes fair? Floods damage crops and homes... but nature (especially wetlands) benefits from extra water. How do we balance food, people, and nature? So far, my research focused mainly on water quantity, but there’s more to explore with modelling, for example the links with biodiversity, water quality, food production, etc. My research is about exploring these trade-offs with models, and help finding sponge strategies that can tackle droughts and floods together. Floods Flood drought Droughts