Managing water excess and deficit in agriculture: Subsurface drainage and irrigation systems

Water is essential in our daily lives: for nature, agriculture, drinking water and industry. However, the demand for freshwater is increasing while the availability is declining. In addition, climate change is increasingly causing extreme weather conditions, such as droughts and heavy rainfall. To anticipate both dry and wet conditions in agriculture is therefore becoming increasingly important. One measure is to convert existing subsurface drainage systems into controlled drainage with subirrigation (CDSI). CDSI aims to drain groundwater when necessary, retain water when possible, and supply water where feasible. In my PhD research, I investigated the hydrological opportunities of CDSI in the Dutch Pleistocene uplands. Four CDSI field pilots at the Dutch Pleistocene uplands (America, Haaksbergen, Lieshout and Stegeren) were set up. The field measurements showed that CDSI can raise groundwater levels and thereby increase crop yields. A simulation study with the Soil- Water-Atmosphere-Plant model (SWAP) shows that a minor part of the supplied water contributes to increased plant transpiration. Most of the supplied water leaves the system as ditch drainage and as downward seepage to the deeper groundwater. The distribution between these components largely depends on the adjacent surface water level and geohydrological characteristics.The required water supply (subirrigation) was high in the field experiments, because a fixed and relatively high groundwater level was maintained throughout the growing season. Water supply can be reduced by applying smarter control, whereby the CDSI system contains a dynamic crest level and dynamic pump management. The water level in the control pit is then calculated via an algorithm based on actual soil moisture conditions and weather forecasts. Water can also be saved in areas with a higher resistance to downward seepage (location characteristics), maintaining a higher ditch water level around agricultural fields (management) or cultivating crop with deeper roots (crop specific). Although water supply can be reduced through smarter applications, a CDSI system still requires water, meaning this local measure influences the regional water system. It is therefore important that stakeholders agree to the extent to which CDSI should be implemented. To support this, a system dynamics model (SDM) was developed. The results show that CDSI upscaling propagates non-linearities in hydrological fluxes with three critical phases. From phase 1 with sufficient regional water availability, through phase 2, a critical zone where water demand begins to limit regional water availability to phase 3, where high water demand significantly impacts the regional water system and CDSI is no longer beneficial for crops. All in all, CDSI could be a valuable measure for a climate robust soil-water system. However, because the local measure CDSI may increase pressure on surface water availability during dry periods, it should be carefully embedded into regional water management strategies. In the short term, it is relatively easy to convert existing drainage systems into CDSI. In the long term, CDSI can serve as a technical measure in areas where soil and water were leading in spatial planning, to mitigate both too wet and too dry circumstances.