Assessing future rainwater lens viability in coastal agricultural regions under climate change

Shallow rainwater lenses are a vital agricultural freshwater resource in many low-lying coastal regions, where groundwater is brackish to saline at shallow depths. This study presents a comprehensive numerical sensitivity analysis assessing the impact of climate change on the viability of shallow rainwater lenses. Using representative conditions from the Southwest Delta in the Netherlands, we evaluated the effects of climate change both independently and in combination with critical environmental parameters: seepage flux, soil type, drain depth, drain distance, ditch distance, surface water level, and crop type. Climate inputs were based on future climate scenarios from the Dutch Meteorological Institute (KNMI). To assess lens viability, five key characteristics were defined: depth of the freshwater interface (D_fresh), mixing zone thickness (W_mix), period of rainwater lens absence (T_absence), recharge deficit preceding disappearance (Trigger deficit), and annual volume of fresh drainwater (V_{fresh drainwater}). These characteristics effectively capture critical feedback mechanisms between soil hydraulic properties and evaporation regimes. Results indicate that rainwater lenses are expected to disappear more frequently and for longer durations under future climate scenarios with drier summers, despite increased winter precipitation. The degree of impact varies across environmental settings. Lenses that already frequently disappear tend to stabilize or deepen slightly, while more persistent lenses become shallower and more sensitive to climatic variability, losing their ability to recover in wetter years. Model results show that optimizing drainage configurations and selecting crops with lower water requirements may enhance the resilience of agricultural systems in coastal regions to climate change.