Storage Dynamics and Groundwater–Surface Water Interactions in a Drought Sensitive Lowland Catchment: Process-Based Modelling as a Learning Tool

Abstract

Groundwater is a key strategic water resource in times of drought, yet climate and land use change are increasing threats; this means that quantitative understanding of groundwater dynamics in lowland catchments is becoming more urgent. Here, we used a spatially distributed numerical groundwater model to simulate seasonal and long-term changes in the spatio-temporal patterns of water storage dynamics and groundwater–surface water interactions in the 66 km² lowland Demnitzer Millcreek catchment (DMC) in NE Germany. DMC experienced a long period of drought following the hot, dry summer of 2018, with groundwater stores becoming depleted and stream flows increasingly intermittent. The architecture and parameterisation of the model domain were based on groundwater observations, hydrogeological mapping and geophysical surveys. Weekly simulations using a single model layer with a 50 × 50 m grid of 15 m depth were able to broadly reproduce observed shallow groundwater dynamics in glacial and post-glacial deposits across the catchment. We showed that most groundwater flow is shallow and focused around topographic convergence zones fringing the channel network in more permeable glaciofluvial deposits. Most stream flow is generated by shallow groundwater in the catchment headwaters, which is relatively young (i.e., ~5 years old). With potential evapotranspiration rates exceeding precipitation, the groundwater balance is very sensitive to hydroclimate at DMC. The past two decades have been dominated by negative anomalies in annual rainfall, causing a general lowering of water tables and persistent storage deficits. Spatio-temporal patterns of recharge are also strongly influenced by vegetation cover, with coniferous forests, in particular, having high evapotranspiration losses that inhibit groundwater recharge. This underlines the importance of developing integrated land and water management strategies in NE Germany where climate change is expected to further reduce rainfall, increase temperatures and decrease groundwater recharge. For an evidence base to guide policy, we need to develop more robust ways to interface groundwater models with ecohydrological models to better characterise the impacts of land use on rechange in groundwater-dominated lowland catchments.