Quantification of climate change sensitivity of shallow and deep groundwater in Denmark

Study region: The study investigates six different geological regions covering 42,111 km or 98% of Denmark. Study focus: Climate change impacts on spatial changes and uncertainties of water balance and groundwater levels are still not well understood, especially, the effect from hydrogeology and geomorphological setting on impact response. In this study, the aim is to quantify the climate change sensitivity of shallow and deep groundwater levels across multiple types of geologies and aquifers for the six Danish regions. 21 climate models under RCP4.5 and RCP8.5 were used to force the National Water Resources Model for 1971–2100. New hydrological insights: Denmark may expect future precipitation increases ranging from + 12% to + 20%, resulting in substantial hydrological changes e.g., actual evapotranspiration (+14–17%), recharge (+11–27%), discharge (+11–33%), drainflow (+16–32%), mean phreatic (+0–18 cm) and deep groundwater levels changes (+2–24 cm). Considerable differences in responses and sensitivity to climate change were found between the regions, and large precipitation changes were not always reflected in substantial groundwater level changes, not even if it led to a large increase in recharge. Climate sensitivities for the groundwater were shown to be low in regions with a shallow phreatic surface linked with restricting drainage networks or heightened actual evapotranspiration. Reversely, climate sensitivities were found to be high in areas with a deep phreatic zone and in high conductive deep chalk aquifers.