Drought-induced land use and land cover change impacts on hydrology: Insights from the Harz mountains, Germany

Abstract. Drought conditions in Europe in 2018 and the following years led to significant land use and land cover (LULC) changes that affect the water balance. One of the regions that experienced strong changes are the Harz mountains in Germany, where drought conditions have led to tree mortality in its forested catchment areas. The aim of this study is to quantify these drought-induced LULC changes and analyze how they affect the water and sediment balance. To this end, remote sensing data and a deep learning model were used to derive annual LULC changes including dead tree areas between 2018 and 2023. These data were used to set up a SWAT+ model with dynamic LULC changes. The model was calibrated at the gauges of three headwater catchments of the Oker river. Kling-Gupta efficiencies indicate varying but at least satisfactory performance at all gauges during calibration (0.75 to 0.82) and validation period (0.63 to 0.79). Areas of dead trees were modeled as bare or sparsely vegetated (recent tree mortality) or as grassland (regrown areas). The LULC change analysis demonstrated strong performance (86–88 % overall accuracy) and revealed a decrease in coniferous trees by up to 46 % in one catchment (19 % and 25 % in the others). The hydrologic impacts were assessed by comparing a model run with LULC changes to a run without LULC changes. The results indicate a continuous decrease of evapotranspiration by up to -7.4 % in 2023 and a continuous increase of water yield by up to 11.3 % in 2023. The spatial assessment of modeled LULC change impacts shows strong increases in water yield and percolation and strong decreases in evapotranspiration associated with tree mortality. The increase in water yield can mainly be attributed to an increase in surface runoff. Changes in sediment yield indicate increased risk of soil erosion at areas associated with tree mortality. It was found that a dynamic model representation of tree mortality is necessary to account for these fast and strong changes and their impacts on hydrology. Moreover, the faster response of the catchment potentially increases the severity of flood events and the flood risk in downstream areas. The results underline that droughts significantly affect hydrology even after the end of the drought event. Therefore, afforestation with climate-resilient trees is needed to improve both flood and drought resilience in regions that suffer from drought-induced tree mortality.