Groundwater recharge in Brandenburg is declining – but why?

Abstract. Brandenburg is among the driest federal states in Germany, featuring low rates of groundwater recharge (GWR) across large parts of the state. This GWR is fundamental to both water supply and the support of natural ecosystems. There is strong observational evidence, however, that GWR has been declining since 1980: first, river discharge (which is almost exclusively fed via GWR) has been significantly decreasing in many catchments (by around 40 % since 1980). Second, groundwater levels in the groundwater recharge areas show a significant long-term decline. In this study, we search for potential reasons behind this decline by investigating five catchments across Brandenburg that we consider largely unaffected by direct anthropogenic interference with the water balance. Using the Soil-Water-Atmosphere-Plant (SWAP) model to simulate long-term trends in GWR, we found that significant increases in air temperature, solar irradiation, and leaf area index (LAI) since 1980 have acted towards a decrease in GWR on the order of −21 to −4 mm a−1 per decade from 1980 to 2023. The Brandenburg-wide LAI trend of +0.1 m2 m−2 per decade was inferred from a recently published, spatio-temporally consistent LAI reconstruction. The contribution of this LAI trend to the decrease in GWR amounted to −5 to −3 mm a−1 per decade. Based on our results, we consider it very likely that the decrease in discharge since 1980 can be explained by a decrease in GWR, which, in turn, was caused by climate change in combination with an increasing LAI. However, we also found that precipitation trends can be highly incoherent at the catchment scale. Even though these precipitation trends are not significant, they can have a fundamental impact on the significance, the magnitude, and even the sign of simulated GWR trends. Given the uncertainty in the precipitation trend, four out of five catchments still appear to exhibit a gap between negative simulated GWR trends and more negative observed discharge trends. We provide a comprehensive discussion of possible reasons and uncertainties to explain this gap, including the effects of the limited length and the inhomogeneity of climate and discharge records, the role of land cover and vegetation change, irrigation water consumption, latent anthropogenic interventions in the catchments' water balance, uncertainties in groundwater table depth, and model-related uncertainties. Addressing these uncertainties should be a prime subject for prospective research with regard to the effects of environmental change on GWR in Brandenburg. Water resource management and planning in Brandenburg should, however, already take into account the possibility of GWR decreasing further. Given the fundamental importance of precipitation trends and their large uncertainty in future projections, we strongly advise against putting our hopes in a future increase in GWR as projected mainly on the basis of expected future increases in winter precipitation.