Groundwater recharge in Brandenburg is declining – but why?

Abstract. Brandenburg is among the driest federal states in Germany, featuring low rates of ground water 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, ground water 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 as largely unaffected by direct anthropogenic interference with the water balance. Using the Soil-Water-Atmosphere-Plant model (SWAP) to simulate long-term trends in GWR, we found that significant increases of air temperature, solar irradiation and leaf area index (LAI) since 1980 acted towards a decrease in GWR in 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 of GWR amounted to -5 to -3 mm a^-1 per decade. Based on our results, we consider it as 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 on the sign of simulated GWR trends. Given the uncertainty of 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 ground water table depth, as well as 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 resources management and planning in Brandenburg should, however, already take into account the possibility of GWR to decrease 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 of GWR as projected mainly on the basis of expected future increases in winter precipitation.