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Abstract

Regional groundwater recharge is usually assessed via hydrological models. However, various studies have shown that most models tend to underestimate long-term trends compared to local observations or to the GRACE mission data. This was systematically investigated via analysis of groundwater data in Northeast Germany, covering an area of more than 50,000 km². About 240 long-term time series of groundwater head and lake water level were studied. It could be shown that at weekly or monthly time scales lake water level dynamics very closely mimicked that of the adjacent groundwater body.A very close correlation between the direction and strength of the trends and the degree of damping of the hydrological input signal was found. The probability of long-term trends systematically increased with the thickness of the vadose zone. This indicates the crucial role of long-term accumulation of soil moisture deficits in the deeper unsaturated zone. In contrast, many models of groundwater recharge consider only the uppermost soil layers. In addition, modellers usually assume initial steady-state equilibrium conditions and thus ignore long-term memory effects in the subsurface. Simulations with different models clearly showed that this resulted in a systematic underestimation of long-term trends. Based on these findings clear recommendations for monitoring and model-based assessment of groundwater recharge will be given.