Impact of groundwater regimes on water balance components of a site with a shallow water table

The management of shallow water table sites is often the subject of discussion between different interest groups, the most controversial topic being the different target groundwater levels. A good level of knowledge about the effects of the different water levels on the water budget components is a precondition for the development of compromises in well-balanced water resource management. We used groundwater lysimeters to investigate the impact of different water level regimes on the water balance. The lysimeters were installed directly within a typical shallow water table site and had a specially designed system to control the lower boundary condition. This enabled us to simulate different management options in a realistic way. Our results show increasing evapotranspiration with higher water levels but only if the vegetation is adapted to these conditions. Adaptation may take place within a few years of higher water levels. High water levels in spring are linked to increased water storage. This can help to compensate for the higher evapotranspiration for some weeks but not for the entire season. The meteorological conditions have a large short-term impact on the water budget, which is difficult to compensate for using long-term water management strategies or slow-responding vegetation development. Our results underline the complex ecohydrological dependencies in such site conditions and could be an important basis for the development or improvement of ecohydrological models.