Model-based estimation of runoff changes in a small lowland watershed of north-eastern Germany

In the small lowland watershed of the Fredersdorfer Muhlenfliess, NE Germany, a decreasing runoff associated with temporal hydrological droughts are observed in the last two decades without considerable changes in precipitation. In order to estimate reasons for the frequent droughts in this basin, a composite lumped physically-based model was developed to describe transient hydrological processes with special emphasis on the surface–subsurface interaction. The model consists of several sub-models taking into account precipitation, evapotranspiration, soil and land use, groundwater recharge and runoff combined within a post-connected balance model. Within scenario simulations for a time-period of more than 20 years different effects on the runoff are studied: (i) ratio of surface–subsurface basin area, (ii) stream water leakage in dependence on the groundwater level drawdown, and (iii) surface water withdrawal and allocation. Observed hydrographs are well reproduced by the model, showing that the occurrence of hydrological droughts in this area is mainly controlled by baseflow and, especially, the decline of groundwater level has an increasing influence because stream water leakage could diminish the runoff seriously. On this account, water balance models for lowland watersheds with similar hydrological and geological conditions without considering subsurface flow components have to be judged as to be not capable of adequately simulating drought.