Comparison of a simple and a spatially distributed hydrologic model for the simulation of a lowland catchment in Northern Germany

This paper describes the application of two simple models for runoff simulation of the Kielstau, a small wetland catchment in Northern Germany. The first one is SIMPEL, a one-dimensional spreadsheet model for soil water balance and runoff for small catchments; the second one is KIDS, Kielstau discharge simulation model, which is written in the PCRaster dynamic modelling language. KIDS uses the algorithms of SIMPEL, but applies them to a spatial database.The basic version of both models fails to reproduce the measured hydrograph for a flat region in Northern Germany with ca. 30% wetlands. Additionally, the water balance of the region cannot be closed, the relative discharge is lower than in the neighbouring catchments which means that ET must be higher. After the creation of an additional wetland compartment module where ETa equals ETp, the hydrographs could be matched. In a second step, we used inverse modelling with the SIMPEL model to evaluate the range of input parameters and possible sources of the low outflow of the catchment. We found that the low discharge can only be modelled if the evaporation is increased substantially. A wetland fraction of 0.3 produced the best results in inverse modelling runs, this value was also estimated based on the available maps.