Combined watershed and groundwater modelling to investigate lowland runoff processes – illustrated for the Lietzengraben basin (Germany)

Since the last decades in several lowland watersheds of north-east Germany a decreasing runoff combined with temporal hydrological droughts are observed. This paper makes the case for a modular organised, fully distributed, comprehensive, multi scaling model, which falls between the fully distributed, physically based hydrological modelling system of the type of the MIKE SHE model and the lumped, conceptual rainfall-runoff modelling system. This is achieved by integrating the watershed model ArcEgmo with the groundwater model ASM. Both of them are fully distributed models and their interaction is organised on the same raster-grid. The advantage of this approach is the use of readily available data, the handling of watershed heterogeneities (multiple land uses, soil layers, vegetation structures) and the significantly increased flexibility in handling stream-aquifer interactions. The mechanics of integrating the components are outlined, and model calibration, validation and simulation of real-world scenarios are briefly presented. Because of the fact, that main part of this basin was influenced by additional water supply more then 80 years, the aim of these scenarios was to derive management strategies to support the water balance of the catchment area, especially taken into account the interaction between surface and ground water. These applications demonstrate the practicability and versatility of this relatively simple and conceptual clear approach, making public acceptance of the integrated watershed modelling system much easier.