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A 3d regional density-driven flow model of a heterogeneous aquifer system at the German North Sea Coast is set up within the joint project NAWAK (“Development of sustainable adaption strategies for the water supply and distribution infrastructure on condition of climatic and demographic change”). The development of the freshwater-saltwater interface is simulated for three climate and demographic scenarios. Groundwater flow simulations are performed with the finite volume code d3f++ (distributed density driven flow) that has been developed with a view to the modelling of large, complex, strongly density-influenced aquifer systems over long time periods.

In large mountainous catchments, shallow unconfined alluvial aquifers play an important role in conveying subsurface runoff to the foreland. Their relatively small extent poses a serious problem for ground water flow models on the river basin scale. River basin scale models describing the entire water cycle are necessary in integrated water resources management and to study the impact of global climate change on ground water resources. Integrated regional-scale models must use a coarse, fixed discretization to keep computational demands low and to facilitate model coupling. This can lead to discrepancies between model discretization and the geometrical properties of natural systems. Here, an approach to overcome this discrepancy is discussed using the example of the German-Austrian Upper Danube catchment, where a coarse ground water flow model was developed using MODFLOW. The method developed uses a modified concept from a hydrological catchment drainage analysis in order to adapt the aquifer geometry such that it respects the numerical requirements of the chosen discretization, that is, the width and the thickness of cells as well as gradients and connectivity of the catchment. In order to show the efficiency of the developed method, it was tested and compared to a finely discretized ground water model of the Ammer subcatchment. The results of the analysis prove the applicability of the new approach and contribute to the idea of using physically based ground water models in large catchments.

Abstract. Hydrological models are the decisive tools to evaluate the effect of global change upon the water cycle. But the applied hydrological models have to be a trade-off between their degree of complexity and manageable structures and data requirements. This paper compares the advantages and disadvantages of integrating a spatially-distributed process-based groundwater flow model in the context of the calibration of a catchment runoff concentration model. The multi-objective optimisation and the GLUE method are used to analyse the performance and the parameter identifiability of both model structures.