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Institut für Wasser- und Umweltsystemmodellierung - IWS

Abstract

 
   

"Analysis of Regional Aquifer Systems in Large Mountainous Catchment Areas and Their Discretisation for Large-Scale Groundwater Models"

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Modern distributed hydrological models allow for the detailed description of the hydrological and energy cycle in large catchment areas and play an important role in modern integrative water resources management. The groundwater component is a decisive part of such a hydrological model and hence has tomust be accurately modelled very accurately. This may be achieved with a deterministic numerical groundwater model., hHowever, such a complex models haves the drawback, that its their calculation times are very high in comparison to the calculation time of other components of the hydrological model is very high.One method to reduce the calculation time is to restrict the underlying conceptual model to the main geological layers and to apply a finite difference approach on a coarse grid. While this leads to a faster simulation and calibration of the groundwater model, it may also cause serious physical and numerical problems. This is especially apparent while when applying the finite difference method to the modelling of aquifer boundaries. Large catchment areas usually show a characteristic interaction between mountain areas as water suppliers, and their forelands as water consumers. In those the forelands, the groundwater flow pattern is often characterized by highly permeable gravel aquifers, which transport the surplus water from the mountains to the outflow of the catchment. However, the horizontal extent of these aquifers may be very small as compared to the grid size chosen to model the catchment area within a reasonable time frame. Therefore, it is very difficult to model the impact of these small but very important aquifers correctly, especially, moreover if their exact layout is not well known or has been modified due to interpolation of the input data. After the identification of the regional aquifer system and the creation of a hydrogeological conceptual model it is important to implement this concept into the groundwater model such that a stable numerical solution of the model is attainable. The main problem is to achieve a connected aquifer system, which is able to receive the groundwater recharge in the mountain areas and which yields a reasonable base flow at existing gauging stations in the forelands. Due to the discrepancy between the finite difference cell size and the extentd of the narrow, highly permeable aquifers, additional highly permeable cells have to be “added” in order to achieve a closed solution for groundwater flow using a finite difference scheme. In addition, it has to be einsured, that each cell of this “virtual” aquifer has at least one neighbouring cell (in the direction of groundwater flow) with a lower base to guarantee the conductivity of the aquifer. With such an algorithm, cells whose permeability needs to be adjusted can be detected and added to the modelled aquifer layer. The proposed algorithm was applied to the catchment of the Upper Danube (gauge Achleiten near Passau). The Upper Danube catchment shows the typical groundwater flow pattern described above. Highly permeable sand and gravel aquifers with a small horizontal extent of a few hundred meters to a few kilometres (FD cell size = 1 km2) transport the groundwater from the Alps to the Danube valley. The modelling results of a finite difference groundwater model in this area using an adjusted aquifer geometry are very promising. Measured groundwater levels in the gravel aquifer can be modelled with an accuracy of less than two meters. Without a proper investigation of the regional aquifer system and the application of the presented algorithm for the discretisation of such a system, the modelling of regional groundwater flow on a coarse finite difference grid would not be possible at all.