A Modelling Framework to Simulate the Dynamics of the Groundwater, Hydrologic, and Ecologic System in an Alpine floodplain

Most surface flows to the 20-km-long Maggia Valley in Southern Switzerland are impounded and the valley is being investigated to determine environmental flow requirements (EFRs). The long-term aim of the investigation is the development of a modelling framework that simulates the joint dynamics of the groundwater, hydrologic, and ecologic systems. A nested modelling framework is selected for the purpose. Large scale models are first developed to provide the boundary conditions for more detailed models of ecologically interesting reaches. The initial (large-scale) groundwater (GW) model is constructed using MODFLOW-2000 and its surface water package. The aquifer is modelled with two confined aquifers and the water table is considered iteratively. Parameters are defined to represent the areal recharge, the hydraulic conductivity of the aquifer (up to 5 classes), and the streambed hydraulic conductivity. Several conceptual models are evaluated by changing the number of hydraulic conductivity classes, and one most likely model is identified, which best fits observations with realistic parameter estimates. The model robustness is tested using sensitivity analysis and a cross-validation method, whereas its predictive capability is discussed with a completely independent set of data. The GW model is further evaluated by accounting for the boundary conditions as predicted by a raster-based, physically oriented and continuous in time rainfall-runoff (R-R) model. This model has proven to perform very well in the investigated area and computes among others the sub-surface flow over the entire flooplain boundary domain. It provides thus an observation-like input to the GW model, which can be thus investigated with respect to the changes in the performance when substituting such input for the homogeneous lateral flow obtained through calibration. The subsurface flows simulated for selected periods or events are used to assess the importance of realistic input to the groundwater model in terms of both initial and boundary conditions. Finally, the coupling of the R-R/GW model system with a 2D hydrodynamic model to complete the nested modelling framework required for detailed simulations of the floodplain dynamics is discussed.