Application of the coupled model to the Somme river basin

International audienceHydrological modeling of the Somme river basin situated in the north of France was made with special emphasis on the stream-aquifer interaction. Due to immense groundwater storage in the chalk aquifer, a damaging flood took place in the basin in spring 2001. To best represent the phenomena occurring in the basin, the coupled model also known as MODCOU was selected to be applied to this particular basin. The whole model structure consists of several components, namely, surface model, groundwater model, unsaturated zone model, and the coupled model. In order to run the surface model, meteorological forcing, land use, and soil type data were acquired. By using land use and soil type data, production zones were obtained. The surface model partitions the precipitation between surface runoff, infiltration, and actual evapotranspiration according to the parameters of production zones. The unsaturated zone model computes the recharge of groundwater by using infiltration from the surface using a Nash cascade model. By using the groundwater model, a steady-state piezometric head distribution is obtained to serve as an initial condition to the coupled model. The unsteady groundwater and surface flow simulations are carried out by the coupled model. Initially, spatial information on the basin was extracted by using Digital Elevation Model (DEM) analysis. After acquiring the necessary spatial data, the surface and aquifer grids were generated by using nested grid generators which make refinement on the stream network and subcatchment boundaries in order to increase the accuracy of numerical solution. A very detailed calibration was performed by following a step-by-step procedure in which the best fit in flow and piezometric head hydrographs was sought. In the surface model, concentration time and parameters of production zones are calibrated. For coupled surface-groundwater flows transmissivity, specific yield and discharge coefficient; and for flow in unsaturated zone storage constant were calibrated. During the simulations, intense and interrupted exfiltrations were observed over the basin. Two algorithms were developed in order to correct the issue caused by the coarse resolution of the DEM: (1) a [`]fill' algorithm was used to remove the sinks and obtain a smoother surface; and (2) a correction procedure for river cell elevations is proposed to acquire continuous exfiltration along the stream network. Validation of the model was made using the discharges at two points on the Somme, namely, Hangest-Sur-Somme and Peronne, which had not been used in any stage of the modeling process and satisfactory results were obtained. The groundwater behavior, its effect on the surface flow and the flood of 2001 are satisfactorily represente