Assessment of Management Practices Impact on the Water Quality of the Béja River Watershed Using SWAT Model

Water pollution o by agricultural and human activities has become a hot topic that must be taken into account. Its qualitative and quantitative evaluation has been well demonstrated by the SWAT model. The proposed research is an important step in the study of the agricultural and human pollution, in the various tributaries of Sidi Salem dam. We envisaged the use of the tool SWAT, for one of the main polluting tributaries, Wadi Beja, and planned a qualitative management of this watershed. The application of this model requires a prior study of the hydrological basin of interest and the calibration of a large number of intrinsic factors specific to the area of study. This model has been mainly tested on several watersheds in the Nordic countries, including Canada and France because the model contains a module specific to the glacial melt. Its adaptation to the Mediterranean context has rarely been carried out. In the case of Tunisia, the absence of a long ongoing series of measurement data and hydrodynamics of the soil hampers the application of the SWAT model. In a first step, we adapted this model to the specific context of the study area, in order to understand the hydrological functioning of this watershed. Subsequently, we studied the sensitivity of the model according to the key hydrological parameters, by performing the continuous simulation for more than 8 years. This allowed us to have a better choice of the shimming parameters, in order to properly shim the model. In a second step, we refined the model calibration settings according to 2 years of deep study: wet and dry, though opting for a seasonal cutting for the sake of better describing the pedoclimatic context of the study area. This approach should have a good match of the simulated and observed data, as confirmed by four accuracy measures. In addition, it has been shown that the contents of nitrate and orthophosphate were of concern because of the poor use of agricultural fertilizers. Of this fact, various scenarios of management have been developed and tested by the model in order to reduce the percolation rate of various forms of nitrogen and t phosphorus leaching. These approaches would enable policymakers to monitor the water pollution in the dam area while reducing the burden of various eutrophying elements.