Influence of spatio-temporal variability of soil surface states on the watershed hydrology behaviour Case study of El hnach watershed in Tunisian semi arid

The Tunisian Dorsal appears as a succession of mountainous, extending from north-east to south-western Tunisia, from Cap Bon to the Algerian border. It suffers from both low economic development and environmental degradation in conjunction with increased pressure on natural resources. The tunisian concern remains soil and water conservation that represent the most valuable assets of a national heritage and the factors of the harmonious development of the country. In order to preserve this wealth, a strategy of mobilization of water resources in the two decades (1990-2000 and 2001-2011), included, among other things, the construction of about thousand hill lakes. The objective of this paper is the analysis and understanding of the hydrological behavior of El Hnach catchment in tunisian semi-arid zone. The watershed of El Hnach control an area of 385 ha and a perimeter of 9 km, instrumented since 1993, its lithology is characterized by an alternation of limestone and marl. The land is defined as a percentage of 56% of grassland, 40% annual crops and fallow for the rest. The used model is OpenFLUID, a physically based hydrological model. A sensitivity analysis was conducted to assess the impact of input parameters on the model outputs and showed that the model is particularly sensitive to the saturated hydraulic conductivity (Ks), slightly sensitive to the capillary pressure effective (Hf) and the soil initial moisture content (θi). From the simulations representing different flood scenarios, we could conclude that OpenFLUID reconstitute well the hydrographs events from the soil parameters chosen primarily with Nash close to 75%. This paper shows the performance of the used model, the important role played by soil surface conditions on runoff and infiltration in semi-arid region and the need to have a good estimat