Tunisia is characterized by a semi-arid climate over more than 90% of its territory with irregular and high spatial variability of rainfall. This constitutes a major constraint limiting agriculture development which depends on the availability of water resources. The assessment and the effective management of water resources in a territory require then extensive knowledge of involved hydrological processes as well as their temporal and spatial scales. This paper discusses the issue of

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... on and hydrological functioning of the Rmel watershed to get an efficient management of water resources in case of scarcity or extreme events using modelling tool. This watershed (675 km 2) is situated in NorthEast Tunisia with average annual rainfall of 420 mm and was equipped in 1998 with a dam. Data on rainfall collected at 12 rainfall stations during the period 1998-2013 are analyzed and used to build a coherent series of monthly rainfalls. In a second step, rainfall-runoff modelling using SWAT Model was used to estimate runoff and water budget of the dam. The Nash criterion is about 85% for calibration and validation. For the tow scenarios with an increase of the annual crops and farmland areas, we noticed that the watershed response was affected in the same way, causing an overestimation of water budget and especially the runoff peaks. The use of the model to make prediction of stream flow using downscaled climatic data from ARPEGE for the period 2060-2080 was discussed. For SWAT model, the estimated stream flow shows a tendency to increase the runoff process explained by the increase of the rainfall intensity. We noted that the implementation of SWAT model is able to represent efficiently the actual hydrologic runoff phenomenon and to predict watershed hydrological behaviour in the future. Thus, this work could be considered as a new tool decision support that can contribute to better management of natural resources in semi-arid environment.