Salinity is turned up to be the major problem of water resources in Tunisia. Given the magnitude of this phenomenon, including resource’s deterioration in the economic calculation becomes increasingly required. It is to internalize this externality through the cost adjustment by the economic value of degradation. The choice has focused on one of the so-called indirect methods of evaluation: the method of productivity. This analytical framework has required the estimation of a production function for two zones of which only the quality of irrigation water differs. In addition to production factors, a dummy variable was introduced to characterize the quality of water used for irrigation with a value of 1 indicating the state of degradation. Empirical results have shown that increasing the salinity of 1% causes a decrease in productivity by 15%, which confirms the initial assumptions