Recent studies on the economics of agricultural water management in Tunisia report a low water productivity of some presently widely cultivated crops such as durum wheat. The objective of this study is to estimate water productivity and marginal value of irrigation water applied to durum wheat in central Tunisia, region of Kairouan. We develop a production function, in which the irrigation revenue of farmers per hectare is expressed in terms of the used water volume in addition to other production factors. Results show that 31.7 % of farmers were applying water volumes above the economic optimal volume (more than 2900 mm/ha). Moreover, 50 % of farmers were found to be applying less irrigation water than this optimal volume. Applying water above the optimal volume means that the benefit farmers generate from each supplementary unit of irrigation water is lower than the market price of irrigation water currently applied in the region (0.110 TND/m3). Water is then wasted. However, using less water than this optimal volume means that farmers can make further supplementary irrigations and generate more benefit from it (extra-yield). The study also shows that most of farmers in the study area are not performing the irrigation scheduling and the appropriate irrigation doses. Improving these irrigation performances will surely preserve the water resources and enhance the food security in Tunisia.