Climate changes in agriculture act on various climate variables (precipitation, temperature, etc..) at different times of crop cycles. Many physical and technical relationships have to be represented even when analyzing a limited aspect of farm management. This work employs the net evapotranspiration (ETn) estimated with the EPIC model, as a synthetic index of the physical factors that the farmer considers in decisions on irrigation. The probability distribution of ETn is inserted into a territorial model of DSP that represents farm choices in conditions of uncertainty about water availability and irrigation requirements of crops. Recent trends of ETn suggest that the probability distribution of this variable may appreciably change in the near future. Also, water availability may become more variable due to changed rainfall. These modifications amplify uncertainty of management and, consequently, costs incurred by the farm typologies of the study area, which in many cases suffer an appreciable drop in income.