A water balance model is coupled to a function which estimates midday leaf water potential of cotton plants. The water balance model predicts soil evaporation, plant transpiration, run-off, and drainage. Rainfall and irrigation are inputs. Maximum crop evapotranspiration is estimated with Penman's equation using climatic data. The water balance model simulates the water distribution within the soil profile, and generates plant available water weighted for root distribution. An empirical function relating the weighted plant available water to midday leaf water potential was then calibrated using field data collected in an experiment with several irrigation treatments.