A simplified method of estimating actual evapotranspiration of crops during an irrigation interval in which soil-moisture deficits occur is presented. With some simplifying assumptions, a relation is developed between a crop-dependent critical leaf water potential, potential evapotranspiration rates, and the fraction of available soil-moisture at which reduction in evapotranspiration occurs (Fig. 1). This relation was tested by comparing calculated and measured rates of actual evapotranspiration for maize, alfalfa, sorghum, wheat, potato, and sorghum crops (Fig. 2, A through F). Calculated values of actual crop water use were close to measured ones, except for the potato crop. Sensitivity tests indicated that, for crops with a critical leaf water potential above −7.5 bars, i.e. −0.75 MPa, errors exceeding 10 per cent were made in estimating the fraction of available soil moisture at which reduction in evapotranspiration occurs.