The purpose of the scientific article is to develop and determine the accuracy of the model for calculating evapotranspiration, taking into account the probabilistic nature of the hydrometeorological and water balance information of the irrigated agricultural landscape. Empirical dependences of various crops evapotranspiration and the actual moisture supply of the soil root layer have been determined for the corresponding phases of plant development by long-term experimental studies on the irrigation systems of the South Russia region for various soil and climatic irrigation zones, followed by comparison of the data obtained with the potential evapotranspiration data at the corresponding meteorological stations. Data analysis shows that the calculated absolute data values of the correlation coefficients between the compared values of the studied parameters vary in a wide range from 0.34 to 0.98, and obtaining empirical dependencies is generally described by nonlinear mathematical equations. Thus, it was found that the method for determining the parameters under study according to A M Alpatev equation gives significant errors; according to S I Kharchenko equation - is generally satisfactory, but allows unacceptable deviations in particular solutions; according to model – a good one, with allowable deviations of the calculated values from their actual values.