The paper considers the procedures for calculating the irrigation rates of agricultural crops. The value of irrigation rate consists of water consumption deficits over vegetation period of an agricultural crop, off-season waterings and the amount of irrigation water for leaching regime, if necessary. The water consumption deficit is calculated using the hydrologic equation which consists of total water consumption, efficient atmospheric precipitation, active water storage in soil, and root-inhabited soil layer intake of ground water (at high levels). The values of water storage in soil are established according the results of agricultural meteorological stations and in the absence of these data by calculation methods. It is noted that solving hydroeconomic problems the irrigation rate is predicted for the given percent of probability. Thereto, the results of climatic factors measuring (for continuous series of years) at the nearest weather station are needed. In the absence of the observations for continuous series of years, the irrigation rate for the given percent of probability is proposed to determine as a product of irrigation rate for averaged perennial conditions and modular coefficient. The averaged values of irrigation rate for different exceedance probability (5, 25, 50, 75 and 95 %) for several agricultural crops by zones of humidification in the South and North-Caucasus federal districts of Russia is cited. The constraint equations of irrigation rates and crop yields obtained by processing the results of perennial field studies are shown.
