The objective of the research was to determine the optimal percentage of runoff probability at the design of estuary irrigation. The percentage was calculated on the basis of energy efficiency of forage crop rotation in the Rostov region. Total energy costs for crop growing were compensated by the energy increase in crop yield at estuary irrigation. Based on the preliminary math analysis of the obtained data the relationship between the energy efficiency of estuary irrigation ( Z ( x ) , GJ/ha), and percentage of runoff probability (x, %) was established: Z(x) = - 64,21x²+8783,9x-287168,15, GJ/ha. The optimal percentage of runoff probability for design the system of estuary irrigation in the Rostov region ranges from 60 % to 75 %. The maximum energy efficiency achieved at Ð = 68.5 %, its value was 13239.63 GJ/ha.
