The final level of agricultural plants productivity is determined by optimum of plants' moisture ensuring. In the conditions of irrigated agriculture the intensity of water supply, the uniformity of water distribution over surface and within some period directly forms the final impact. In this article suggest using such concepts as "readiness of system for work" and "system reliability" for irrigation systems design. It is obviously shown, that by implementation of suggested formulas it is possible to calculate the influence coefficients of mentioned factors for additional yields data and respectively the final production. To use the notion of technique downtime during the operation of irrigation equipment is recommended.