Abstract: Abstract: The probability of irrigation water requirement is one of the most important designing parameters for an irrigation system. And the actual rainfall distribution affects the realization degree of probability of irrigation water requirement. There is a possibility that the annual regulation of rainfall resources is overestimated as the typical year is chosen by annual precipitation. In this paper, based on the meteorological data from 1951 to 2015 in Zhengzhou, Henan province, the frequency of annual rainfall, monthly rainfall, and ten-day rainfall were calculated respectively. And the rainfall distribution corresponding to typical years was analyzed under different probability of irrigation water requirement. Then the typical years were selected at different time scales, and the rainfall of typical years under the same probability of irrigation water requirement was compared and analyzed. Taking April, July and August corresponding to the critical periods for water demand of winter wheat and summer maize as the typical periods, the changes of rainfall under the same probability of irrigation water requirement were studied, and corresponding irrigation schedules were formulated according to different calculation time scales. The effect of time scales on irrigation quota and rainfall utilization was discussed. The results showed that: 1) Under the same probability of irrigation water requirement, the annual rainfall in each typical year was similar, the month and ten-day rainfall in each typical year had a significant difference, which was more than 5 times in May. The magnitude of precipitation varied from the largest to the smallest as followed: the annual, monthly and ten-day scales. 2) Using the water balance method, an annual irrigation schedule was formulated for typical winter wheat and summer maize under different time scales. Under the probability of irrigation water requirement at 25%, the annual irrigation quota under the seasonal scale was significantly higher than the daily scale. Compared with the seasonal, monthly, and ten-day scales, the annual irrigation quota on the daily scale was lower by 34%, 23% and 3%, respectively. Irrigation quota decreased with decreasing precipitation calculation scale in dry years, but the difference was not obvious. In conclusion, the utilization of rainfall resources increased as the time scale decreased. Considering that different time scales had different water saving potentials and effects, improving the corresponding standards and their selection methods in irrigation and drainage engineering, and selecting the appropriate calculation time scale would not only help to ensure the probability of irrigation water requirement, but also improve the utilization of various water resources.