LAUSR

Effective precipitation and water requirements throughout a growing period are important for planning irrigation and optimal water resource allocation. In this study, Jize County of Hebei Province was selected as a typical region to calculate the winter wheat, summer maize, and cotton effective precipitation and water requirements, using the Penman⁃ Monteith equation recommended by FAO and the piecewise single value average crop coefficient method. Using the Mann⁃ Kendall test method, we analyzed the trends of changes in effective precipitation and crop water requirements over the 1955—2015 in Jize County. The water requirement of the main crops showed a significant decreasing trend, and the average water requirement of the crops in was in the order of cotton, winter wheat, and summer maize. The maximum water requirement of winter wheat was 593.2 mm in 1968, the maximum water requirement of summer maize was 581.6 mm in 1968, and the maximum water requirement of cotton was 710.4 mm in 1972. An obvious change in water requirements, which was directly related to the improvement of crop varieties, appeared in the 1980s. The different water requirements of crops are related to the physiological characteristics of the crops. The maximum water requirements of winter wheat, summer corn, and cotton were during different growth stages; these were the middle growth stage, initial growth stage, and rapid development stage, respectively. Effective precipitation showed no significant decreasing trend in any of the three crops. The daily precipitation required for crop growth gradually declined over time, and the average effective precipitation of the crops in was in the ascending order of cotton, summer maize, and winter wheat. The maximum effective precipitation of winter