Spatial-temporal variation in irrigation water requirement for the winter wheat-summer maize rotation system since the 1980s on the North China Plain

The irrigation water requirement (IR) is crucial for optimizing agricultural water management and reallocation and for adjusting the planting structure. Based on the datasets derived from 277 meteorological stations and 42 agro-meteorological stations from 1980 to 2012, the simplified water balance equation was employed to estimate the IR in the winter wheat-summer maize rotation system. The results indicated that, for the two crops, the crop coefficients varied with time and space at different growth stages, with low or moderate variability levels. The average values of Kcini, Kcmid and Kcend were 0.69, 1.17, 0.34 and 0.76, 1.13, 0.43 for winter wheat and summer maize, respectively. The region located to the most of northern parts of the Yellow River had reduced precipitation and increased reference evapotranspiration (ETo) during the rotation cycle; moreover, in the southern part of this region, the precipitation increased significantly with distinctly decreased ETo. In the North China Plain (NCP), the IR for the winter wheat, summer maize and rotation cycle all had no significant trend change, for which the multi-year average values were 341.1, 250.5 and 592.5 mm, respectively. The region with higher IR was primarily located in northern Shandong and the most of northern parts of the Yellow River, where the IR level was remarkably aggravated in dry seasons. Additionally, the IR increased in the northern NCP region and in the junction area between Hebei and Shandong, and IR decreased with a trend of 10 mm decade−1 in other areas. In addition, the magnitude of the station and time intervals for abrupt change of IR varied with different growing seasons.