Trend and causes of water requirement of main crops in North China in recent 50 years

Information of crop water requirement and its historical change are important for irrigation scheduling, water resources planning, and future decision-making. Crop water requirements of winter wheat and summer corn in North China in recent 50 years were calculated by the FAO approach, which equaled to crop coefficient multiplied by reference crop evapotranspiration. Results suggested that water requirements of winter wheat and summer corn in most locations showed a downtrend in the past 50 years except Beijing, which respectively decreased by 0.9～19.2 mm and 8.3～24.3 mm for every 10 years. The decrease for summer corn exceeds that for winter wheat. Zhengzhou, the southern part of North China, ranks the first in terms of decreasing value, 19.2～24.3 mm per 10 year. The correlations among crop water requirement and sunlight, wind speed, temperature, humidity and rainfall revealed that the change of crop water requirement and the downtrend of sunlight and wind speed was coincident. Since decreased sunlight resulted in reduced energy reaching earth, and decreased wind may weaken the water and energy exchange between the earth and atmosphere, the downtrend of crop water requirement in North China mainly attributed to the reduction of sunlight hours and wind speed.