Trends in reference crop evapotranspiration and possible climatic factors in the North China Plain
华北平原参考作物蒸散量变化特征及气候影响因素

Reference crop evapotranspiration (ET0) is the key factor to estimate crop water requirement and guide farming irrigation scheduling. Under the climate change background, daily reference crop evapotranspiration at typical stations across the North China Plain (NCP) during the period from 1961 to 2007 were calculated by the Penman-Monteith approach. The seasonal and annual characteristics of ET0 were analyzed from the view of energy balance and dynamics. Using the mathematical statistics, primary and secondary climatic impact factors were studied for reference crop evapotranspiration and its constituents (ETrad and ETaero) respectively, for guiding farming water management more effective in the NCP. The results showed that the annual reference crop evapotranspiration and its constituents (ETrad and ETaero) were significantly declined, while annual mean temperature was significantly increased, annual mean sunshine hour, related humidity and wind speed were significantly decreased. The highest reference crop evapotranspiration and ETrad were in summer, and lowest in winter, while the spring ETaero value was highest across the NCP. The relationship between annual ETrad and ETaero was slightly negative, while was positive in spring and summer as well as negative in autumn and winter. The main climatic factors for the change of ETrad were sunshine hour, wind speed and mean temperature, and the impact of wind speed was negative to ETrad. However, the main climatic factors for the change of ETaero were wind speed, related humidity and mean temperature, and the impact of related humidity was negative. Therefore, the change of reference crop evapotranspiration was synthetically impacted by sunshine hour, relative humidity, difference of maximum and minimum temperature and wind speed. In addition, the correlation between rainfall and reference crop evapotranspiration was significantly negative, while the decline rate of rainfall was a little more than that of reference crop evapotranspiration.