Modeling crop evapotranspiration using remotely sensed vegetation data: A case study of winter wheat in the North China Plain

The North China Plain (NCP) is a vital granary of China, with an area of 140 000 km² of arable land which produces some 20% of the nation's grain. As local precipitation is not enough to meet crop water requirements in the NCP, irrigation is widely used to increase yields and to ensure food supply. Agriculture accounts for about 70% of water consumption in the NCP, over 75% of which is from groundwater. Groundwater resource has steadily been depleted due to persistent over-pumping, posing tremendous challenges for sustainability. Meanwhile, excessive use of groundwater has resulted in severe environmental problems in the region. Thus, in order to achieve sustainable agricultural water management in the NCP, it is extremely important to explicitly estimate crop evapotranspiration in recent decades. In this paper, a model based on crop coefficient and vegetation remote sensing data was developed to estimate crop evapotranspiration in the NCP. The model not only estimated crop evapotranspiration, but also the spatial distribution of crops and soil moisture in the region. Moreover, the model was applicable to other studies such as water use efficiency and estimation of water footprint. With the model, winter wheat evapotranspiration was estimated for 2000?2013 in the NCP. The results of the model were consistent with those of ETWatch, a remote sensing evapotranspiration estimation product (R2=0.952, RMSE=1.3×10⁷ m³). The results were as follows: 1) Winter wheat evapotranspiration was higher in the southern than in the northern region of the NCP. Evapotranspiration was in excess of 400 mm in the piedmont region of Taihang Mountains and in the irrigation region of Henan-Shandong Yellow River. It was less than 350 mm in the middle plains and even less than 200 mm in coastal areas. 2) Irrigation water consumption by winter wheat was higher in the southern than in the northern region of the NCP. Irrigation was in excess of 250 mm in the piedmont region of Taihang Mountains and in the irrigation region of Henan-Shandong Yellow River. It was less than 100 mm in northern Hebei Plain. 3) There was a significant declining trend in the cultivated area of winter wheat and in irrigation water consumption in northern Hebei Plain in recent 14 years. Along with declining irrigation water consumption was also the slowing in the downward trend in regional groundwater level. Thus the effect of water-saving on the regional groundwater level was obvious. The proposed model in this study was applicable in estimating regional crop water consumption and irrigation water management.