Estimation of Crop Evapotranspiration Under Standard Conditions for Winter Wheat in the Huang-Huai-Hai Plain

Crop evapotranspiration under standard conditions (E_c) is defined as the evapotranspiration from disease-free, well-fertilized crops grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic conditions. The calculation of E_c considers crop and local surface conditions. E_c is the theoretical upper limit of actual evapotranspiration for actual local surface coverage, ensuring objective analysis on crop water requirements and agricultural drought. To summarize the spatial and temporal distribution characteristics and their causes of E_c, daily E_c is calculated based on Penman-Monteith method using meteorological data and satellite remote sensing data from 2000 to 2015. The meteorological data are provided by 27 meteorological stations in the winter wheat growing area of the Huang-Huai-Hai Plain. The satellite remote sensing data are extracted from NASA MODIS products (LAI (MOD15A2) and Albedo (MCD43C3)) at the corresponding location of 27 meteorological stations. E_k is calculated based on single crop coefficient approach recommended by FAO. Results show that daily dynamic changes of E_c and E_k are consistent in the regional tie scale. However, compared with E_k, E_c has a spatial distribution corresponding to the objective reality. The growth period of winter wheat is divided into five stages:Before wintering stage, wintering stage, returning green-jointing stage, heading stage and milky maturity-maturity stage. With the spatial distribution characteristic of higher in the south and lower in the north, the average daily E_c in the whole winter wheat season, wintering stage and returning green-jointing stage is 1.95 mm, 0.46 mm and 2.74 mm, respectively. The average value of E_c is 1.23 mm before wintering stage, and the whole fluctuation of E_c in the Huang-Huai-Hai Plan is small. There is no significant fluctuation in E_c in heading stage and milky maturity-maturity stage except for the middle part of the Huang-Huai-Hai Plain. The average value of E_c is 4.71 mm and 3.72 mm in these two growth stages, respectively. In terms of spatial distribution, extremely significant positive correlation is shown between LAI and E_c in all growth periods. In wintering stage, returning green-jointing stage and milky maturity-maturity stage, E_c also shows a higher significant negative correlation with albedo. During the whole growth period of winter wheat, E_c has a higher partial correlation coefficient with LAI and water vapor pressure. These results can provide basic data for drought monitoring and wet or dry climate zoning in China, and also provide a new idea for the actual evapotranspiration estimation.