A comparison and discussion of observational studies on farmland evapotranspiration in the North China Plain

The North China Plain is an important grain-producing area in China, and the water resources in this region are limited; therefore, accurately measuring the evapotranspiration (ET) of farmland is of great scientific significance and practical value for the region to reasonably allocate water resources and improve the efficiency of agricultural water use. In this study, ET was measured using the eddy covariance (EC), water balance (WB), and large-scale lysimeter (LYS) methods in a typical winter wheat and summer maize double-cropping agroecosystem in the North China Plain from October 2013 to September 2018. The results showed that the seasonal trends of ET measured by the three methods were generally consistent and were significantly positively correlated among the different methods with a correlation coefficient (r) of more than 0.90; the value of ET measured using the LYS method (ET_LYS) was the highest, while the values of the WB method (ET_WB) and the EC method (ET_EC) were lower. The correlation coefficients between ET_WB and ET_LYS and ET_EC were both approximately 0.94; thus, the WB method was more suitable for validating ET changes at different scales. The average annual ET_WB and ET_LYS were 788.6 mm and 906.4 mm, which were 9.8% and 26.2% higher than ET_EC (717.9 mm), respectively. The ET of winter wheat during the key growth periods (jointing, heading, and filling stages) accounted for 57.3%−61.5% of the total ET during the growing season; for summer maize, the ET in the key growth periods (heading, flowering, and filling stages) accounted for 58.5%−61.6%. In summary, in studies of ET changes and their effects, appropriate observation methods should be selected based on specific objectives and the advantages and disadvantages of the various methods.