Irrigation‐Induced Potential Evapotranspiration Decrease in the Heihe River Basin, Northwest China, as Simulated by the WRF Model. Issue 2 (22nd January 2020) - Zhang, Xuezhen
Abstract: Evapotranspiration occurs via land‐atmosphere coupling. To understand the impacts of irrigation on local potential evapotranspiration, we carried out a couple of simulations with the Weather Research and Forecast (WRF) model, where an irrigation scheme was incorporated, for the irrigated area in the middle reaches of the Heihe River, Northwest China. By comparing the irrigation simulation (hereafter, the IRRG case) with the simulation excluding irrigation (hereafter, the NATU case), we found that irrigation may lead to a decrease in reference evapotranspiration (ET0 ) during the growing season (May to September), which is a metric of potential evapotranspiration, by approximately 1.05 mm d −1, accounting for approximately 19% of NATU ET0 . The ET0 in the IRRG is closer to the ground measurements than the NATU ET0, with a root‐mean‐square error of 0.75 mm d −1 in the IRRG and 1.86 mm d −1 in the NATU cases. Meanwhile, irrigation leads to an actual evapotranspiration (ET a ) increase of approximately 2.1 mm d −1 for croplands. Such an asymmetric relationship between ET0 decrease and ET a increase was found over the irrigated croplands. The ET0 decrease was mainly induced by the moistening and cooling effects of irrigation through intensified evapotranspiration and reduced sensible heat. This study highlights the importance of considering land‐atmosphere coupling when assessing the impacts of climatic change on irrigation water requirements. Key Points: Irrigation intensifies evapotranspiration and leads to cooling and moistening effects, as well as to potential evapotranspiration decreases The increase in irrigation‐induced actual evapotranspiration and the decrease in potential evapotranspiration follow the complementary principle It is necessary to consider land‐atmosphere coupling when assessing the impacts of climate change on irrigation water requirements