Irrigation-induced decreases in reference evapotranspiration over the North China Plain

As a pivotal factor influencing crop water requirement, reference evapotranspiration (ET0) plays a crucial role in agricultural water planning and management. Irrigation can exert distinct influences on ET0 by modifying meteorological conditions through land-atmosphere coupling. While the climatic effects of irrigation in the North China Plain (NCP), the most intensively irrigated area over China, have been explored, the specific impact of irrigation on ET0 remains unclear. To address this gap, we here employ a regional climate model coupled with a well-validated irrigation scheme to perform idealized simulations to investigate the responding behaviors of ET0 to irrigation in the NCP from 2005 to 2014. We find that the incorporation of irrigation notably enhances the model performance in modeling ET0 in the NCP. Irrigation induces an ET0 decrease of -0.32 mm day- 1 (-29.4 mm year- 1) in spring and -0.38 mm day- 1 (-35.0 mm year- 1) in summer and an actual evapotranspiration (ETa) increase of 0.49 mm day- 1 (45.1 mm year- 1) in spring and 0.40 mm day- 1 (36.8 mm year- 1) in summer, respectively. The irrigation-induced changes in ET0 and ETa adhere to the complementary principle, which stems from the land-atmosphere coupling. The decline in ET0 is attributed to decreases in surface air temperature and wind speed, coupled with an increase in relative humidity due to irrigation, with the latter playing a paramount role. Furthermore, irrigation decouples the relationship between precipitation and ETa and weakens the traditional dependence of ETa on local precipitation. This study underscores that the related change in ET0 due to the climate feedbacks of irrigation should be taken into account in agricultural water planning and management.