Impacts of irrigation on hot extreme in the Yangtze River Basin using observed analysis and model simulation

Irrigation is one of the land management types with significant biophysical impact on local climate. Former studies have shown that irrigation has cooling effect on temperature, especially hot extremes. However, most of the studies are based on simulations from regional or global climate models, and the simulated irrigation-induced cooling remains largely unvalidated. In this study, two individual methods, observational analysis and the WRF model with irrigation module are used to assess the impacts of irrigation on hot extremes in the Yangtze River Basin (YRB) of China, a typical humid/semi-humid region with intensive irrigation. The average extreme high temperature in the YRB decreased by 0.3 °C with the highest decrease of 1.4 °C due to irrigation. We find a threshold for the cooling benefits of irrigation expansion in the YRB. Namely, the cooling intensity increases with the expansion of the irrigated area fraction, but diminishes once the irrigated area fraction exceeds the threshold value of 0.3. This can be explained by using model simulation, where irrigation primarily achieves the cooling effect on extreme high temperatures by increasing humidity and latent heat fluxes while reducing sensible heat fluxes. The study highlights the urgent need to understand the cooling effect of irrigation in such regions for efficient land management and emphasizes the necessity of considering the differences in irrigation impacts in future climate predictions.