Effects of Irrigation on Seasonal and Annual Temperature and Precipitation over China Simulated by the WRF Model

In this study, we developed a realistic irrigation scheme in version 3.6 of the Weather Research and Forecasting model (WRF3.6) with version 4 of the Community Land Model (CLM4) land surface scheme to investigate the effects of cropland irrigation on regional climate in China. Irrigation may occur throughout the year in most croplands with good thermal conditions to cultivate crops for more grain production, known as multiple cropping (MC). However, MC has been considered less in previous studies investigating the climatic effects of irrigation. In addition, the effects of cropland irrigation on seasonal climate in China have been less studied. The climatic effects of irrigation are assessed by comparing observations and model simulations with and without irrigation from 2001 through 2010. Results showed that the simulation with irrigation reduced mainly biases of land surface temperature (LST), surface air temperature (SAT), and precipitation over the irrigated areas. The simulation with irrigation also reproduced reliable annual irrigation water use and reasonable spatial distribution patterns of seasonal irrigation amounts. Both annual LST and SAT decreased 0.6 °C averaged over irrigated areas due to the irrigation-induced cooling effect. Additionally, the decreased surface temperature in the spring led to a reduced land-sea heat contrast that suppressed summer precipitation. The results indicated that a realistic irrigation scheme is important for accessing the climatic effects of irrigation. More broadly, including MC in the irrigation scheme may be useful for other assessments of the climatic effects of irrigation.