Numerical simulation of the impacts of groundwater irrigation over the North China Plain on regional climate

In this study, a dynamic irrigation scheme that considers the groundwater processes is developed and incorporated into the Noah land surface model with multi-parameterization options (Noah-MP), which is coupled with the Weather Research and Forecasting (WRF) model. Two experiments (with and without considering irrigation) are conducted to investigate the impacts of groundwater irrigation over the North China Plain on regional climate. The model is run continuously from 1 January 2001 to 1 January 2011. Results show that groundwater irrigation affects the groundwater table, which declines in the North China Plain, particularly during the dry season when the cropland needs large amounts of irrigation water. In the irrigated area, the increase in soil moisture induced by irrigation affects the partitioning of surface energy (latent heat flux increases and sensible heat flux decreases), which leads to decrease in 2 m air temperature (T_2m) by 0.6-1.0℃. Therefore, the summer warm bias in T_2m simulation is reduced after considering the effects of irrigation. Irrigation causes wetting and cooling in the planetary boundary layer (PBL) above the irrigated area, and these influences are confined to PBL in spring but can reach up to above 5 km in summer. The influence of irrigation on precipitation is much stronger in summer than in spring. The wetting and cooling effects of irrigation have positive and negative feedbacks on summer precipitation, respectively, which jointly affect the precipitation in the irrigated area. The influence of irrigation on water vapor transport also results in precipitation changes in non-irrigated area. The increases in summer precipitation over the middle and lower reaches of Yangtze River Basin may be relevant to the southward shift of the westerly jet core induced by irrigation.