Simulating the Impacts of Irrigation and Dynamic Vegetation Over the North China Plain on Regional Climate

A dynamic irrigation scheme that considers groundwater processes and dynamic vegetation was constructed and incorporated into Noah land surface model with multiparameterization options coupled with the weather research and forecasting model. Multiple experiments (with and without irrigation) were designed to simulate the impacts of irrigation over the North China Plain (NCP) on regional climate. The simulated irrigation rate of the NCP was close to observations, and its interannual variations were clearly associated with dry or wet years. The groundwater table affected by irrigation varied in the NCP irrigated areas during simulation period but declined overall. The irrigation-induced increases in green vegetation fraction led to increases in transpiration, indicating that the influences of irrigation on latent heat flux should contain the changes of transpiration besides changes of soil evaporation. The modified surface energy budget caused by irrigation affected surface air temperature (SAT), and the spring and summer SAT in the areas with larger irrigation rate decreased significantly by 0.8-1.6 and 1.2-2.6 degrees C, respectively. The experiment that considered irrigation showed obvious improvement in simulating several key variables (gross primary productivity, green vegetation fraction, sensible heat flux, and summer SAT). In the NCP, irrigation increased water vapor and caused cooling in boundary layer, which had opposing influences on precipitation, resulting in heterogeneous changes in summer precipitation. The changes of precipitation in nonirrigated areas were related to the influences of irrigation on wind fields. The contribution of direct effects of irrigation to blended influences of irrigation and dynamic vegetation on spring (summer) SAT was 70.9% (63.8%). Key Points