The climatic effects of irrigation over the middle and lower reaches of the Yangtze River, China

Characterizing climatic effects of agricultural irrigation is of great importance and challenging to understand the full influence of water management on the Earth's environment and hydroclimatic cycle. The investigation of the local and remote climatic responses to irrigation in the middle and lower reaches of the Yangtze River (MLRYR), a typical humid region, was previously unidentified. A novel combined subgrid-scale irrigation scheme for wheat (dynamic irrigation) and rice (fixed irrigation) was developed and incorporated into the Weather Research and Forecasting (WRF) modeling framework. The comprehensive climatic effects on energy balance, temperature, precipitation and the planetary boundary layer, were assessed by numerical experiments with irrigation off/on for ten crop growth seasons from 2007 to 2016. Both the simulated irrigation amounts of wheat and rice were generally in agreement with actual values. Surface energy and water budgets have been substantially altered by irrigation, and the magnitudes were dependent on the irrigation amounts. Irrigation increased latent heat flux and reduced sensible heat flux, especially during summer. The warm bias in the simulation without consideration of irrigation was reduced by the cooling effects of irrigation. The mean surface air temperature decreased significantly by 0.12-0.24 degrees C and 0.4-0.8 degrees C for spring and summer, respectively. Meanwhile, irrigation presented stronger cooling effects on daytime temperature than nighttime temperature, resulting in the reductions of diurnal temperature range in irrigated area. Moreover, the irrigation-induced increase of geopotential height and anomalous anticyclones led to the decreases in precipitation over the MLRYR. The changes of precipitation in remote areas were generally related to the irrigation induced large-scale circulation change. Furthermore, although the climatic effects of irrigation in grid scale were not relatively high due to the relatively low irrigation fraction, they exhibited apparently stronger in the subgrid scale, which can be used to investigate the impacts of climatic effects of irrigation on crop growth.