Simulation of climate effect of large-scale irrigation in Haihe River Basin

The Haihe River Basin is one of the most important grain production bases in China, such as winter wheat and summer corn. Natural precipitation cannot fully meet the water demand of crops in this region. Supplementary irrigation has been widely used to meet the growth needs of crops. The annual irrigation water volume has exceeded 20 billion m³, in order to ensure national food security. Large-scale irrigation has posed a significant impact on the climate, thus altering the local ecological environment, crop cultivation and growth in the irrigation area. Therefore, there is a high demand to explore the impact of irrigation on the climate, particularly for a comprehensive understanding of regional climate change, water resource management, and ensuring food security. In this study, a new irrigation scheme was developed in the land surface module of the Weather Research and Forecasting (WRF) model, in order to simulate the climatic effects of large-scale irrigation in the Haihe River Basin. Actual statistical irrigation water volume data was used with the proportion of grid unit cultivated land area to total cultivated land area as the weight to spatially distribute the irrigation water volume. Then, the irrigation water volume was distributed to the month, according to the irrigation area and irrigation system of the main crops. The results indicated that the irrigation caused an increase in the soil moisture in farmland, leading to an increase in the evaporation, and latent heat flux, whereas, there was a decrease in the sensible heat flux. Irrigation led to an average increase of 10.76W/m² in the latent heat flux, and a decrease of 7.19W/m² in the sensible heat flux from April to June. The increase in the latent heat flux was consistent with the decrease in the sensible heat flux in both time and space. The increase in the latent heat flux was greater than the decrease in the sensible heat flux. The larger the irrigation area and month were, the greater the increase in latent heat flux and the decrease in sensible heat flux. Irrigation had a significant local cooling effect. There was a 0.30 ℃ decrease in temperature from April to June, with the largest decrease in May, reaching 0.45 ℃. The decrease in temperature in space was positively correlated with the amount of irrigation water, and the cooling from April to June in plain areas mostly exceeded 0.4 ℃. In addition, the cooling effect of irrigation was more limited to the local area, indicating a relatively small impact on the surrounding areas. There was a certain degree of rain increasing. An average increase of 6.07mm was found in the annual precipitation, which was mainly concentrated from April to June, with an increase of 4.83mm. There was an increase in the precipitation in the Ziya and Daqing River Basin from April to June, with the largest increase in the precipitation in the Daqing River Mountain area, ranging from 15 to 30mm. The dominant impact on convective precipitation, with the relatively small changes in the non-convective precipitation. There was the effect of irrigation on the regional climate, meteorological factors, and supported water security in the basin on both supply and demand sides. Regional precipitation also increased for the available water in the basin. At the same time, the irrigation increased the water vapor content near the surface, whereas, there was a decrease in the temperature, and the reference crop evapotranspiration in the watershed by 38.14mm/a, resulting in a decrease in crop water demand per unit area. There was the increasing precipitation while the less crop water demand after irrigation. Therefore, large-scale irrigation can be expected to alleviate the contradiction between water supply and demand in the Haihe River Basin.