华北平原地下水灌溉对区域气候影响的数值模拟

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

在耦合模式WRF/Noah-MP中加入考虑地下水过程的动态灌溉方案,设计两组试验(分别为考虑和不考虑地下水灌溉),连续模拟10 a(2001—2010年),来研究华北平原地下水灌溉的区域气候效应。结果表明,地下水灌溉导致华北平原地下水位下降,在少雨的季节灌溉量大,水位下降较快。在灌溉期(3—9月),灌溉引起的土壤湿度升高影响了地表能量的分配(潜热增加,感热减少),导致2 m气温显著降低0.6—1.0℃,同时也降低了灌溉区夏季模拟偏高的气温。灌溉对灌溉区边界层大气有升高湿度和冷却降温的作用,对春季的影响局限在边界层内,而夏季的影响持续到5000 m以上。夏季灌溉对降水的影响远强于春季,灌溉的升高湿度和冷却效应分别对夏季降水产生正反馈和负反馈,共同影响灌溉区的降水变化。灌溉通过对水汽输送的影响,引起非灌溉区降水的变化,而长江中下游流域夏季降水的增多可能与高空西风急流轴位置南移有关。

In this study, a dynamic irrigation scheme that considers the groundwater processes is developed and incorporated in to 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 im pacts 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 sensi- ble heat flux decreases）, which leads to decrease in 2 m air temperature （T2m） by 0.6 - 1.0℃. Therefore, the summer warm bias in T2m 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 nonirrigated 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.