A numerical study for climatic effects of irrigation in the Fergana basin，Central Asia
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摘要: 中亚地处干旱气候区，农业生产高度依赖灌溉，然而灌溉对当地气候影响的认识还较为薄弱。为此，针对多雨（2009年）、少雨（2008年）及正常（2007年）年景下中亚典型农业区—费尔干纳盆地暖季（5—9月）的气候，利用嵌入灌溉过程参数化方案并更新土壤参数的WRF模式，分别进行了考虑灌溉过程（称为IRRG试验）与不考虑灌溉过程（称为NATU试验）的模拟试验，并通过对比IRRG与NATU试验之差揭示了灌溉对区域气候的影响。研究发现：（1）灌溉致使暖季地面潜热增加（79.2 W/m2）、感热减少（−61.3 W/m2），日均温降低1.7℃，空气比湿增加2 g/kg（约为NATU的36%），因5—6月为雨季，7—8月为旱季，故7—8月的灌溉量大，冷湿效应略强于5—6月；（2）冷湿效应主要出现在灌溉区域，降温达2℃，增湿达2.4 g/kg，灌区外甚微，同时从地面到高层大气，冷湿效应越来越弱，在约500 hPa（距地面约4000 m）以上冷湿效应消失；（3）在盆地中央平原地区，因灌溉而致空气湿度增加产生的潜在增雨效应与地面冷却产生的对流抑制作用相互抵消，灌溉与无灌溉情景下当地降水无显著差异；灌溉可导致盆地南、北两侧山区降水增加（约0.6 mm/d）；（4）不同年景之间灌溉量差异主要出现在5—6月，少雨年比多雨年灌溉量偏多20 mm/月，日均温降幅偏大0.3℃，空气比湿增幅偏大0.5 g/kg，但山区降水增幅偏小0.6 mm/d。Abstract: Central Asia is located in the arid climate zone and agricultural production is highly dependent on irrigation. However, the effects of irrigation on local climate remain unclear. The effects of irrigation on warm season (May-September) climate in the Fergana basin are studied using the Weather Research and Forecast model (WRF), in which an irrigation parameterization scheme is implemented and soil data are also updated. A pair of simulations, i.e., the IRRG that includes irrigation and the NATU that excludes irrigation, are carried out for the years of above normal rainfall (i.e. 2009), below normal rainfall (i.e. 2008) and normal rainfall year (i.e. 2007), respectively. The climatic effects of irrigation are then studied by comparing the IRRG and NATU simulations. The results are as follows. (1) Irrigation leads to an average increase in surface latent heat flux (79.2 W/m2) and an average decrease in surface sensible heat flux (−61.3 W/m2). As a result, daily mean temperature cools by 1.7℃ and specific humidity increases by 2 g/kg (accounting for about 36% of NATU) in the warm season. Since May to June is rainy season and July to August is dry season, the irrigation demand from July to August is larger than that from May to June. Thereby, the cooling and wetting effects of irrigation are also larger from July to August. (2) The cooling and wetting effects mainly exist in the irrigation area with cooling of 2℃ and specific humidity increment of 2.4 g/kg, while they are too weak to be detected beyond the irrigation area. Meanwhile, from the ground to the upper atmosphere, the cooling and wetting effects gradually weaken and cannot be detected above the pressure level of 500 hPa (about 4000 m above the ground). (3) In the central plain of the basin, the potential rainy effect caused by air humidity increment and the potential rainless effect caused by convective inhibition offset each other. As a result, local precipitation shows no detectable differences between the IRRG and NATU simulations. However, irrigation lead to precipitation increment (~0.6 mm/d) in remote areas over the northern and southern mountains surrounding the basin. (4) Differences in irrigation demand and their effects on local climate under different climate backgrounds mainly exist in May to June. The irrigation demand in 2008 is more than that in 2009 by 20 mm per month. Consequently, the cooling effect is stronger by 0.3℃ and the wetting effect is stronger by 0.5 g/kg in 2008 than those in 2009. Meanwhile, the rainy effect is weaker in 2008 than that in 2009 by 0.6 mm/d.
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Key words:
- Fergana basin /
- Farmland irrigation /
- Climate effect
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图 1 模拟区域范围与地形 （色阶，海拔高度）、研究区地理位置 （红色多边形） 及气象站分布 （红色空心圈，蓝色数字表示NOAA气候数据集中气象站的编号）
Figure 1. Model domain and topography （shaded，altitude），geographical location of the Fergana Basin （red polygon） and meteorology stations （red circles，blue numbers denote station IDs in the NOAA climate dataset）
图 2 研究区IRRG试验550个农田网格 （a） 平均的逐月灌溉量与 （b） 多雨、少雨、正常3类年景平均的生长季 （5—9月） 灌溉量空间分布
Figure 2. Average monthly irrigation amount of IRRG simulation over 550 cropland grids in the study area （a） and spatial distribution of averaged monthly irrigation amount in the growth season （May to September） for years of above normal rainfall，below normal rainfall and normal rainfall （b）
图 3 2007年5—9月灌区地表辐射平衡与能量收支 （a. 地表接收短波辐射，b. 地表反射短波辐射，c. 地表净吸收短波辐射，d. 地表净吸收长波辐射，e. 地表净辐射，f. 感热通量，g. 潜热通量，h. 土壤热通量）
Figure 3. Surface radiation balance and energy budget over irrigation area from May to September in 2007 （a. shortwave radiation reaching the surface，b. shortwave radiation reflected by the surface，c.net shortwave radiation absorbed by the surface，d.net longwave radiation absorbed by the surface，e. net surface radiation，f. sensible heat flux，g. latent heat flux，h. ground heat flux）
图 4 IRRG与NATU模拟的2007年5—9月感热通量 （a） 与潜热通量 （b） 之差 （黑点表示置信水平为95%）
Figure 4. Differences between IRRG and NATU simulations in sensible heat flux （a） and latent heat flux （b） from May to September in 2007 （the black dots denote the 95% confidence level）
图 5 IRRG与NATU模拟的3类年景平均5—9月 （a、b） 及5—6月 （c、d） 比湿 （a、c） 与地表气温 （b、d ） 之差 （黑点表示置信水平为95%）
Figure 5. Differences between IRRG and NATU simulations in specific humidity （a，c） and surface air temperature （b，d） from May to September （a，b） and from May to June （c，d） for the three years of different rainfall anomalies （black dots denote the 95% confidence level）
图 6 IRRG与NATU模拟的少雨年 （a、b） 及多雨年 （c、d） 5—6月比湿 （a、c） 与地表气温 （b、d） 之差 （黑点表示置信水平为95%）
Figure 6. Same with Fig. 5 but for May to June in the years of below normal rainfall （a，b） and above normal rainfall （c，d）（the black dots denote the 95% confidence level）
图 7 正常 （a1、b1）、少雨 （a2、b2）、多雨 （a3、b3） 年景下气象站 （a1—a3. 38611站，b1—b3. 38618站） 观测 （OBS） 与模拟的地表气温
Figure 7. Observed and simulated surface air temperature at two meteorological stations （a1—a3. Site ID 38611，b1—b3. Site ID 38618） for the normal rainfall year （a1，b1），below normal rainfall year （a2，b2） and above normal rainfall year （a3，b3）
图 8 IRRG与NATU模拟的3类年景平均5—9月 （a、b） 及5—6月 （c、d） 比湿 （a、c） 与大气温度 （b、d） 之差的垂直剖面 （黑点表示置信水平为95%）
Figure 8. Mean vertical profiles of differences between IRRG and NATU simulations in specific humidity （a，c） and air temperature （b，d） from May to September （a，b） and from May to June （c，d） for the three years of different rainfall anomalies （the black dots denote the 95% confidence level）
图 9 少雨年 （a、b） 及多雨年 （c、d） IRRG与NATU模拟的5—6月比湿 （a、c） 与大气温度 （b、d） 之差的垂直剖面 （黑点表示置信水平为95%）
Figure 9. Same as Fig. 8 but for May to June in below normal rainfall year （a，b） and above normal rainfall year （c，d）（the black dots denote the 95% confidence level）
图 10 NATU模拟的3类年景平均5—9月 （a、b） 与5—6月 （c、d） 降水量 （a、c） 及IRRG与NATU模拟的降水量之差 （b、d）（黑点表示置信水平为95%，红框表示降水增加典型区域）
Figure 10. Mean NATU simulated precipitation （a，c） and differences between IRRG and NATU simulations in precipitation （b，d） from May to September （a，b） and from May to June （c，d） in the three years of different rainfall anomalies （the black dots denote the 95% confidence level，red polygons denote typical areas where precipitation increases）
图 11 NATU模拟的少雨年 （a、b） 与多雨年 （c、d） 5—6月降水量 （a、c） 及IRRG与NATU模拟的降水量之差 （b、d）（黑点表示置信水平为95%，红框表示降水增加典型区域）
Figure 11. Same as Fig. 10 but for precipitation from May to June under below normal rainfall year （a，b）and above normal rainfall year （c，d）（the black dots denote the 95% confidence level，red polygons denote typical areas where precipitation increases）
图 12 NATU模拟的少雨年 （a、b） 与多雨年 （c、d） 5—6月水汽通量 （a、c） 及IRRG与NATU模拟的整层水汽通量之差 （b、d）（红框表示降水增加典型区域，箭头表示水汽输送方向）
Figure 12. Mean NATU simulated column-integrated water vapor flux （left panel） and the differences between IRRG and NATU （right panel） for May to June under below normal rainfall years （top panel） and under above normal rainfall years （bottom panel）（red polygons indicate typical areas where precipitation increases，arrows denote direction of water vapor transportation）
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