摘要:
滹滏平原光、热及土壤资源优越, 是华北平原重要的粮食生产基地, 灌溉是该区农业获得稳产高产的重要保障, 持续抽取地下水和无节制利用地表水已经引起了严重的水资源危机, 合理高效利用有限水资源进行农业生产势在必行。本文利用单源梯形遥感蒸散发模型(a single-source trapezoid model for evapotranspiration, STME)和中等分辨率成像光谱仪MODIS(2011-2012年共115期)地表温度和反射率产品估算区域地表土壤缺水状况及实际蒸散量, 并利用中国科学院栾城农业生态系统试验站(以下简称"栾城站")和赵县梨园涡度相关系统地表水热通量的观测值对STME模型估算结果进行验证。结果表明该模型可以很好地估算区域蒸散量, 误差在可接受范围内。赵县梨园净辐射Rn的观测平均值为4.10 mm, 估算平均值为4.69 mm, 均方根差RMSD为0.80 mm; 赵县梨园蒸散量观测平均值为2.86 mm, 估算平均值为3.01 mm, 均方根差RMSD为0.95 mm; 栾城站蒸散量的观测平均值为2.67 mm, 估算平均值为2.44 mm, 均方根差RMSD为0.87 mm。将STME模型应用到滹滏平原估算日蒸散量, 明确了区域尺度蒸散发的时空变化特征: 10月份果园生态系统蒸散量多于农田生态系统; 11月份区域蒸散量整体小于1 mm; 第2年春季小麦返青、拔节期, 农田生态系统蒸散量多于果园生态系统蒸散量; 5月份处于植被生长旺盛期, 农田和果园生态系统的蒸散量相差不大; 6月份小麦收获, 玉米播种, 农田生态系统蒸散量少于果园生态系统; 7月份整个区域蒸散量达到最大, 蒸散量不仅与植被长势相关, 而且与土壤湿度相关; 8、9月份随着植被的成熟和收获, 区域蒸散量整体变小。不同时期区域水分亏缺指数不同, 可根据其指导区域灌溉量。STME模型继承了基于数理计算确定梯形顶点的方法和水分亏缺指数, 使得计算过程得以简化且物理机制明确。
Abstract:
The Hufu Plain is a vital grain production base, with sufficient solar radiation, energy, fertile soils as well as intensive agricultural management. In the region, irrigation has ensured stable and high crop yields over the years. The continuous extraction of groundwater and surface water has induced severe water shortage in the Hufu Plain region. Therefore reasonable and efficient use of the limited water resources was necessary for sustainable agricultural production. Here in this study, we developed a remote sensing evapotranspiration (ET) model, STME (a single-source trapezoid model for evapotranspiration), and took 115 MODIS (moderate resolution imaging spectrometer) images (local surface temperature and reflectance) for October 1, 2011 to September 30, 2012 to estimate regional surface water deficit index (WDI) and evapotranspiration in the Hufu Plain. Two typical farmland ecosystems were selected as the investigated objectives, one was cropland ecosystem in Luancheng, the other one was orchard ecosystem in Zhaoxian. We compared the estimated ET by STME with observations by the eddy covariance system. Results suggested that the STME model well estimated daily ET. The average observed net radiation (Rn) was 4.10 mm and the average estimated Rn was 4.69 mm, with a root mean square difference (RMSD) of 0.80 mm for Zhaoxian orchard ecosystem. The average observed daily ET was 2.86 mm and the average estimated daily ET was 3.01 mm, with RMSD = 0.95 mm for Zhaoxian orchard ecosystem. Also the average observed daily ET was 2.67 mm and the average estimated daily ET was 2.44 mm, with RMSD = 0.87 mm for Luancheng cropland ecosystem. To interpret the temporal and spatial variations in regional ET, we used the STME model to estimate regional ET in the Hufu Plain study area. Furthermore, the water deficit index (WDI) reflected the severity of drought in the region. ET for orchard ecosystems in October was more than that for cropland ecosystems. The regional ET was less than 1 mm in November. In April, ET for cropland ecosystems was more than that for orchard ecosystems. Then in May, ET for cropland and orchard ecosystems was somehow similar as there was vigorous vegetation growth. In June, ET for cropland ecosystems was less than that for orchard ecosystems because wheat of cropland was harvested and maize just planted. In July, ET for the whole region was the maximum. This suggested that ET was not only related to vegetation growth, but also associated with soil moisture. In the months of August and September, ET decreased as crops matured and were harvested. WDI varied with time and place and it was practicable in guiding irrigation. STME, a mathematically-based model for calculating the vertex of trapezoid framework and water deficit index, simplified and clarified ET estimation process.
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