运用表观热惯量遥感地表蒸散--以河北省易县崇陵流域为例

Estimating Evapotranspiration through Application of Apparent Thermal Inertia based on Remote Sensing Technology:A Case Study in Chongling Watershed of Yixian County of Hebei Province

现有遥感地表蒸散的方法是基于地表热量平衡方程,先求出地表净辐射通量、土壤热通量和显热通量,再用余项法求出潜热通量。地表净辐射通量和显热通量的计算需空气温度、风速和地表粗糙度等非遥感参数,并且涉及到空间插值问题,由此增加了估算的复杂性并降低了估算的精度。引入反映地物固有热特性的物理参数-表观热惯量,结合地表辐射温度,充分利用遥感较易获取的这两个参数来估算地表蒸散,可弥补上述不足。利用河北省易县崇陵流域卫星过境同步观测的地面资料,以2007年5月28日过境的LANDSAT/TM作为数据源,尝试利用表观热惯量来遥感地表蒸散的新途径。首先根据像元表观热惯量和地表辐射温度等遥感参数反演出像元的大气下行辐射,进而估算出地表净辐射通量;其次,利用表观热惯量估算出波文比,再根据波文比对地表净辐射通量进行切割,然后代入地表热量平衡方程中,直接估算出地表潜热通量;最后,计算出崇陵流域日地表蒸散量,并获取其空间分布图。这种方法避开了估算显热通量的中间环节,可操作性更强。

Existing methods for estimating land surface evapotranspiration use remote sensing data and surface heat-balance equation to estimate surface net radiation fluxes, soil heat flux and surface sensible heat fluxes, and finally to get evapotranspiration. These involve such parameters as air temperature, wind speed, land surface roughness as well as spatial interposition problems, and increase the complexity of operation and reduce the accuracy of estimation results. these disadvantages in estimating evapotranspiration. In Application of thermal inertia method can avoid this paper, the experimental data acquired from Chongling watershedof Yixian County of Hebei province at the time of Landsat/TM passing on May 28, 2007 and the TM images captured are used. First, the downward atmospheric long-wave radiation data were calculated according to the apparent thermal inertia and surface radiometric temperature data were retrieved from the TM images. Secondly, surface net radiation fluxes were estimated. Thirdly, Bowen ratio was estimated by using the apparent thermal inertia and used to divide surface net radiation fluxes. Finally, the balance equation of surface energy was employed to obtain surface heat fluxes. This way reduced the steps for estimating surface sensible heat fluxes and is easily operable. The results show that 1 ） The apparent thermal inertia in Chongling watershed ranges from 0.1 to 0.2 cal·cm^-2 ·s^-1/2 ·℃^ -1 ;2） The Bowen ratio in Chongling watershed ranges from 0.1 to 0.7. It means that in the study area, there is no dry-point that surface latent heat fluxes equal to zero and no wet-point that surface sensible heat fluxes equal to zero. In other words, the surface latent heat fluxes are well matchable with surface sensible heat fluxes;3） The instantaneous surface latent heat fluxes in Chongling watershed ranges from 300 - 582.1 W·m^-2; 4） The daily evapotranspirations in the study area on 28 May 2007 ranges from 3mm to 6. 70 mm. Results illuminate that the estimating land surface evapotranspiration through apparent thermal inertia based on Remote Sensing is feasible and preponderant. The two innovations were conducted in this paper： 1） The application of apparent thermal inertia avoided parameter limitations in terms of air temperature, wind speed, surfce roughness and spatial interposition; 2）The surface radiometric temperature was introduced to estimate evapotranspiration, which avoiding the problems in determinating surface emissivity. These two innovations make the method more universal, propagable and easily operable.