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Estimation of vegetation water content is central to the understanding of water cycle processes. The information of vegetation water content presents the healthy condition of the plant. Various methods were used to extract vegetation water content in semiarid area, however, Spectral indices were still widely used. In this paper, a global sensitivity analysis (GSA) using PROSPECT model was used to understand and quantify vegetation water content effectsed on the signal measured at leaf level. The NIR region was therefore required in combination with SWIR to retrieve equivalent water thickness (EWT). An index EWTsparsecanopy was created to provide an operational method for quantitatively retrieving vegetation water content at satellite scale in a rapid and reliable fashion for sparsely vegetated and area based in tarim river basin,. Compared with EWTcanopy, the former one had a better relationship to normalized difference infrared index(NDII), With the R-2=0.553. Finally, the regression equation(Y=0.05552 +0.53512*NDII) was used to estimate EWTsparsecanopy from the Landsat TM imagery.