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The trapezoidal relationship between land surface temperature (<i>T</i><sub>s</sub>) and Vegetation Index (VI) was used to estimate soil moisture in the present study. An iterative algorithm is proposed to estimate the vertices of the <i>T</i><sub>s</sub> ~ VI trapezoid theoretically for each pixel, and then Water Deficit Index (WDI) is calculated based on the <i>T</i><sub>s</sub> ~ VI trapezoid using MODIS remotely sensed measurements of surface temperature and enhanced vegetation index (EVI). The capability of using WDI based on <i>T</i><sub>s</sub> ~ VI trapezoid to estimate soil moisture is evaluated using soil moisture observations and antecedent precipitation in the Walnut Gulch Experimental Watershed (WGEW) in Arizona, USA. The result shows that, the <i>T</i><sub>s</sub> ~ VI trapezoid based WDI can capture temporal variation in surface soil moisture well, but the capability of detecting spatial variation is poor for such a semi-arid region as WGEW.