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The commonly applied surface temperature-vegetation index (T-s-VI) triangle method is used to estimate regional evapotranspiration (ET) in and and semi-arid regions. A practical algorithm based on the T-s-VI triangle method is developed to determine quantitatively the dry and wet edges of this triangle space. First, the T-s-VI triangle method is reviewed. Assumptions involved in this method are highlighted, and advantages, disadvantages and applicability are discussed. Then, an experimental use of the T-s-VI triangle method is developed and applied to several MODIS/TERRA datasets acquired during the Heihe Field Experiment from May 20th to August 21st, 2008. The sensible heat fluxes retrieved using MODIS data from a grassland located in the middle reach of Heihe river basin, Northwest China, are in good agreement with those measured from a Large Aperture Scintillometer (LAS). The Root Mean Square Error of this comparison is 25.07 W/m(2). It is shown that determination of dry and wet edges using the proposed algorithm is accurate enough at least in most cases of our study for the estimates of regional surface ET. (C) 2009 Elsevier Inc. All rights reserved.