Evapotranspiration Estimation for an Oasis Area in the Heihe River Basin Using Landsat-8 Images and the METRIC Model

The Heihe River Basin is the second largest inland river basin in arid northwestern China. Accurate estimation of regional evapotranspiration (ET) and its spatial distribution in the oasis area plays an important role for water resource management in the whole basin. The objective of this study was to estimate the temporal and spatial variations of ET for different land cover types in the oasis. To estimate ET values in the growing season of 2013, 14 Landsat-8 images of the study area were processed using the Mapping Evapotranspiration at High Resolution with Internalized Calibration (METRIC) model. A new approach selected hot extreme pixels separately for the desert and oasis sub-areas due to large differences in soil thermal characteristics. Results indicated large temporal-spatial variations in ET in the oasis. The largest ET values occurred over water bodies, followed by arable land, low-lying land and forest. The desert-oasis transition zones (shrubland, sparse forest, medium- and low-coverage grassland) had lower ET values, while the lowest ET values occurred in unused lands (salt meadows, sandy and Gobi deserts). Monthly ET values of all land cover types increased from April, peaked in July, before decreasing until October. Overall, METRIC can provide reasonable ET estimates in a heterogeneous land use types under advective environmental conditions with the selection of appropriate extreme pixels.