| 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-desert area plays an important role for water resource management and allocation 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-desert area. To estimate ET values in arid area, the Mapping Evapotranspiration at High Resolution with Internalized Calibration(METRIC) model were processed using Landsat-5 TM and Landsat-8 OLI/TIRS images. The eddy covariance measured ET and Hydrus-1D simulated ET were used to evaluate the performances of remote sensing based ET estimation models. The main results are outlined as follow:(1) A new approach selecting hot extreme pixels separately for the desert and oasis sub-areas due to large differences in soil thermal characteristics. The estimated H values increased with the decreases in the Ts of the “hot” pixels, i.e., the H increased by about 10-30 W/m2 with a 2 K decrease in Ts. Besides, pixels of different land-cover-type responeded differently to various “hot” pixel selections. Among the different land cover types, the water body had the smallest differences in the H estimations, followed by arable land, forest, and meadow, while salt meadow, and Gobi and sandy deserts exhibited relatively large differences.(2) Large temporal-spatial variations in ET in the oasis-desert area were presented. 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.(3) This paper evaluated the performance of three contextual remote sensing based models for ET estimation(METRIC; the Ts-VI triangle model; and SSEB) in an oasis-desert region during a growing season under advective environmental conditions. Comparisons among model outputs were then conducted on a pixel-by-pixel basis for three main land-cover types(farmland, transition zone and desert). Overall, METRIC outperformed both the Ts-VI triangle and SSEB models; the Ts-VI triangle model tended to overestimate and the SSEB to underestimate at higher values of λET. ET estimations by SSEB and the Ts-VI triangle model are more sensitive to estimated surface temperature and available energy than those from METRIC. Two daily ET extrapolation methods were evaluated with the EC measured daily ET. The results suggest that the constant reference ET fraction(ETrF) method be used over well-watered areas due to the regional advection effect; the constant evaporative fraction(EF) method tends to give better outputs for other areas. Pixel-wise comparisons showed the greatest consistency between the Ts-VI triangle model and METRIC outputs in farmland with an R2 of 0.98 and an RMSE of 13.69 W/m2.(4) Seasonal ET variations were compared at field scale between METRIC outputs and Hydrus-1D simulated ET from 63 soil moisture sampling points. Good correlations were obtained at different time scale(daily, monthly and seasonally) for farmland. Obviously mistakes were made when estimating daily ET in desert area during the period between two satellite overpass dates. But the mistakes can be cancelled out to some extent when summed to monthly and seasonal ET. Besides, Hydrus-1D could provide accurate ET at field scale, however, regional ET map interpolated from Hydrus-1D simulated field ET values using Kriging interpolation method could not depict the spatial variations of ET in arid oasis-desert area, effectively. |
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