Spatial and Temporal Variations of Surface Evapotranspiration in the Source Region of Rivers in Southwest China

Source Region of Rivers in Southwest China (SRRSC), which is situated in the eastern Qinghai-Tibet plateau, is the area with the most abundant water resources in China and even in South and Southeast Asia. Surface evapotranspiration (ET) is a key parameter to understand the spatial and temporal variations of the water resources. Due to the extremely complicated environmental characteristics of SRRSC, dense ground measurements of surface ET are impossible. In order to better understand the surface ET and it driving factors, this study conducts a time series analysis of surface ET in the SRRSC. In this study, two surface ET products with good qualities, including the 8-day MODIS surface ET product (i.e. MOD16) with a 1-km resolution developed by Mu et al. (2011) and the monthly ET product with a 5-km developed by Chen et al. (2014) (termed Chen-ET hereafter) from 2001 to 2013 are utilized. The MOD16 ET was monthly composited to generate a new monthly ET dataset. Additionally, MOD16 ET and Chen-ET were yearly composited. Then Empirical Orthogonal Function (EOF) was applied to these four ET datasets to decompose the surface ET into spatial modes and the corresponding temporal coefficients. Results show that the surface ETs derived from MOD16 and Chen-ET are quite similar in both spatial pattern and temporal variation, though ET from the latter product is slightly higher than the former product. At the annual scale, the surface ET in the SRRSC has uneven spatial distribution, namely higher in the southern part (i.e. the middle reaches of the Yarlung Zangbo River - YZR) and the eastern part while lower in the northern part. Different parts of the SRRSC have different inter-annual variations: the western part (i.e. the upper reaches of the YZR) has higher surface ET in 2001-2002, 2004, 2007-2008, 2011, and 2013 than in other years; the middle part has higher surface ET in 2001-2005, 2008, and 2011-2013 than in other years. At the intra-annual scale, the surface ET has evident seasonal variation: the western and southern parts (i.e. the YZR) has higher surface ET in spring and autumn than in summer and winter, while the eastern part has higher ET in spring and summer than in autumn and winter. The spatial and temporal variations of surface ET in the SRRSC are closely related to the climate, precipitation, and land cover.