Abstract: Soil moisture is the core content and link of the groundwatersoil wateratmospheric water circulation system. Evapotranspiration(ET) is the important driving force of this system. The study of distribution of soil moisture and its effect on ET from regional scale has great significance on ecoenvironmental protection in arid areas. Based on MODIS and GLDAS data, the monthly soil moisture from June to September in 2014 was estimated in plain area in Qaidam Basin after downscale treatment of 0-10 cm GLDAS surface soil moisture data using Apparent Thermal Inertia(ATI) method. The retrieval results were validated by combination of field measurement and NDVI. Furthermore, the mean daily ET was estimated in 9 drainage basins of plain using the Surface Energy Balance System(SEBS), and the relationship between soil moisture and ET was also analyzed in this study. The result indicated that the correlation between ATI soil moisture and 0-10 cm downscaling GLDAS data was good and the determination coefficient R2 is more than 07. The determination coefficient R2 between retrieval result of soil water content and field measurement is 0791, and the R2 between soil water content and NDVI is 0954. Therefore, it is reliable to retrieve soil moisture in Qaidam Basin based on downscale treatment of GLDAS data using ATI method. The daily ET is positively correlated with soil moisture and the determination coefficient R2 is more than 096. The impact of soil moisture on ET is much more than other impact factors.

Key words: soil moisture, evapotranspiration(ET), Apparent Thermal Inertia(ATI), Surface Energy Balance System(SEBS), MODIS, GLDAS, Qaidam Basin