## 2 Study Area and Datasets
### Datasets
#### 2.2.1 Remote Sensing Data  
Three kinds of remote sensing data were used in this study, specifically high spatial resolution data from the Chinese HJ-1B satellite, low spatial resolution MODIS data and the ASTER Global  
Figure 1: Location and maps of the study area. (**a**) Location of the study area in China; (**b**) False color composite of an HJ-1B image of the Zhangye oasis acquired on 22 August 2012 (R/G/B = 4/3/2); (**c**) Locations of ground sites (5.5 km \(\times\) 5.5 km matrix). Digital Elevation Map (GDEM), which was used as auxiliary data. The HJ-1 A/B satellites, which were launched in September 2008, carry a total of four CCD cameras (i.e., CCD1 and CCD2 for HJ-1 A and similar for HJ-1 B). These satellites have quasi-sun-synchronous orbits, an altitude of 650 km, and a swath width of 700 km [30]. Apart from the first band of the HJ-1 CCDs (B1 = 475 nm), which is \(\approx\)10 nm wider than that of Landsat TM/ETM+ (B1 = 485 nm), the other three bands are identical (B2 = 560 nm, B3 = 660 nm and B4 = 830 nm). The spatial resolution of the four CCD sensors matches the first four bands of Landsat TM/ETM+ (see Table 1). The side rotation (\(\pm 30^{\circ}\)) feature of the HJ-1 A/B CCDs confers an advantage over Landsat, in that the revisit time is shortened from 16 days to 48 h or less [31; 32]. The images produced by the HJ-1 A/B CCDs are stable, and the performances of each band are balanced [33]. After correction (i.e., geometric correction, radiometric calibration and atmospheric correction), the quality of the images is very similar to that of Landsat-5 TM [34]. The potential for high-quality ET or LE monitoring using the fine spatial resolution of the HJ-1 CCD sensors has already been demonstrated [24; 35]. The data from the HJ-1 CCD sensors are freely available from the China Center for Resources Satellite Data and Application (CRESDA) website ([[http://218.247.138.121/DSSPlatform/index.html](http://218.247.138.121/DSSPlatform/index.html)]([http://218.247.138.121/DSSPlatform/index.html](http://218.247.138.121/DSSPlatform/index.html))). We used the HJ-1B image data that cover the HRB region in 2012. Because many algorithms for estimating evapotranspiration using surface parameters require clear-sky conditions, we combined available information on data quality with visual interpretation to select satellite images without clouds. The selected images made up eleven groups, where a group consists of the matching IRS and CCD data covering the study area. The data making up these groups were collected on 19 June, 30 June, 8 July, 27 July, 3 August, 15 August, 22 August, 29 August, 2 September, 13 September, and 14 September. The HJ-1B satellite data covering the HRB were pre-processed using methods including geometric correction, radiometric calibration, and atmospheric correction [24]. To estimate the regional LE, various surface variables, including the downward shortwave radiation [36], the downward longwave radiation [24], the surface albedo [37; 38], the land surface temperature [24], the emissivity [39], the leaf area index (LAI) and the fractional vegetation coverage (FVC), were retrieved by applying the lumped method to the HJ-1B data (CCD/IRS) [24]. The MODIS sensor was launched on 18 December 1999 aboard the Terra satellite and has been in operation for nearly 18 years. It provides global data that are used to monitor long-term regional land surface heat and water vapor fluxes and changes in the Earth system [40; 41; 42]. The MODIS data have high temporal resolution (i.e., at least once every day) and high spectral resolution (i.e., 36 bands in total). The parameters used in the estimation of heat fluxes, such as NDVI, albedo, LSTs and emissivity, can be freely downloaded ([[https://ladsweb.nascom.nasa.gov/data/search.html](https://ladsweb.nascom.nasa.gov/data/search.html)]([https://ladsweb.nascom.nasa.gov/data/search.html](https://ladsweb.nascom.nasa.gov/data/search.html))). The