## 2 Study Area and Data
### Data Description  
The MOD10A1 and MOD09 GA (Surface Reflectance Daily L2G Global 500 m) datasets were used to determine the influence of SZA on snow cover and to calculate the daily SCA in the Tarim River Basin  
Figure 1: Elevation map of the Tarim River Basin and environs showing the locations of the key subwatersheds, sample area, meteorological stations, and coverage of HJ-1A/B satellite images.  
from 1 October 2002, to 30 September 2012 [24; 25]. MOD10A1 data are based on a snow-cover-mapping algorithm that relies mainly on the normalized difference snow index (NDSI). Snow maps have been produced automatically on a daily basis at the National Snow and Ice Data Center Distributed Active Archive Center of the National Aeronautics and Space Administration (NASA) since 1999. This dataset has 500-m spatial resolution and it is gridded to a sinusoidal map projection of 7600 tiles of MOD10A1 data, which are compressed to a Hierarchical Data Format-Earth Observing System format [8]. For this study, tiles h24v4, h25v4, h24v5, and h25v5 that covered the entire Tarim River Basin area were downloaded.  
SZA data and reflectance data obtained from the MOD09 GA dataset were downloaded from NASA's Earth Observing System Data and Information System's ECHO using the Reverb client. The MOD09 GA provides seven bands of 500-m resolution daily surface reflectance data, and 1-km resolution observation and geolocation statistics. The latter low-resolution datasets were used to obtain the SZA degree data, which were represented as 16-bit signed integers to denote the SZA values in each tile [26].  
Meteorological data were downloaded from the National Meteorological Information Center of China website ([[http://data.cma.cn/](http://data.cma.cn/)]([http://data.cma.cn/](http://data.cma.cn/))). Here, we used the dataset titled \"Chinese terrestrial climate data,\" which contains daily meteorological data obtained at ground stations since 1951. Relevant records for our study comprised daily precipitation (including snow depth information), and daily average, maximum, and minimum temperatures. Most meteorological stations in the Tarim River Basin are located in its snow-free interior, although some are peripheral to the snow-covered areas. As such, the small amount of meteorological data collected at the periphery of the basin cannot effectively assist in identifying the degree to which MOD10A1 data overestimate snow cover. Nonetheless, data for precipitation (snow) and temperature recorded by those stations closest to our study area (i.e., Tashkurghan and Pishan) were used as auxiliary data to understand the effects of snowfall within the region.  
The Huanjing (HJ) satellite constellation was launched by the Chinese government on 6 September 2008, to monitor disasters and environmental conditions. The multitemporal, multispectral charge-coupled device (CCD) data and infrared scanner (IRS) data collected by HJ-1A and HJ-1B (two optical satellites with 30-m spatial resolution) were used as references to validate the MOD10A1 snow cover data (green boxes in Figure 1). In theory, the four-day return period of each optical satellite could be used to provide daily, continuous CCD images by integrating two or more paths, which would validate the MOD10A1 snow cover data; however, the 30-m spatial resolution IRS data obtained by HJ-1A/B were also considered sufficient for validation purposes and were used as such herein. HJ-1A/B images acquired on 14-17 May 2011, were downloaded from the China Centre for Resources Satellite Data and Application website ([[http://www.cresda.com](http://www.cresda.com)]([http://www.cresda.com](http://www.cresda.com))), and their sensor types and path/row numbers are listed in Table 1. The snow map method proposed by [PERSON] [7; 8] was applied in our work in order to produce snow cover maps from the HJ-1A/B images. Sixty additional HJ-1A/B images were used to validate the modified product; their acquisition dates, orbit numbers, coordinates, and accuracies are listed in Table S1.