## Materials and Methods
### Satellite-Derived MODIS Products and Processing  
The Moderate Resolution Imaging Spectroradiometer (MODIS) GPP and ET products have been widely used to assess the regional or even global carbon sequestration and water consumption by terrestrial ecosystems [33; 34]. Both MOD17A2 GPP (V5.5) and MOD16A2 ET (V1.05) were separately downloaded from the website [[http://www.ntsg.umt.edu/project/](http://www.ntsg.umt.edu/project/)]([http://www.ntsg.umt.edu/project/](http://www.ntsg.umt.edu/project/)) (developed by the Numerical Terradynamic Simulation Group of the University of Montana) in eight-day composite and 1 km resolution from 2013 to 2015. Based on a simplified light-use efficiency (LUE) model, the MODIS GPP algorithm generated the first satellite-driven product to monitor vegetation productivity across Earth's entire vegetated areas [35; 36; 37], and then experienced continuous improvements [38]. It mainly presumed that vegetation GPP was proportionally related with the amount of absorbed photosynthetically-active radiation (APAR) under ideal environmental conditions. The maximum LUE parameter was obtained from a biome-specific look-up table, the fraction of photosynthetically active radiation (FPAR) was from the MOD15A2 product, and the meteorology datasets were consistently derived from the NCEP/NCAR Reanalysis II. Meanwhile, two simple linear scalars are incorporated to account for the consequences of temperature and water stresses, respectively. The MODIS ET algorithm adopted a linear form of the Penman-Monteith equation [39] to estimate ET [40; 41]. It includes the evaporation from wet and moist soils, the precipitation intercepted by the vegetation canopy, and the transpiration through stomata from plant leaves to stems. These improvements encompassed: (1) estimating the soil heat flux as the radiation partitioned from the land surface; (2) calculating the land surface evaporation as the sum of the saturated and moist soil surface; (3) separating the dry canopy from the wet that caused water evaporation due to the intercepted rain water and plant transpiration; (4) including the daytime and nighttime components; and (5) improving the estimation of canopy and aerodynamic resistances, and the vegetation cover fraction. Ecosystem WUE is then calculated by dividing GPP by ET according to the definition [5].  
In addition, the eight-day composite Land Surface Reflectance product (MOD09A1, V5, with the resolution of 500 m) from a 3 \(\times\) 3 pixel area centered on the two flux towers are downloaded from the Oak Ridge National Laboratory's Distributed Active Archive Center ([[http://daac.ornl.gov/MODIS/](http://daac.ornl.gov/MODIS/)]([http://daac.ornl.gov/MODIS/](http://daac.ornl.gov/MODIS/))). The enhanced vegetation index (EVI) developed by [PERSON] et al. [42] has been demonstrated to be effective in monitoring large-scale vegetation phenology and growth status [43; 44], ecosystem productivity [45], and evapotranspiration [46]. The detailed calculation method was described in [PERSON] et al. [47]. The time-series MODIS EVI data are used to analyze the correlation with seasonal dynamics in ecosystem WUE, and to develop a new approach for monitoring grassland and cropland WUE in the dry regions of China.