## Materials and Methods  
### Study Sites  
This study used the EC-based flux data from a grassland site (Arou) and an agricultural site (Daman) located in the Heihe River basin in the arid and semi-arid areas of Northwest China, which is part of the integrated research network entitled \"Heihe Watershed Allied Telemetry Experimental Research\" (HiWATER) that aims to improve our understanding of hydrological and ecological processes, to establish a world-leading watershed observing system, and to enhance the applicability of multi-source remotely-sensed data in managing scarce water resources in dry regions [22; 24]. The Arou superstation has been set up since the end of 2012 to measure the land-atmosphere exchanges of carbon and water at the Alpine meadow ecosystem (the maximum height of grass is approximately 0.2-0.3 m during the summertime). This site is placed in a valley oriented in an east-west direction with a maximum width of 3 km from south to north in the upstream Heihe River, with mean annual temperature and annual total precipitation of 0.9 \({}^{\circ}\)C and 403.1 mm (1960-2000), respectively [25]. The terrain around Arou site is relatively flat with a gentle decline from the southeast to the northwest, and the soil texture is sand mixed with silt. However, the Daman site is located in the middle reaches of the Heihe River, with the total annual precipitation and average annual temperature of 126.7 mm and 7.2 \({}^{\circ}\)C from 1960 to 2000, respectively [26]. The EC system was installed in a typical oasis with very flat terrain, about 8 km southwest of Zhangye City, and started to work from September 2012.  
heights of the EC sensors were 3.5 m and 4.5 m above the ground for grass and maize ecosystems, respectively. Each EC system is comprised of a three-dimensional sonic anemometer (CSAT3, Campbell Scientific, Logan, UT, USA) and a Li-7500A open path CO\({}_{2}\)/H\({}_{2}\)O gas analyzer (LI-COR Inc., Lincoln, NE, USA). Raw data are continuously recorded at a frequency of 10 Hz on a CR5000 (Campbell Scientific) data logger. Then, the post-processing procedures including spike detection and despiking, two-dimensional coordinate rotation, time delay removal of H\({}_{2}\)O/CO\({}_{2}\), virtual temperature correction, density effects (WPL correction) and frequency response corrections were completed using the improved EdiRe software package (developed by the University of Edinburgh) in order to produce half-hour flux dataset [27; 28]. Currently, these data are released through the data management and sharing platform of the Environmental and Ecological Science Data Center for West China ([[http://westdc.westgis.ac.cn/](http://westdc.westgis.ac.cn/)]([http://westdc.westgis.ac.cn/](http://westdc.westgis.ac.cn/))). However, because of instrument malfunctions, power failure, and various poor weather conditions, about 25% of the one-year observation data are missing. Thus, it is necessary to fill these gaps with a standardized gap-filling algorithm in order to obtain daily-, monthly- or annually-integrated values. The mean diurnal variations (MDV) method proposed by [PERSON] et al. [29], but also considering the temporal auto-correlation and the co-variation of these fluxes with meteorological variables [30], are implemented for gap filling. This study used the daytime-based flux partitioning algorithm [31] to partition net CO\({}_{2}\) exchange (NEE) into the main components: gross primary production (GPP) and ecosystem respiration (R\({}_{\text{e}}\)). This method applied a modified light-response curve based on the [PERSON] model [32] to explain the sensitivity of R\({}_{\text{e}}\) to temperature with the VPD limitation of GPP. The magnitudes of fluxes are calculated from daytime values, with only the parameters that define the temperature sensitivity of R\({}_{\text{e}}\) needing to be inferred from night-time data. Therefore, the estimates of R\({}_{\text{e}}\) based on the night-time measurements and GPP values estimated from the day-time observations are recognized as statistically independent.