## 4 Discussion  
Natural grassland and irrigated cropland are the major ecosystem types in the arid and semi-arid areas of Northwest China. Currently, only a few flux sites in this region continuously measure ecosystem C and water exchanges between the biosphere and the atmosphere with limited site-years of data. However, it provided a direct and effective tool to monitor the flow of carbon, water and energy quantitatively across different time scales. Comparing with the results of [PERSON] et al. (2012) that WUE at the Dangxiong alpine meadow-steppe ecosystem with short, sparse vegetation and the Neimeng temperate steppe ecosystem under semi-arid climate, were 0.41 g kg\({}^{-1}\) and 0.80 g kg\({}^{-1}\), respectively, the values in this study (Arou site) just fell in between (Figure 2). [PERSON] et al. (2015) revealed the latitudinal trends in multi-year mean annual WUE of six cropland sites ranging from 1.05 g kg\({}^{-1}\) and 2.36 g kg\({}^{-1}\), it reflected that ecosystem WUE in the dry areas of China (Daman site) was apparently lower than that of other regions. Previous studies ([PERSON] et al., 2017) also implied that under normal precipitation, C\({}_{4}\) plant (maize) usually owned higher ecosystem WUE than C\({}_{3}\) plant (soybean) because of different physiological pathways, at about 1.46 g kg\({}^{-1}\) and 0.68 g kg\({}^{-1}\), respectively. Precipitation events, in terms of timing and amounts, generally affect the fluxes of CO\({}_{2}\) and water vapor that consequently impact ecosystem WUE variability in different ways. [PERSON] et al. (2012) found that precipitation addition in a temperate steppe stimulated GPP and ET by 70.4% and 37.5% in dry 2007, and by 24.8% and 12.6% in normal 2008, respectively, leading to the increase in ecosystem WUE. Although few studies examined the response of grassland WUE to natural precipitation, positive correlations between rain-use efficiency and annual total rainfall over time have been reported in temperate grasslands in China ([PERSON] et al., 2017). There was also a clear trend for less WUE during the peak growing season (Figure 2), which can be explained by the poor soil moisture and high temperature in this period ([PERSON] et al., 2017; [PERSON] et al., 2017). It predominantly reflected a sharp decrease in GPP, but no significant influence on ET, which resulted in the reduction of ecosystem WUE under a warm temperature. Because of stomatal  
Figure 5: Seasonal patterns and the scatter plots of ecosystem water use efficiency between flux tower measurements (WUE\({}_{\mathrm{EC}}\)) and the MODIS estimates (WUE\({}_{\mathrm{GR}}\)) from time-series EVI data at the two flux sites in 2015. (**a,b**) represent the Arou grassland site and the Daman cropland site, respectively. The black dashed line and red line represent 1:1 line and linear fit, respectively. regulation, lower GPP rates can generate a water-saving adaptive mechanism to a drier environment induced by warming climate [49]. In spite of a positive correlation between VPD and ecosystem WUE found in this study (Table 2), WUE declined obviously with the increase of VPD in many terrestrial ecosystems under well-watered conditions [5; 50]. Global climate change has exerted considerable effects on the biogeochemical and hydrological processes in terrestrial ecosystems [51]. Especially in the arid and semi-arid regions of Northwest China, our understanding of climate change-terrestrial feedbacks must be improved to manage the fragile ecosystems in the face of extreme weather events. Meanwhile, increased frequency and intensity of drought events are among the prospects that we are facing. How ecosystem WUE and flux dynamics respond to and cope with droughts will be crucial in the terrestrial feedback to changing climate. Ecosystem responses to drought, however, are highly variable in time and space [52; 53]. [PERSON] and [PERSON] [54] revealed that ecosystem WUE increased when the intensity of drought was moderate while it tended to decrease under severe drought. [PERSON] et al.