## 4 Results and Discussion
### Spatial Characteristics of ET, GPP, and WUE for Midstream Oasis Agricultural Areas
This synchronous linear decrease may be because the vegetation cover in the oasis area inhibits soil evaporation to an extent, even when the vegetation declines. In the case of insufficient agricultural water resources, more attention should be paid to the temporal variation of water use during the growing season in order to achieve a transformation from traditional high-yield irrigation to water-saving and high-quality irrigation [100]. However, in-depth investigation and research work are still required to determine the feasibility of this approach. Regional yearly ET, GPP, and WUE, as derived from remote sensing methods, can illustrate their spatial variations in the oasis (Figure 9a-c). Annual ET, GPP, and WUE varied from 300 to 700 mm (accounting for 90%), 200 to 650 gC/m\({}^{2}\) (accounting for 90%), and 0.5 to 1.0 gC/kgH\({}_{2}\)O (accounting for 95%), respectively, over the entire irrigation agriculture district under different cropping patterns and water resource conditions. The highest ET was observed for reservoirs and ponds and the watercourses of the Heihe River within the oasis, while GPP was the lowest. Due to the existence of roads and residential areas, and shortages in vegetation cover and water supplies inside the oasis, annual ET and GPP were notably lower. Thus, higher ET and GPP were obtained in the cropping oasis areas. Annual ET, GPP, and WUE in 2011 and 2014 were significantly smaller than the multi-year averages of 580 mm, 500 gC/m\({}^{2}\), and 0.86 gC/kgH\({}_{2}\)O, respectively. The characteristics of their spatial distributions did not change significantly over the seven-year study period (2010-2016), except for the eastern part of the oasis (results of that mapping not shown here). Land cover changes greatly affected ET and GPP patterns, and much higher ET and GPP values were observed at fine spatial resolution under the eastern part of the oasis, where they reached their highest levels. This fine-grained estimation of increasing annual variation indicates the expansion of the oasis agriculture area and/or human disturbances. There was a linear relationship between annual oasis cropland ET and GPP. When ET was greater than 400 mm, it was slightly greater than the GPP value (Figure 9d). However, annual WUE presented a nonlinear relationship with ET and GPP (Figure 9e,f). Whenannual ET and GPP were greater than 550 mm and 500 gC/m\({}^{2}\), annual oasis cropland WUE had strong invariability and was maintained at approximately 0.85 gC/kgH\({}_{2}\)O. This is an important finding and indicates the factors that describe cropland WUE at the annual scale over the irrigation oasis agricultural area of an arid region. When WUE was high (\(\geq\)0.85 gC/kgH\({}_{2}\)O), annual oasis cropland ET was maintained at 500 mm, which means the use of irrigation water should be reduced as much as possible. Generally, the variations and constraint relationships between annual ET, GPP, and WUE in the oasis of an arid area are attributed to changes in the water supply, crop planting patterns, surface cover type, and anthropogenic fertilizer application. The strong correlation between GPP and ET demonstrates that the carbon and water cycles are a tightly coupled system [101; 102]. This is particularly true of our findings in arid oasis croplands (Figures 8 and 9). The direct determinants of the cropland WUE are the GPP and ET. Specifically, the cropland WUE varied depending on the irrigation amount, period, variety, and methods [103; 104]. In general, some varieties more easily produce high yield and high WUE under lower irrigation conditions [105; 106]. For example, studies have shown that appropriate deficit irrigation will not significantly affect wheat yield but will significantly reduce irrigation water and crop water consumption, which improves the irrigation efficiency and WUE for wheat [107; 108], whereas exceeding the suitable irrigation water consumption reduces the WUE [109; 110].