## 4 Results and Discussion
### Spatial Characteristics of ET, GPP, and WUE for Midstream Oasis Agricultural Areas  
Spatial patterns at the fine-grain scale for monthly and yearly ET, GPP, and WUE over the midstream oasis of the HRB during the growing season (April to October) are variable and complex and based on different net irrigation water distributions, cropping patterns, and seasonal weather conditions. Remote sensing and spatial-statistical mapping give the spatial distributions of monthly and yearly ET, GPP, and WUE from the modeled daily values through the entire midstream oasis area from April to October, which are shown to reveal temporal changes at the regional scale. The data from 2013 were selected as an example to map the spatial characteristic information (Figures 8 and 9). Figure 7: Comparison of multi-year (**a**) ET, (**b**) GPP, and (**c**) WUE for 12 irrigation districts over seven-year study period (2010–2016) for midstream oasis. Figure 8: Spatial time series for monthly ET (row 1), GFP (row 2), and WUE (row 3), and bivariate histograms (count) between ET and GFP (row 4) over nakstream coasis agricultural areas. Black plus sign indicates location of DMS. Data from 2013 were selected as an example of detailed spatial distribution information. Figure 9: Spatial distribution of annual (a) ET, (b) GPF, and (c) WUE, and bivariate histograms (count) between (d) ET and GPF, (e) ET and WUE, and (f) GPF and WUE over midstream costs agricultural areas. Black plus sign indicates location of DMS. Data from 2013 were selected as an example of detailed spatial distribution information. The monthly ET and GPP maps using remote sensing models (Figure 8, rows 1 and 2) show that the spatial distribution pattern of ET and GPP changed from April to October in 2013. While the monthly ET and GPP had minor responses in the early growth season (April and May), there were slightly larger changes for ET in the riverway and GPP in the wheat-growing areas of the oasis, for example, around the southern part of Zhangye City. In June, monthly ET and GPP values increased significantly for croplands in the midstream oasis. These larger values are associated with maize seeding in late May, as the vegetation begins to germinate and turn green later in the spring, which is followed by irrigation and solar radiation enhancement. In July and August, crop water consumption (i.e., ET) increases rapidly as the vegetation growth rate and biomass (i.e., GPP) accumulation rate is the highest in these two months. Late in the growth season, the oasis crops mature, and frost begins to occur. Thus, there is a significant decrease in ET and GPP for September. In October, the solar radiation decreases significantly, and ET and GPP are further reduced. As shown in Figure 8, row 3, the spatial characteristics of monthly WUE are significantly different from ET and GPP from April to October. In April and May, the monthly WUE throughout the entire oasis area was generally low. In June and October, WUE was generally around 0.80 gC/kgH\({}_{2}\)O, while in July to September, during the vigorous growth of oasis crops, monthly WUE remained at around 1.25 gC/kgH\({}_{2}\)O. This phenomenon can be seen more intuitively from the bivariate histograms between monthly ET and GPP. This behavior can be explained to some extent. In the spring in arid areas (early growing season), enhanced radiation and irrigation activities lead to rapidly increasing water consumption (ET) of the oasis, and there is more soil evaporation at this time. As oasis crops are in the sowing and seedling stage at this time, there is less transpiration and a slow biomass increase (Figure 8, row 4, April and May). From July to October, ET and GPP were relatively small, and WUE was nearly stable at higher values ranging from 1.00 to 1.35 gC/kgH\({}_{2}\)O. This is because the synchronization of monthly ET and GPP decreased linearly during this period (Figure 8, row 4, June to October).