## 4 Results and Discussion
### Assessment of Daily ET, GPP, and WUE at Footprint Scale
In addition, several factors reduce the prediction quality of surface parameters with ESTARFM, such as temporal variance, cloud coverage, spatial heterogeneity, and view angle effects. Figure 3: Scatterplots of observed (**a**) evapotranspiration (ET), (**b**) gross primary productivity (GPP), and (**c**) water-use efficiency (WUE), and associated estimates obtained from remote sensing model over cropland (maize) cover types at Daman superstation (DMS). Comparing the observed ET or GPP with the estimated WUE based on a dataset from one site could bring uncertainties as a result of differences in the observation orientation and environmental conditions at a given in situ station [95]. In addition to the inherent uncertainties in the validation data of EC and GPP, the mismatched spatial locations between estimated ET, GPP, and EC measurements will also affect the accuracy [88,89]. Figure 4 shows the time series data of estimated and observed daily ET, GPP, and WUE and the response of temporal variations in daily temperature (Ta), daily precipitation, daily incident shortwave radiation (SWd), daily albedo, and NDVI (16 days) under cropland cover conditions in the midstream oasis area of the HRB during the multi-year study period (2010-2016). The daily temporal variations in estimated and observed values showed similar patterns in midstream oasis agricultural areas over the study period, especially during the growth seasons. Of the three, estimated and observed WUE performed the worst. The temporal variations in daily ET, GPP, and WUE indicate some key factors when determining ET, GPP, and WUE, including Ta, precipitation, SWd, albedo, and NDVI (Figure 4a-c). When the daily temperature was below 5 \({}^{\circ}\)C, the daily ET was relatively small and the daily GPP and WUE were near 0. Thus, warmer temperature mainly determined daily ET, GPP, and WUE over the midstream oasis area of the HRB. Although precipitation did not solely determine daily ET, GPP, and WUE, their peak durations were a direct response to rainfall/cloudy events in the growing season. The duration of peak stages was influenced by the time distribution characteristics of precipitation for the year. For example, in 2011 and 2016, the profiles for ET, GPP, and WUE were significantly abnormal due to the concentrated and relatively low rainfall (Figure 4a,d,f). In addition, Figure 4b shows that the seasonal variations of NDVI were basically consistent with estimated ET, GPP, and WUE. In midstream oasis areas with relatively low precipitation, the rain gauge-observed precipitation (TE525 MM, Texas Electric, USA) at the DMS (Figure 4a) also showed high daily ET (5-8 mm/day), GPP (5-8 gC/m\({}^{2}\)), and WUE (1-1.5 gC/kgH\({}_{2}\)O) during the  
Figure 4: Comparison of estimated daily ET, GPP, and WUE with ground measurement data and response of temporal variations in daily temperature (Ta), daily precipitation, daily incident shortwave radiation (SWd), daily albedo, and normalized differential vegetation index (NDVI) (16 days) from Daman superstation in midstream oasis area of HRB from 2010 to 2016. Black circles indicate measurements from EC at Daman superstation. growing season. This is because the midstream oasis area has irrigation water supplied from the surface and groundwater (Figure 4d,f). In general, high GPP values correspond directly to high ET values, and vice versa, and the trends of WUE over the midstream oasis area of the HRB were similar but larger discrepancies were seen (Figure 4). There were large differences in the early and late periods of the growth season and periods of precipitation. The estimated and observed ET and GPP values increased rapidly in the initial stage of the growth season and reached maxima in the middle of July as the plants matured, before declining gradually in late August through the subsequent senescent stage. As WUE is a ratio using GPP and ET, their results directly determine WUE estimations, particularly its magnitude.