## 4 Results
### Temporal Dynamics of Annual Cumulative ET in the Study Area  
Significant spatial discrepancies are highlighted in Figure 4, which depicts the regional characteristics in yearly cumulative ET in the Heihe River Basin from 2000 to 2020. There were significant variations in ET between the midstream irrigation area and the downstream Gobi Desert (up to 600 mm/year), with the annual total ET in the study region ranging from 100 to 750 mm/year. The midstream irrigated oasis zone showed the highest yearly cumulative ET, while the downstream Gobi Desert region showed the least ET. There are a great deal of summer precipitation, significant fluctuations in temperature, and rapid evaporation in the upstream watershed of the Qilian Mountains region. The midstream Hexi Corridor region experiences a drier climate than the upstream regions. Despite being in a dry and semi-arid area with little precipitation, the midstream artificial oasis region (around 40\({}^{\circ}\)N) has the strongest ET. The reason for this is that there are many  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Site** & **Metrics** & **TSEB-SM** & **PML-V2** & **SEBAL** \\ \hline \multirow{4}{*}{Arou} & R\({}^{2}\) & 0.79 & 0.76 & 0.22 \\  & RMSE(mm/d) & 0.76 & 0.78 & 0.88 \\  & Fitted equation & y = 0.91x + 0.32 & y = 0.70x + 0.15 & y = 0.32x + 0.32 \\ \hline \multirow{3}{*}{Huazhaizi} & R\({}^{2}\) & 0.38 & 0.39 & 0.05 \\  & RMSE(mm/d) & 0.46 & 0.47 & 0.74 \\  & Fitted equation & y = 0.46x + 0.28 & y = 0.49x + 0.17 & y = 0.23x + 0.20 \\ \hline \multirow{3}{*}{Daman} & R\({}^{2}\) & 0.78 & 0.73 & 0.34 \\  & RMSE(mm/d) & 0.57 & 0.62 & 0.74 \\  & Fitted equation & y = 0.90x + 0.07 & y = 0.86x \(-\) 0.18 & y = 0.28x + 0.12 \\ \hline \multirow{3}{*}{Sidaojao} & R\({}^{2}\) & 0.55 & 0.53 & 0.05 \\  & RMSE(mm/d) & 0.73 & 0.75 & 0.58 \\ \cline{1-1}  & Fitted equation & y = 0.42x + 0.39 & y = 0.21x + 0.13 & y = 0.07x + 0.21 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Metrics of the fit of the three ET products to the observations.  
irrigated agricultural regions that are supplied with water from reservoirs in the Qilian Mountains, and these dry climatic circumstances lead to an intense need for atmospheric evaporation. All of the downstream areas are covered by the Gobi Desert except for a small amount of oasis existing along the Heihe River, and the downstream area has low precipitation and strong evaporation, so ET is mainly provided by soil evaporation, but the ET here is the lowest in the study area. The interannual fluctuations in ET in the Heihe River Basin from 2000 to 2020 were not very significant, with little year-to-year variation. In addition to the relatively stable climate and precipitation in the study area, it is also inseparable from the active water management initiatives undertaken by the local government. In May 2000, in response to ecological problems such as the drying up of rivers and lakes, death of forests, and rampant sandstorms in the Heihe River Basin, the government departments concerned carried out water dispatching work for the main stream of the Heihe River to reasonably manage water usage along the Heihe River Basin, thus reducing the occurrence of cutoffs. In September 2001, a severe drought was experienced in the midstream region, but the impact of this drought on the midstream region was reduced by a reasonable allocation of water.