## 3 Results
### Inter-Annual Variation of WUE  
The WUE time series at the 8-day interval scale from 2002 to 2021 was decomposed using the STL decomposition method into three components: the trend term, the seasonal term, and the stochastic fluctuation term (Figure 7). The trend term was centered on its mean value to facilitate the observation and comparison of the three components. Figure 5: Spatial variation of WUE in the study area. Figure 6: Comparison of multi-year average WUE of different vegetation types. According to the trend term, the WUE displayed a fluctuating decreasing trend from 2002 to 2021. WUE varied between 0.53-0.71 gC kg\({}^{-1}\) H\({}_{2}\)O and decreased at an average rate of 0.002 gC kg\({}^{-1}\) H\({}_{2}\)O/a. The change in WUE can be divided into three time stages to discuss its inter-annual variation pattern. From 2002 to 2010, WUE fluctuated at a relatively high level, reaching two peaks in 2004 and 2009 at 0.69 and 0.71 gC kg\({}^{-1}\) H\({}_{2}\)O, respectively. From 2010 to 2017, WUE fluctuated at a relatively low level, reaching peaks in 2013 and 2016, respectively. During the period 2017-2021, WUE increased continuously from 0.53 gC kg\({}^{-1}\) H\({}_{2}\)O to 0.68 gC kg\({}^{-1}\) H\({}_{2}\)O. According to the results of the Mann-Kendall test (Table 2), the results show that the \(p\)-value for each zone was close to zero (<0.01). Combining the \(Z\) value, it can be found that the WUE in each zone showed a significantly decreasing trend at a 0.01 confidence level. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Zone** & \(Z\)**Value** & \(p\)**-Value** & _sen_**Value** & \(H_{0}\) \\ \hline Eastern Qilian Mountains & \(-6.1\) & \(1.32\times 10^{-9}\) & \(-4.5\times 10^{-5}\) & Rejection \\ Central Qilian Mountains & \(-3.5\) & \(3.94\times 10^{-4}\) & \(-1.2\times 10^{-5}\) & Rejection \\ Western Qilian Mountains & \(-3.4\) & \(7.83\times 10^{-4}\) & \(-1.1\times 10^{-5}\) & Rejection \\ Corridor Plain & \(-15.3\) & \(2.2\times 10^{-16}\) & \(-2.2\times 10^{-4}\) & Rejection \\ The whole study area & \(-10.3\) & \(1.95\times 10^{-24}\) & \(-5.8\times 10^{-5}\) & Rejection \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Result of the Mann-Kendall test. Figure 7: Characteristics of WUE changes during 2002–2021. We used the Sen method to investigate the spatial heterogeneity of the WUE change by analyzing the trend term of WUE time series. The spatial distribution of WUE change magnitude and the significance test results were calculated (Figure 8). The area with a valid WUE value was analyzed, excluding the non-vegetated regions such as deserts, Gobi, and glaciers. The magnitude of WUE change ranged from \(-1.36\) to \(1.18\) gC kg\({}^{-1}\) H\({}_{2}\)O over the last 20 years. The proportion with positive change was 37%, while that with negative change was 63%. WUE decreased significantly in the eastern and central HC and the surroundings of Qinghai Lake. Furthermore, the significant increase in WUE was concentrated in central QM. The WUE in Dunhuang and Guazhou Oasis, located in the western HC, also showed an upward trend. Statistical analysis was conducted based on the results of the _Sent_ trend and the M-K test (Table 3). The results show that the percentage of slightly and significantly improved areas was 6.4%, while the percentage of slightly and severely degraded areas was 18.2%. The average Hurst index in the WUE of the whole study area reached 0.72. The area with a Hurst index of less than 0.4 accounted for a minor proportion (0.4%) and was mainly located in the central QM, from Hala Lake to Qinghai Lake and Qilian County. WUE there will change from the current upward trend to a downward trend in the future, showing a strong anti-continuity. The area with a Hurst index greater than 0.6 was primarily located in the eastern QM, the HC oases, the surroundings of Qinghai Lake, and the west of Hala Lake, accounting for 32.4%. WUE showed strong continuity in the future. Figure 8: Spatial distribution of the magnitude of WUE change (**a**) and M-K test result (**b**).