## 4 Results and Analysis
### Temporal and Spatial Trend Variations of WUE and Various Factors  
The Theil-Sen Median method in combination with the Mann-Kendall test can be used for analyzing the trend in the WUE, GPP, and ET values in the basin from 2001 to 2020. The results are shown in Figure 3. The types of changes in the WUE, GPP, and ET values in the basin were classified into nine categories against the trend values and confidence intervals in Table 1. Figure 3a shows the trend in spatial and temporal changes in GPP; the overall trend in GPP values in the basin was increasing, with the areas having an increase accounting for about 86.12% of the total area of the basin. The proportions of insignificant, slightly significant, significant, and extremely significant increases were 33.74%, 8.17%, 16.13%, and 28.06%, respectively; the proportion of the areas with almost no change accounted for about 0.1%; and the remaining 13.78% consisted of the areas with significant decreases, with the area proportion of insignificant decrease > significant decrease > slightly significant decrease. In terms of spatial  
Figure 2: Spatial distribution of GPP, ET, and WUE in the Li River Basin, 2001–2020. distribution, most of the non-significantly increasing and decreasing areas of GPP were located in the northern part of the basin, with a few areas scattered in the southern part; the significantly increasing areas showed an increasing trend from north to south, which was opposite to that of the significantly decreasing areas. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Trends** & **M-K Test Confidence Level** & **Type of Change** \\ \hline \hline \multicolumn{2}{c}{V \(>\) 2.58} & Extremely significant increase \\ \hline \(\beta>0\) & 2.58 \(\geq\) V \(>\) 1.96 & Significant increase \\ \hline \multicolumn{2}{c}{1.96 \(\geq\) V \(>\) 1.65} & Slightly significant increase \\ \hline \multicolumn{2}{c}{V \(\geq\) 1.65} & Insignificant increase \\ \hline \(\beta\) & V & No change \\ \hline \multicolumn{2}{c}{V \(\geq\) 1.65} & Insignificant decrease \\ \hline \multicolumn{2}{c}{1.96 \(\geq\) V \(>\) 1.65} & Slightly significant decrease \\ \hline \(\beta<0\) & 2.58 \(\geq\) V \(>\) 1.96 & Significant decrease \\ \hline \multicolumn{2}{c}{V \(>\) 2.58} & Extremely significant decrease \\ \hline \end{tabular}
\end{table} 
Table 1: Classification of WUE trend types. Figure 3: Spatial distribution of trend changes in WUE, GPP, and ET in Li River Basin from 2001 to 2020. Figure 3b shows the trend in ET values in the basin over the past 20 years. Similar to the GPP values, the ET values in the basin mostly showed a continuous increase, but the difference was that the area with an extremely significant increase in the ET values accounted for more than that of the same type of GPP values, which accounted for 48.81% of the whole study area, followed by the area with a significant increase, which accounted for 22.79%; the areas with almost no change in ET values in the basin were similar to that of GPP values, accounting for the lowest percentage. The percentage of areas with a decreasing trend were lower than that of GPP, which was only 2.1%. The spatial ET values showed a continuous increasing trend from north to south and reached the highest percentage in the part in Pingle. The areas with significantly decreasing values were mainly concentrated in Lingui, with a few distributed in the urban areas of Guilin and the neighboring junctions. From Figure 3c, it can be seen that 9.27% of the area had an increasing trend in WUE, about 90.23% of the area had a decreasing trend, and the remaining 0.5% was the unchanged area. A further analysis of Figure 3c showed that the area with an extremely significant decrease was the largest, accounting for 45.8%, while the area with an extremely significant increase was the smallest, accounting for about 0.22%. The areas with decreasing WUE were mainly distributed in the northeast and southeast of the basin, and most of the increasing areas were in the middle and western regions.