# The Contributions of Climate and Human Activities to Water Use Efficiency in China's Drylands
## 4 Discussion
### Sensitivity of WUE and GPP to Climate Change  
The reactions of WUE and GPP to climatic variables varied across both monthly and yearly time frames. At the monthly scale, Prec and VPD were the primary drivers of the monthly changes in GPP (Figure 4). Prec served as the primary source of surface soil water in the drylands, which in turn was the main driver of drought stress in the ecosystems [48], suggesting that drought stress had a strong impact on plant organic matter production in the drylands. In terms of the degree of response, the correlation coefficients for spring and fall were slightly larger than those for summer, indicating that Prec in spring and fall had a more significant effect on increasing GPP, which was primarily due to the fact that Prec in the drylands was mainly concentrated in the summer and mostly occurred in the form of storm rainfall. The effect of water stress on vegetation growth was more pronounced during the spring and fall seasons when Prec was sparse [49]. Fall Prec was observed to improve WUE, whereas spring Prec was associated with a decline in WUE. Previous studies have also indicated that the enhancement of spring rainfall was a key factor in promoting vegetation regeneration in arid regions, which played a crucial role in revitalizing the western drylands [29; 50]. Climatic factors exhibited distinct responses to WUE across different seasons (Figure 4), suggesting that vegetation employed varying water utilization strategies at different developmental stages. Throughout the year, the two meteorological factors that had the most significant impact on fluctuations in WUE were Tem and VPD, and both exhibited a positive correlation. Tem showed a highly significant negative correlation with ET (\(p<0.01\)), while it did not demonstrate a significant correlation with GPP. This suggested that the influence of Tem on monthly changes in WUE was primarily realized through its effect on ET. On the scale of interannual variability, except for the cold drylands of the Tibetan Plateau, Prec had the highest contribution to the increase in GPP among the four factors, amounting to 31.47%, and showed a strong positive correlation. The relative importance of the other three climatic factors to GPP was ranked as Solra\(>\)VPD\(>\)Tem, indicating that the increase of Prec was an important reason for the increase of GPP in the drylands of China. Moreover, the increase of Prec enhanced the effectiveness of water and thus GPP, and also verified the conclusion that the role of water stress in the drylands was significant [49]. At the same time, the Prec was one of the most important factors affecting the ET in the study area, and the contribution rate was 33.31%, which this indicates that the increase of ET was closely related to the increase of rainfall. Meanwhile, the negative correlation between \begin{table}
\begin{tabular}{c c c} \hline \hline
**Area** & **Time** & **WUE Trend** \\ \hline Inner Mongolia [44] & 2001–2020 & Decrease \\ Tibetan Plateau [45] & 1979–2010 & Increase \\ Three-North Region of China [46] & 2001–2017 & Increase \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Trends in WUE in related studies. Prec and WUE was dominant, indicating that in the drylands of China, the increase of Prec promoted the growth of GPP and increases the ET at the same time to lead to the decrease of WUE. Moisture changes had a more significant effect on GPP than heat, and the potential for vegetation to fix carbon was greater when the Tem reached the requirements for vegetation growth and development, so GPP increased significantly when Prec increased. This aligned with findings from earlier research [51; 52; 53]. Solra predominantly showed a positive correlation with both WUE and GPP, indicating that Solra mainly acted on WUE by affecting the fixation of carbon in plants (Figure 5). [PERSON] et al.