# Spatio-Temporal Variations of Ecosystem Water Use Efficiency and Its Drivers in Southwest China
## 4 Discussion
### Effects of Meteorological Factors and EVI on Changes to Ecosystem WUE  
This study focused on the effects of temperature, precipitation, and EVI on ecosystem WUE. Temperature, precipitation, and EVI were generally positively correlated with ecosystem WUE and the largest proportion of the area of the pixel exhibited a positive correlation between EVI and ecosystem WUE. EVI was significantly increased in 65.83% of the total study area, demonstrating a significant increasing trend in ecosystem WUE. EVI was the dominate factor influencing ecosystem WUE variations throughout most of the study area (Figure 5). These findings suggest, at least to some extent, that vegetation restoration was associated with increased ecosystem WUE. The government of China has conducted large-scale afforestation activities since 2000 to combat stone desertification in the study area, which has accelerated recovery of regional vegetation [13] and continues to improve [48]. EVI as a biophysical parameter of the leaf canopy increases rapidly with biomass on a long-term scale and is highly correlated with GPP. Hence, both EVI and GPP are important indicators of the effectiveness of ecological projects, such as vegetation restoration [49]. In this study, the cumulative area of afforestation was positively correlated with EVI from 2003 to 2017 (Figure 7). Thus, the ecological projects implemented in the southwest have resulted in good recovery of vegetation coverage, thereby effectively improving ecosystem WUE. Meanwhile, afforestation has led to major restoration of the karst regions of southwest China and has positively influenced the ecosystem WUE in southwest China [16], consistent with the findings of the present study. In this investigation of the variation to ecosystem WUE with latitude, longitude, and altitude, two meteorological elements, temperature and precipitation, were found to influence the spatial distribution of ecosystem WUE, in agreement with the findings of previous studies [34; 50]. Both temperature and precipitation are commonly cited meteorological factors and both were positively correlated with ecosystem WUE throughout most of the study area. [PERSON] et al. [51] also showed that ecosystem WUE was particularly sensitive to temperature. A previous study [17] concluded that ecosystem WUE is negatively correlated with precipitation in southwest China. This inconsistency may be due to differences in study areas and research methods, as the study area in the previous report included eight provincial administrative regions in southwest China and employed simple correlation analysis, while the study area in the present report included three provincial administrative regions in southwest China, which resulted in differences area share statistics for positive and negative correlations. The influence of other variables was eliminated when calculating the correlation coefficient between any two variables by partial correlation analysis, as applied in previous studies with more reliable results [52; 53]. Although precipitation is abundant in southwest China, karst landforms have unique geological conditions that impede water storage and are susceptible to drought [54], which seriously affects the availability of water to sustain vegetation growth [55; 56]. On the other hand, the relatively excessive precipitation and cloudiness of non-karst landforms can lower temperatures and solar radiation [57; 58], which is detrimental to vegetation growth [59]. Thus, precipitation \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Type of** & **GPP Average** & **GPP Variation** & **ET Average** & **ET Variation** & **WUE Average** & **WUE Variation** \\
**Landform** & **GPP Average** & **Rate** & **Rate** & **Rate** & **Rate** \\ \hline Karst & 1792.71 & 25.82 ** & 673.40 & 4.01 * & 2.61 & 0.20 ** \\ Non-karst & 2254.26 & 20.23 ** & 770.49 & 4.58 ** & 2.86 & 0 \\ \hline \multicolumn{6}{c}{**\(p<0.001\), * \(p<0.05\).**} \\ \end{tabular}
\end{table}