# Spatio-Temporal Variations of Ecosystem Water Use Efficiency and Its Drivers in Southwest China
###### Abstract  
Water use efficiency (WUE) has garnered considerable attention at global and regional levels. However, spatio-temporal variations of WUE and related influencing factors in the complex karst landforms of southwest China require further elucidation. Herein, the ratio of gross primary productivity (GPP) to evapotranspiration (ET) obtained through the PML-V2 product was used to characterize ecosystem WUE, the spatio-temporal variations to ecosystem WUE, and responses to temperature, precipitation, and the enhanced vegetation index (EVI) in southwest China. The results showed that: (1) The ecosystem WUE in southwest China decreased with increasing latitude and altitude. Spatially, the ecosystem WUE fluctuates in a \"W\" pattern with increasing longitude because of the karst landforms' distribution patterns. (2) The non-significant trend in increased ecosystem WUE during 2003-2017 may be associated with significant increases in the ET offsetting part of the GPP contribution to ecosystem WUE. Spatial distribution of changes in WUE is similar to GPP owing to the dominant role of GPP in changes to ecosystem WUE. (3) The multi-year average ecosystem WUE was lower in karst than in non-karst landforms; however, vegetation restoration projects have contributed in significantly increasing variation rate of ecosystem WUE in karst than that in non-karst landforms. (4) Temperature, precipitation, and EVI were generally positively correlated with ecosystem WUE and were important factors for the increase in ecosystem WUE. EVI characterized vegetation restoration indicators showed that the ecological engineering construction in the study area was effective and was the dominant factor of ecosystem WUE change in 59.59% of the study area. The results of this study are important for further understanding carbon and water cycling processes in karst regions. water use efficiency; southwest China; karst region; spatio-temporal variations; influencing factor 2023123971  
was responsible for the increase in WUE of most regional ecosystems, and precipitation and soil moisture also contributed positively to it [7]. In arid and water-scarce northwest China, precipitation is the primary driver of increased WUE, whereas secondary drivers vary among different landforms [8]. The above studies show that climate change and the significant differences in the subsurface have resulted in different characteristics and drivers for spatio-temporal variations to WUE at the regional level. Southwest China is the largest contiguous karst region globally, which is characterized by fragile ecosystems because of extensive stone desertification and soil erosion [9]. To mitigate the ecological issues in this region, the government of China has conducted a series of critical projects aimed to restore the ecosystems and increase vegetation coverage [10; 11; 12]. These projects have achieved remarkable success in significantly increasing vegetation coverage [13; 14]. In addition, the spatial distribution of meteorological elements in the mountainous areas of southwest China is notably heterogeneous. Therefore, to gain a comprehensive understanding of the spatio-temporal variations to WUE and related influencing factors, it is imperative to conduct research in the karst region of southwest China. Thus far, studies of WUE in southwest China have been somewhat fruitful. For example, [PERSON] et al. [15] found that WUE increased with elevation at the Gongga Station located on the eastern slope of the Tibetan Plateau and gradient and temperature are among the main factors driving variations in WUE. [PERSON] et al. [16] studied four provincial administrative regions in southwest China using Breathing Earth System Simulator (BESS) products and reported that ecosystem WUE therein had significantly increased from 2001 to 2015, which was driven by ecological restoration projects, such as afforestation and planting of grasslands. In contrast, [PERSON] et al.