# The Contributions of Climate and Human Activities to Water Use Efficiency in China's Drylands
## 1 Introduction  
Plant water use efficiency (WUE) is a measure of the amount of carbon fixed per unit of water consumed [1, 2]. At the ecosystem level, the WUE is typically calculated as the ratio of gross primary productivity (GPP) to evapotranspiration (ET) [3, 4]. As a vital indicator, WUE elucidates terrestrial ecosystems' adaptation to environmental shifts, spotlighting the carbon-water nexus [5, 6]. In the context of global warming, drylands exhibit a more pronounced warming trend than humid regions [7]. Consequently, adverse effects of warming are anticipated to occur more frequently and intensely in drylands. Drylands, home to over a third of the global population and covering nearly half the land, rank among the most climate-vulnerable ecosystems [8]. Previous studies have revealed that variations in WUE in drylands are sensitive to climate change and human activities. Therefore, monitoring WUE dynamics is critical for unraveling dryland ecosystems' carbon and water cycles under changing environmental conditions [9; 10]. Natural factors and human actions significantly influence WUE [11; 12]. Previous studies have demonstrated that WUE is closely related to the composition and diversity of plant communities but is more sensitive to climate change than plant structure [13; 14; 15]. Furthermore, the mechanisms by which dryland vegetation responds to various climatic factors exhibit significant heterogeneity across regions [7; 16]. Precipitation (Prec) and temperature (Tem) stand out as primary influencers of WUE variability. For example, a study reported that WUE was closely related to Prec in temperate and tropical drylands [17], while the limiting effect of Tem on WUE was more pronounced in high-latitude regions. In addition, certain studies have revealed the existence of a crucial Tem threshold for WUE variability, beyond which increases in Tem result in decreased WUE [18]. The expansion of arid regions due to rising Tems contributes to greening of these areas, but it also exacerbates atmospheric desiccation. This affects changes in WUE in drylands. Under global warming, VPD plays a significant role in WUE by affecting ET. Low plant VPD affects WUE by altering plant transpiration and soil evaporation, whereas high VPD induces stomatal closure, thereby reducing plant photosynthesis in ecosystems [19; 20], particularly in drylands. Solar radiation (Solra) plays a significant role in determining the capacity of ecosystems to sequester carbon by directly controlling vegetation photosynthesis, which can significantly affect WUE [21]. Some scholars have found that there was a threshold for the role of Solra in increasing WUE [18], beyond which WUE decreased with further increases in Solra. Human interventions, notably through land cover changes, profoundly affect WUE by altering ecosystems' carbon sequestration and water dynamics [22]. On agricultural land, human activities, such as irrigation and fertilizer application, as well as an increase in crop yields, can result in alterations to the carbon and water cycles, which can increase WUE [23]. Simultaneously, overgrazing has a significantly negative impact on grassland ecosystems, particularly alpine meadows, causing grassland degradation and desertification [24; 25]. Several studies have demonstrated that certain ecological measures such as reforestation initiatives in China have contributed to the greening trend in the northern regions [26]. This has also led to an increase in water consumption. Moreover, there were regional uncertainties in the direction of the change in WUE. China's drylands, spanning over 2.4 \(\times\) 10\({}^{6}\) km\({}^{2}\), face critical environmental challenges. However, extensive restoration efforts since 1978 have significantly mitigated, if not reversed, desertification [27]. Concurrently, large-scale afforestation, as a primary restoration measure, has resulted in considerable changes in WUE for dryland vegetation by affecting GPP and ET [28].