# Quantifying Soil Moisture Impacts on Water Use Efficiency in Terrestrial Ecosystems of China
## 1 Introduction  
The water use efficiency (_WUE_) of an ecosystem is the ratio of carbon sequestration to water consumption, which couples the terrestrial carbon and water cycles and closely links photosynthesis and evapotranspiration (_ET_) processes in the ecosystem [1, 2]. Although there are many ways to express the _WUE_ of an ecosystem, the ratio of the net primary production (_NPP_) to _ET_ is widely used [3]. The variation of _WUE_ controlled by both biotic and abiotic factors [4, 5] and recent research showed that a vapor pressure deficit (_VPD_) and canopy conductance are two dominant factors of _WUE_ in response to drought [6]. On the global scale, _WUE_ is positively related with drought in the majority of regions [7]. In China, most forest ecosystems exhibit strong drought resistance by improving their _WUE_[8]. Under the influence of the negative feedback relationship between _ET_ and _WUE_ temperature [9, 10], the variation in _WUE_ is usually positively correlated with airtemperature [11]. Many studies have shown that, in the past 30 years, the vegetation cover, gross primary productivity (_GPP_) and _ET_ in western China have been low, and the demand for evaporation has been high [12]. In the context of climate change, many studies focus on the response process and mechanism of _WUE_ to the constraints of drought or water conditions [7; 13; 14]. On a global scale, _WUE_ generally decreases with an increase in drought or geographic latitude [15]. However, studies have shown that _WUE_ generally has a positive relationship with the drought degree. Drought events can slightly increase the _WUE_ in the majority of forest, grassland, and shrub ecosystems [16; 17]. In addition, the response of the _WUE_ to drought has spatial heterogeneity at different spatial scales and in different ecosystems. _ET_ has been shown to control the _WUE_ response to drought in semi-arid and high-latitude regions, and _GPP_ dominated the response _WUE_ to drought in tropical forest regions [7]. Thus, a quantitative study of _WUE_ and its spatial variation along drought gradients is essential for understanding the regional environmental and physiological functions of heterogeneous landscapes [16; 18]. As an important indicator of drought, soil moisture (_SM_) often plays a vital role in the ecosystem carbon-water relationship [19; 20; 21; 22]. Studies have shown that _SM_ can indirectly affect the _WUE_ of terrestrial ecosystems by controlling the _NPP_ and _ET_ processes [21]. Soil water loss in sub-humid, semi-arid, and arid regions can reduce the total primary productivity by 40% [23]. [PERSON] et al. showed that when the soil relative extractable water (_REW_) was below 0.4, the _GPP_ decreased significantly and the total ecosystem respiration did not decrease until the _REW_ continued to decrease to 0.2 [24]. However, another study showed that because water stress has less limitation on carbon assimilation than _ET_, _ET_ decreases especially on cloudy days, thereby resulting in a significant increase in _WUE_[25]. In the global forest ecosystem, the variations in ecosystem _WUE_, _GPP_, and _ET_ are correlated with _SM_, and the sensitivities of the _WUE_ to the soil water content significantly increase with the increase in the vapor pressure deficit [26]. The seasonal fluctuation of _SM_ caused by local rainfall is the main factor determining the _GPP_/_ET_ relationship in the tropical Amazon rainforest [27; 28].Given the above results, some studies think that soil water use efficiency is an adequate representative and indicator of _WUE_[15; 29]. Therefore, studying the influence mechanism of _SM_ on _WUE_ is of great significance to the evolution of terrestrial ecosystems and the response to climate change. Many studies have shown that a decrease in soil moisture or an increase in drought severity usually increases the _WUE_[7; 25; 30; 31]. A recent study showed that _SM_, rather than _VPD_, dominates dryness stress on ecosystem production globally [32].