# Responses of Water Use Efficiency to Drought in Southwest China
## 1 Introduction  
Water use efficiency (WUE) is the ratio of carbon gain (or gross primary production, GPP) to water consumption (or evapotranspiration, ET). As an important indicator of the coupling between the carbon and water cycles, WUE closely links biological processes (e.g., photosynthesis, transpiration) with physical processes (e.g., evaporation) [1, 2]. WUE measures the tradeoff between carbon uptake and water consumption of terrestrial ecosystems, and is an important indicator for understanding the responses of terrestrial ecosystems to climate change [3]. Ecosystem WUE is influenced by many biological and abiotic factors. For example, a previous study based on eddy covariance data [4] suggested that forest WUE increased because of the elevatedatmospheric carbon dioxide concentrations while this hypothesis was challenged by a recent study [5] based on 30-year tree ring records of carbon and oxygen isotope measurements, basal area increment, and eddy covariance measurements. This new study [5] disentangled the relative effects of photosynthesis and stomatal conductance on rising forest WUE, and found that rising forest WUE was mainly due to enhanced photosynthesis and partly due to reductions in stomatal conductance. WUE also decreased with the increase of elevation and solar radiation and increased with temperature, precipitation, and leaf area index (LAI) [6]. Moreover, WUE varies with vegetation type. For example, a previous study showed that the WUE of deciduous broad forests was higher than that of evergreen needle forests; stomatal conductance mainly controlled the increase of WUE in deciduous broad forests while the increase of GPP led to the increase of WUE in evergreen needle forests [7]. In addition, WUE is affected by drought, and the responses of WUE to water stress vary over space. For example, [PERSON] et al. [8] found that drought generally reduced WUE in central China but increased WUE in central Inner Mongolia and Northeast China. [PERSON] et al. [9] found that the WUE increased in Northeast China, Northeast Inner Mongolia, and some regions with abundant forests in South China, and declined in Northwest and Central China during drought periods. Drought has become one of the main obstacles to the health of ecosystems by reducing soil water availability and plant carbon uptake [10; 11]. With the acceleration of global climate change, the frequency and intensity of drought have been increasing regionally and globally [12; 13; 14; 15]. Drought can limit plant growth and lead to forests fires, crop yield reduction, land degradation, and biological invasion [3; 16; 17]. Meanwhile, drought can reduce plant transpiration and disturb the global water balance [18]. Up to now, the impact of drought on ecosystem WUE has received growing attention in scientific research [19; 20; 21; 22; 23; 24; 25]. A previous study found that annual WUE decreased in moderate and extreme drought but slightly increased in severe drought [8]. In addition, [PERSON] et al. [10] found that drought increased WUE in arid areas and decreased WUE in semi-arid and sub-humid areas, and different ecosystems had different sensitivity to the changes of hydrological and climatic conditions. However, another study showed that the global WUE had a negative response to drought in arid areas while it had both positive and negative responses in humid areas [26]. [PERSON] et al. [27] found that the 2009-2010 drought in Southwest China inhibited the growth of vegetation in Yunnan, northern Guangxi, Guizhou, and eastern Sichuan and led to the decline of primary productivity and carbon uptake. Although the effects of drought on WUE have been well studied [28; 29], it remains unclear how the responses of WUE vary with drought severity. In this study, we examined the responses of WUE to drought in Southwest China from 2000 to 2017 using the scPDSI (Self-calibrating Palmer Drought Severity Index) [30] and MODIS (Moderate-resolution Imaging Spectroradiometer) GPP and ET data.