## 1 Introduction  
Persistent droughs reduce vegetation greenness, biomass, productivity, and increase tree mortality [1; 2; 3; 4; 5; 6; 7; 8]. The frequency, severity and incidence of droughs are expected to increase with climate change [9; 10; 11]. Extreme climate events and their ecological impacts have been observed in the United States, Europe, Brazil and China [1; 12; 13; 14; 15]. The sustainability of terrestrial carbon and water cycles is strongly affected by the frequency and intensity of large-scale drought events, and all these disturbances have substantial impacts on ecosystem structure, species composition and function [16; 17; 18]. Hence, they are of great importance in exploring the impacts of droughts on the regional carbon budget and water resources and improving our ability to refine water management. Water availability controls plant photosynthesis and influences plant growth, and thus affects net primary productivity (NPP), evapotranspiration (ET) and then water-use efficiency (WUE) [19; 20]. The productivity of forest ecosystems is vulnerable to drought and temperature extremes [21] and many studies have reported a drought-induced reduction in NPP [1; 3; 22; 23; 24]. At the global scale, large-scale drought has reduced regional NPP from 2000 to 2009, especially in the Southern Hemisphere [3]. Regionally, the effect of drought on carbon dynamics has been focused in different regions such as Europe and the Amazon. In Europe, the drought and heatwave in 2003 substantially reduced vegetation productivity, leading to a large net carbon release into the atmosphere [1; 22]. The 2005 and 2010 drought events had a total biomass carbon impact in Amazon forests [23; 24]. ET, the sum of water lost to the atmosphere from the soil surface through evaporation and from canopy via plant transpiration, is a central process of the hydrological, energy and carbon cycles [25; 26; 27; 28]. Given a similarity in responses of ET and photosynthesis to stomatal closure, ecosystem ET is also impacted by drought and has feedbacks upon productivity accelerating with climate change [18; 27]. Evidence has shown that drought reduced both ecosystem NPP and ET simultaneously [27; 29; 30]. However, the magnitudes of NPP and ET responding to drought depend on forest types and environmental stress [31; 32]. WUE, defined as the ratio of NPP to ET, measures the trade-off between the carbon gain and water loss of terrestrial ecosystems and highlights the linkage between biological processes (i.e., photosynthesis and transpiration) and physical process (i.e., evaporation). In general, drought suppresses both ecosystem productivity and ET simultaneously and, in turn, has a great impact on WUE [32]. Based on long-term flux data and the satellite product from MODIS, [PERSON] et al. (2013) found the responses of WUE to changes in water availability were different in drought and wet years, with WUE increasing in drought years across all biomes [33]. A model study by [PERSON] et al. (2010) suggested that the sensitivity of WUE to water availability depended on ecosystem types, which indicated that WUE response to an increase in precipitation is higher for forests [34]. A global study based on MODIS data also showed that increases of WUE were found in forests under drought impact [18]. Thus, evaluating shifts in WUE over drought stress and sensitivities of its component (i.e., NPP and ET) helps us better understand the potential impacts of climate change on ecosystem carbon budgets and water resources. Drought frequency has been increasing in southern regions of China which have usually received ample precipitation in the last decade [6; 35]. The 2009/2010 drought in south-western China was the most severe drought in the last five decades [36]. The spring drought occurring in south-western China reduced regional annual gross primary productivity and net primary productivity in 2010 by 65 and 46 Tg C yr\({}^{-1}\), respectively [37].