## 1 Introduction  
Drought is an intermittent disturbance when water supply does not satisfy the demand for a long time [1; 2], which can bring profound effects on carbon-water fluxes in terrestrial ecosystems at global and regional scales [3; 4]. Drought-induced water stress could constrain vegetation growth, trigger wildfire and lead to crop yield reduction, grassland degradation and biological invasion [5; 6]. Both frequency and intensity of global drought would increase in the context of climate warming until the year 2100 [7; 8; 9]. Knowledge about how terrestrial ecosystems adapt to and recover from drought stress is obviously critical for governmental agents to implement effective measures for sustainable ecosystem development and water resources management under future drought disturbances [10; 11]. Water use efficiency (WUE) is a widely accepted parameter in describing carbon-water flux coupling between terrestrial ecosystems and the atmosphere and linking biological (e.g., vegetation transpiration and photosynthesis) and physical (e.g., soil evaporation) processes [12; 13]. It reflects how much water is consumed by an ecosystem while it obtains carbon unit, which can be quantified as the ratio of carbon uptake (measured by GPP--gross primary productivity, NPP--net primary productivity or NEP--net ecosystem productivity) and water consumption (measured by ET--evapotranspiration or precipitation) [14; 15; 16]. Many previous studies have investigated the spatiotemporal changes of WUE. For example, [PERSON] et al. (2017) showed that underlying WUE (GPP \(\times\) VPD\({}^{0.5}\)/ET) increased slowly during 1901-1975 but increased rapidly from 1976 to 2010 over the world based on terrestrial biosphere models [17]. [PERSON] et al. (2014) found that the WUE had a similar latitudinal trend on earth, increasing from the subtropics to about 51\({}^{\circ}\) N and then decreased in higher latitudes [18]. [PERSON] et al. (2013) indicated that the WUE varied with water availability at the global scale, with an increasing trend from moderate to extreme drought and a decreasing trend in wet conditions [19]. The ecosystem WUE is vulnerable to climate change because various abiotic factors (e.g., radiation, wind, precipitation and temperature) can have a huge impact on it [11; 20; 21]. Drought disturbances (less water available than in normal conditions) can influence plant transpiration and soil evaporation directly, and ultimately affect ecosystem WUE and productivity [2; 22]. In the last three decades, the spatiotemporal responses of WUE to drought have been widely investigated. For example, [PERSON] et al. (2015) calculated ecosystem WUE in China using the Boreal Ecosystem Productivity Simulator (BEPS) model, which indicated that WUE increased in Northeast China and central Inner Mongolia but WUE was reduced in Central China when drought happened from 2000 to 2011 [23]. [PERSON] et al. (2008) indicated that WUE estimated from site measurements enhanced in drier regions (relative to precipitation-abundant regions [24]) in eastern China because plants decreased stomatal conductance to adapt to water-limited conditions [25]. Moreover, [PERSON] et al. (2016) investigated the relationship between global WUE and wetness index (WI) and found that WUE rose in arid ecosystems but declined in semi-arid/sub-humid ecosystems during drought periods mainly because of distinct sensitivities of different ecosystems to changes in hydro-climatic conditions [26]. However, [PERSON] et al. (2017) suggested that the global WUE negatively (both positively and negatively) responded to drought in arid (humid) ecosystems during 2000-2014 based on the spearman correlation between the WUE and Standardized Precipitation-Evapotranspiration Index (SPEI) [20]. The inconsistency may arise from distinct data source, time periods, biomes and the way WUE is defined in different studies.