Several global and regional studies have been carried throughout the world [8; 9], and have shown that the spatiotemporal distribution of the WUE is affected by many factors, such as latitude, surface temperature, and plant species; however, water availability (hydrological conditions) is the main factor affecting the spatial and temporal distributions of WUE through changes to the global water balance and water availability [10; 11]. Therefore, research on the spatial and temporal distribution of drought and the WUE plays an important role in understanding the impacts of climate change in terrestrial ecosystems. The response of the WUE in an ecosystem to drought has become a popular topic because of climate change. In general, a common mechanism is that the WUE will increase in water-deficient or arid environments at the vegetation leaf and observation site scale, as the leaf stomatal conductance will decrease under drought conditions, resulting in decreased water loss. This regulation of stomatal behavior is widely used in ecosystem models [12; 13]. However, studies have shown that under severe drought conditions, the WUE decreases rapidly at the leaf scale, because severe drought causes plant activity losses, which changes the effects of drought on vegetation from stomatal limitation to nonstomatal limitation. However, this relationship between WUE and drought has not been applied in ecosystems modeled under severe drought conditions [14; 15; 16]. In recent studies, the relationship between ecosystem WUE and drought showed a large divergence across different ecosystems [6]. Therefore, there is an urgent need to improve the knowledge to explain and understand the relationship between WUE and drought, especially under severe drought conditions. An improved understanding of the effects of drought on ecosystems can be obtained by understanding the changes in ecosystems before and after drought. In general, ecosystems exhibit a certain degree of resilience after a drought, that is, the ability of an ecosystem to return to its predrought state [17]. For example, in grasslands, drought can rapidly reduce the aboveground biomass of grassland vegetation, but the belowground parts are rarely affected, which serve as preparations for post-drought recovery [18; 19]. Many trees have a resprouting ability and are able to return to their pre-drought state [20; 21]. However, severe drought may lead to vegetation death as a result of the threshold of ecosystem resilience being exceeded and changes to different ecological processes at various spatiotemporal scales, such as tree regeneration and growth and ecosystem productivity. In addition, there is a body of empirical and theoretical evidence that sustained drought can cause herbaceous vegetation losses and increase bare land areas, which may trigger the arid and semiarid ecosystems to switch to an alternative steady state, which is characterized by low water infiltration and high runoff [22]. The results of these studies indicated that if there is no human intervention in the plant community, it will be impossible to rebuild to the previous state [23]. Therefore, it is necessary to study the changes in ecosystem states before and after drought, which can help us understand ecosystem processes under drought disturbances. In previous studies, scholars have defined several ways to calculate WUE based on the definition of ecosystem WUE. Among these methods, ecosystem carbon sequestration is often determined using several indicators, including net primary productivity (NPP) [24], gross primary productivity (GPP) [4; 14], aboveground NPP (ANPP), and net ecosystem productivity (NEP) [25], of which GPP is the most used to characterize carbon absorption of ecosystems. At the leaf scale, water loss is characterized by the leaf evaporation rate. Theoretically, the leaf evaporation rate at the leaf scale should be used to determine the water loss of an ecosystem; however, because of measurement difficulties, evapotranspiration (ET) is often applied to indicate water loss [26].