## 1 Introduction  
Climate change, particularly global warming, has significantly impacted agricultural production, food security, species survival, human health, and social stability. The series of problems arising from global warming has become a global threat [1]. Since the Industrial Revolution, the sharp increase in the concentration of greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, driven by human activities, has led to drastic changes in the global climate [2]. Vegetation plays a crucial role in maintaining the carbon water balance of terrestrial ecosystems and mitigating global warming [3]. Changes in energy cycle processes, including fluctuations in water availability and heat, have a significant impact on plant communities [4]. Vegetation growth is highly sensitive to changes in water and heat conditions [5]; therefore, climate change affects the physiological and ecological processes, functions, and structures of terrestrial ecosystems [6]. Climate change affects vegetation growth in two primary ways. First, studies indicate that climate change affects the water absorption of vegetation [7]. Environmental changes [8], such as increased CO\({}_{2}\) concentration, altered soil moisture, higher temperatures, and enhanced ultraviolet radiation, have caused vegetation to green earlier [9], delayed the yellowing period, and prolonged the growing season. Climate change affects plant productivity, which subsequently affects vegetation carbon flux and ecosystem water use efficiency [10]. Second, extreme climatic conditions can significantly reduce vegetation productivity, with persistent droughts potentially leading to ecosystem decline or collapse, thereby exacerbating global warming [11]. Thus, how vegetation responds to climate change remains uncertain. Ecosystem water use efficiency (WUE) is defined as the amount of dry matter produced per unit of water consumed in an ecosystem [12]. Scholars often use gross primary productivity (GPP) to represent carbon fixation in plants and evapotranspiration (ET) to represent water consumption, with the ratio of GPP to ET serving as a measure of ecosystem WUE [13,14]. This ratio is an important indicator for assessing the interaction between carbon and water. In the context of global climate change, especially frequent drought events, WUE characterizes vegetation's response and feedback to climate variability [15, 16, 17, 18]. As a core indicator in the study of carbon water coupling mechanisms, WUE has been the subject of extensive research and debate, primarily focusing on its definition, calculation methods [19], data uncertainties [20], and the complexity of its driving mechanisms [21]. Across various scales, ranging from individual leaves, plant canopies, and communities to regional and global levels, significant differences exist in the concept and methodologies of WUE research [16, 17, 18]. Climatic factors, such as atmospheric CO\({}_{2}\) concentration, extreme climatic events, and differences in vegetation types, exert diverse effects on ecosystem WUE [22, 23, 24]. Studies have demonstrated that different vegetation types exhibit distinct water use strategies. Moreover, plant attributes, including vegetation categories, root structures, and plant morphology, play a critical role in determining WUE [25, 26, 27]. Additionally, drought events [28, 29]--including drought severity [30], timing [31], duration [32], and lag effects [33]--all impact the WUE of terrestrial ecosystems. Currently, discrepancies in ecosystem WUE studies are largely attributed to errors in the simulation of GPP and ET, as well as spatial heterogeneity. To address these issues, this study introduces two key improvements. First, the spatial patterns of GPP across the QTP were reassessed using flux observations within the study area. Second, a new ET fusion product was developed by integrating multiple ET datasets and grounding them in flux-based ET observations.