###### Abstract
Traditional studies of WUE have mainly focused on crop leaves or individual levels, and have used gas exchange methods and field monitoring to estimate WUE [16]. In recent decades, due to the development of observation facilities and theoretical models, the investigation scale of WUE has been extended to ecosystems, including those at the local scale [17], regional scale [18], and global scale [19; 20]. Because the influencing factors of WUE change with the research scale and vegetation type, significant variability exists in the characteristics of the spatiotemporal variation in WUE. For example, the average WUE in China's ecosystem during 1979-2012 obtained by a process-based model was negatively correlated with the averages of annual precipitation and annual temperature [21]. Recently, [PERSON] et al. [20] reported that variations in terrestrial ecosystem WUE at the global scale are mainly related to soil moisture. According to reports published by the government's climate change committee, the global average surface temperature will rise by 1.8-3.7 \({}^{\circ}\)C and global precipitation will change greatly by the end of the 21 st century; these variations will influence carbon and water balancing processes in the ecosystem [22]. In addition to the drivers mentioned above, drought is a typical hydmeteorological phenomenon and the main environmental factor that affects the functioning of the ecosystem [23]. Drought events are becoming more frequent and more severe in the context of climate change [24], and these changes have been considered to be a dominant factor responsible for increasing plant mortality [25] and ecosystem degradation [26; 27]. However, the consequences of droughts on ecosystem WUE differ depending on whether they occur in dry or wet zones, the drought severity, and the vegetation type. Although it has been reported that drought can decrease ecosystem WUE [28], other studies reported increases in WUE due to drought [29]. For instance, at the global scale, WUE showed a negative relationship with drought in arid areas, but showed both positive and negative responses in humid areas [30]. Another study reported that the trend of increasing drought resulted in a decrease in WUE in Central Asia during 2000-2014 [31]. It can be concluded that the response of WUE to drought is obviously different in different climate zones. Desert, oasis, and alpine ecosystems are widely distributed in the arid and semi-arid regions in the western part of China [3]. These areas are of vital importance to agriculture management and ecological protection. The Tianshan Mountains (TMS), located in the hinterland of Eurasia, are the main water source in the middle section of the Silk Road Economic Belt. The water circulations of the TMS are characterized by significant spatiotemporal variations, complex water-generation mechanisms, multiple runoff compositions, and fragile water systems. Ecosystem WUE is very sensitive to small changes in hydrological elements caused by climate change and human activity. Therefore, MODIS products were applied to investigate the spatiotemporal variations in the relationship between meteorological factors and the temperature vegetation dryness index (TVDI), and the changes in WUE under different vegetation cover types, in the terrestrial ecosystems of the TMS during 2000-2020. The specific objectives of the present work were to: (1) investigate the temporal and spatial variations in annual NPP, ET, WUE and drought in the TMS in the past two decades and the trends in the annual values; (2) clarify the spatial distribution characteristic in WUE under different land-use types; (3) and investigate the relationship between WUE and meteorological factors (temperature, precipitation, sunshine duration), drought, NDVI, NPP, ET, and WUE during the past two decades.