# Water Use Efficiency Spatiotemporal Change and Its Driving Analysis on the Mongolian Plateau
## 1 Introduction  
Global warming, caused by human activities, is becoming increasingly serious, and the global water cycle is intensifying [1, 2]. Water shortages are a serious problem in arid and semiarid areas [3, 4]. Owing to the progress of civilization and the intensification of human activity, speeding up the greenhouse effect, all countries experienced a water crisis at the beginning of the 21 st century. This crisis was particularly prominent in arid and semiarid areas, such as the Mongolian Plateau. As an important indicator, the water use efficiency (WUE, gC/mm\(\cdot\)m\({}^{2}\)) can be used to understand the relationship between water and the carbon cycle in ecosystems, as it reveals the balance between carbon storage and water loss [5, 6]. Determining the spatiotemporal variation characteristics and control mechanisms of the WUE of regional-scale ecosystems helps to evaluate and predict the response of an ecosystem's carbon-water cycle to global changes. At the ecosystem scale, water use efficiency is generally defined as the ratio of carbon storage (such as net primary production, NPP, or gross primary production, GPP) to water loss (such as transmission, T, or evapotranspiration, ET) [7, 8]. On a large regional scale, a model integrated with remote sensing data is usually used to calculate the productivity (GPP, NPP) and ET to estimate the WUE, which is calculated as the ratio of the two (defined as WUE = NPP/ET in this study) [9, 10]. Recently, research on WUE has attracted much attention, especially the correlation between water use efficiency and meteorological variables and technology, seeking to improve vegetation's WUE [10, 11, 12, 13]. Previous studies have shown that the rise in carbon dioxide concentrations in many areas has led to an increase in the utilization rate of water resources [14, 15, 16]. Simultaneously, owing to the increase in the aerosol load in global terrestrial ecosystems, the WUE continues to increase every year [8]. Low precipitation in the arid and semiarid areas of the Mongolian Plateau has led to perennial droughts, whereas the intensification of climate change has rendered already fragile ecosystems more vulnerable to damage. Therefore, it is imperative to study the long-term spatiotemporal dynamics of water resource utilization efficiency in the Mongolian Plateau and explore the impact of the unique climate of the Mongolian Plateau on this efficiency. However, flux observation stations and model construction are unsuitable for large-scale WUE research. For example, numerous scholars have conducted research on WUE using flux observation data; however, these studies are typically limited to small spatial scales [17, 18, 19]. Additionally, some researchers have employed models to simulate the GPP and ET to investigate long-term and large-scale WUE [20]. However, these studies are often accompanied by inherent uncertainties within the models themselves [21, 22, 23]. Fortunately, low-cost satellite remote sensing can be used to monitor the biological characteristics of various ecosystems. The latest-generation GIMMS NDVI (INDVI B v1.0) has been proven to have the highest time consistency and provides the NDVI with the longest time scale (>30 years) [24, 25, 26]. Moderate-resolution imaging spectrometry (MODIS) provides the mod13 NDVI product and has a high spatiotemporal resolution. This instrument has greatly promoted research on the regional-scale WUE, and, in the past, remote sensing data have been effectively used to study water use efficiency [27, 28]. However, research on the water resource utilization efficiency at the ecosystem scale is still in the primary stage, and the process of quantifying the water resource utilization efficiency using remote sensing data remains insufficient. In the past, most of the factors considered to affect WUE were meteorological factors. However, WUE is affected not only by the climatic environment but also by vegetation changes.