###### Abstract
The transition from forest to grassland, low vegetation cover, a poor ability to contain water, and unreasonable human deforestation due to farming, and mining [6], makes the Loess Plateau increasingly subject to soil erosion and one of the most seriously affected regions in the world [7]. As an important base for agricultural production and animal husbandry development in China, it is important to manage and protect the ecological environment of the Loess Plateau in a timely manner [8]. Since the implementation of the project of \"returning crop to forest and grass\" in 1999, many policies have been introduced to control soil erosion on the Loess Plateau. After 2000, the Loess Plateau was in the ecological restoration stage. The large-scale process of returning crops to forest and grass areas and the construction of green water and green mountain areas in this phase greatly improved the ecological environment of the Loess Plateau. However, with the excessive restoration of vegetation, a series of ecological problems have also arisen [9], motivating people to examine the problems of inefficient water use and unbalanced resource distribution in the ecological management and governance process. The water-carbon cycle and its interactions change greatly in the context of rapid vegetation changes on the Loess Plateau. Therefore, it is of great importance to investigate the dynamics of the ecosystem WUE on the carbon-water coupling relationship on the Loess Plateau. To further maintain the ecological balance of the Loess Plateau and promote the modernization of the ecological restoration management system and governance capacity for the future, it is important to understand the WUE and carbon-water coupling mechanism. As an important indicator for evaluating drought in ecosystem carbon-water connections, the Crop Water Stress Index (CWSI) typically plays a significant role [10]. According to studies, the GPP and ET processes are impacted by the CWSI, which in turn has an indirect impact on a terrestrial ecosystem's WUE [8]. Numerous research studies have examined the mechanisms and processes of the WUE reactions to water shortages or droughs in the context of climate change [11; 12; 13]. When drought intensity or geographic latitude increase on a worldwide scale, the WUE often declines [14]. However, research has shown that the amount of drought and WUE are typically positively connected. In most of the forest, grassland, and shrub ecosystems, drought events can marginally raise WUE [10; 15]. Furthermore, the response of the WUE to drought is spatially heterogeneous across spatial scales and across ecosystems. Therefore, studying the mechanism of the CWSI's effects on the WUE is important for terrestrial ecosystem evolution and climate change responses. In recent years, many scholars have used remote sensing methods to study plant WUE. Numerous studies have demonstrated that different spatial scales and ecosystems exhibit spatially variable water usage efficiency responses to drought [16; 17]. The WUE often rises when soil moisture levels drop, or the severity of a drought intensifies [18]. Most places throughout the world show a positive correlation between the WUE and drought [19]. Most forest ecosystems in China have considerable tolerance to drought by raising their WUE [20]. It has been demonstrated that ET regulates how water usage efficiency responds to drought in semi-arid and high-latitude locations, while the GPP predominates in tropical forest zones [21]. Therefore, the CWSI plays a role in the WUE, both in GPP and ETpathways. However, in previous studies, it has been difficult to investigate the complex spatial heterogeneity of the Loess Plateau, meaning they could not investigate the carbon-water coupling mechanisms of different vegetation types.