## 1 Introduction  
The water use efficiency (WUE) plays a crucial role in energy transfer and water movement, serving as a vital link among the interactions of the geosphere, biosphere, and atmosphere [1, 2, 3]. WUE serves as a metric for assessing carbon-water interactions within ecosystem vegetation; using Gross Primary Productivity (GPP) and Evapotranspiration (ET) data to study WUE is a common method in the fields of ecology and geography [4, 5]. In natural ecosystems, WUE significantly affects plant distribution, ecosystem structure, and the dynamics of both carbon and water cycles [6, 7, 8].  
With the intensification of global climate change, precipitation patterns are becoming increasingly erratic, and the frequency of droughts is increasing [9, 10]. Enhancing WUE in crops and ecosystems is essential for optimizing water resource allocation and utilization, thereby promoting sustainable management in the face of dwindling water resources [6, 7, 11]. WUE can effectively reflect the variations and interactions of carbon and water cycles [12, 13]. Consequently, investigating the spatiotemporal variations in ecosystem WUE and its underlying factors will advance our understanding of the carbon cycle. This research will also offer valuable insights for the optimal management of regional water resources and ecosystem services [14, 15, 16].  
In recent years, the escalating effects of climate change have become increasingly apparent [17, 18, 19]. Research on the WUE in terrestrial ecosystems is providing new insights for mitigating greenhouse gas emissions and addressing the challenges associated with climate change [20, 21, 22, 23, 24]. [PERSON] et al. [25] found that WUE was higher in semi-arid regions compared to humid areas, indicating that temperature and precipitation significantly influence changes in ET and WUE. [PERSON] et al. [26] showed that both GPP and WUE exhibited significant increasing trends, with rates of 10.53 gCm\({}^{-2}\) yr\({}^{-1}\) and 0.01 gC\(\cdot\)kg\({}^{-1}\) H\({}_{2}\)O yr\({}^{-1}\), respectively. Furthermore, GPP played a crucial role in WUE changes, accounting for over 79.51% of the total area. [PERSON] et al. [4] observed a declining trend in vegetation WUE in Xinjiang from 2000 to 2014, with temperature having a significant impact on WUE (87.7%). [PERSON] et al. [2] identified the Leaf Area Index (LAI) as the primary factor influencing variations in grassland WUE on the Loess Plateau. [PERSON] et al. [16] observed that the rate of increase in vegetation WUE was highest in the water-scarce regions of the Loess Plateau, with grassland WUE and its growth rate surpassing those of other ecosystem types. Climate change threatens ecosystems and the species; the WUE research is more necessary, as it offers a scientific foundation for the advanced development of ecological systems and the creation of strategies for water resource management.  
The Yellow River Basin (YRB) extends throughout China's dry and semi-arid regions, where the ecological environment is fragile and sensitive to climate change [27, 28]. For a long time, water resource issues have been a critical factor hindering the ecologically sustainable development of the YRB [29]. WUE is a key indicator of how terrestrial ecosystems respond to both climatic and non-climatic factors [30, 31]. However, the trends in WUE within the YRB and its responses to major climatic factors remain unclear. Moreover, there is a shortage of research on the differences in water use efficiency variations before and after the Returned Farmland to Forestry and Grassland Project in 2000 [26, 32, 33]. Therefore, investigating the characteristic changes in WUE and its primary influencing factors across different time periods has become an urgent necessity for ecological protection and water resource management. This research offers valuable recommendations for promoting high-quality development and biodiversity conservation.