## 1 Introduction  
Global climate changes have increased the imbalance of water resources [1], especially in arid regions. The planting of vegetation is regarded as one of the most powerful approaches for combatting global warming [2]. However, newly planted vegetation needs more water to grow [3] while storing carbon and providing biofuel to contribute to climate change mitigation [4], which has a significant impact on the ecological sustainability in arid and semiarid regions. The 2030 Agenda for Sustainable Development set by the United Nations through the Sustainable Development Goals (SDGs) defined the sustainable development of terrestrial ecosystems (SDG15) as one of the important objectives [5]. The large-scale ecological restoration programs (e.g., afforestation, revegetation) implemented in the arid and semiarid areas of northern China are sustainable for ecological development, but there is still controversy [6; 7; 8] regarding the adverse effect of the excessive water consumption. The ecosystem water use efficiency (WUE) is defined as the ratio of the gross primary productivity (GPP) to the evapotranspiration (ET) [9; 10; 11] and is one of the indicators that comprehensively reflects ecological sustainable development [12]. WUE is often used to explore the resilience of vegetation ecosystems to drought [13; 14], as it is a key parameter that measures the exchange between the carbon gain and water loss of terrestrial ecosystems [15]. It provides an important basis for promoting sustainable management of regional vegetation ecosystems [15; 16; 17; 18]. Although many environmental factors have a significant impact on WUE, plants with high WUE may also likely place severe stress on regional water resources [11]. Therefore, quantifying the spatiotemporal variations in WUE and its drivers is crucial for understanding the relationship between ecosystem restoration and water resource shortages [17; 18; 19; 20]. Climate factors, including temperature, precipitation and solar radiation, have significant effects on the WUE in an ecosystem [17; 18]. However, the effects of these factors on the WUE are recognized differently at different spatiotemporal scales [14; 21; 22]. The estimation of the WUE with a process-driven model based on the GPP and ET suggested that the WUE was negatively correlated with the mean annual precipitation and temperature in China over the last three decades [18]. A study that employed long-term measurements of carbon and water fluxes at sites in a forest of the Northern Hemisphere identified a large increase in the WUE during the past two decades, which coincided with an increase in the atmospheric CO\({}_{2}\) from 350 to 400 ppm [22]. The flux measurements from sites showed that the response of the WUE to drought across global ecosystems corresponded well with the climatic zones [14]. The Moderate Resolution Imaging Spectroradiometer (MODIS) GPP and ET products are some of the most widely used remote sensing data in studies of ecosystem WUE [10; 23]. The effects of meteorological drivers on the global patterns of WUE were nonmonotonic based on the MODIS GPP and ET data over the period from 2000 to 2013 [23]. However, studies on the WUE over long time series and large spatial ranges based on remote sensing data are still insufficient. The time range of the Global Land Surface Satellite (GLASS) GPP and ET products is from 1982 to 2015, and it is currently one of the longest remote sensing product sources [24; 25]. Based on the GLASS GPP and ET data, this study explored the spatiotemporal changes in the WUE and the relationship between the climate factors and the ecosystem WUE. The WUE is sensitive to changes in human activities such as land cover changes and land management practices [26]. On agricultural land, the crop yield could be improved by anthropogenic activities such as irrigation, fertilizer application, and tillage because these land management practices modify the carbon and water cycles to improve the WUE [27].