## 4 Discussion  
In this study, we analyzed the spatial and temporal variations in vegetation WUE in Gansu Province from 2000 to 2020 and observed a general decreasing trend. The southern part of the study area includes diverse ecosystems, including forests, undergrowth, and grasslands, which benefit from a balanced river network and high soil moisture levels. These conditions enhance carbon sequestration, contributing to higher overall WUE. In contrast, the Hexi Corridor Plain in central Gansu, known for its developed agriculture, exhibits relatively high GPP and ET values. Gansu Province is situated in the arid and  
Figure 10: Importance of climatic environmental factors in vegetation WUE in Gansu Province. semi-arid region of northwestern China, where groundwater resources are limited and vegetation is sparse. Precipitation is the primary water source, playing a crucial role in sustaining vegetation growth. Among the five vegetation types in Gansu Province, the highest values of WUE were distributed in the Longnan Mountains in southeastern Gansu, and the lowest values were distributed in the grasslands in the desert area of the west corridor of Gansu in Sunan Yugur Autonomous County in Zhangye City. Abundant precipitation and runoff in the Longnan Mountains support vegetation growth and development, and the dense vegetation cover allows for greater dry matter accumulation per unit area, thereby increasing GPP. This is the primary reason that the WUE of forests is higher than that of other vegetation types. The forest WUE in Gansu Province was the highest (1.90 gC\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\)) and was close to the annual mean value of forest WUE in the Shiyang River Basin (1.43 gC\(\cdot\)mm\({}^{-1}\cdot\)m\({}^{-2}\)) calculated by [PERSON] et al. [24]. The WUE of cultivated vegetation was second only to that of forests but higher than that of grasses, which is consistent with the findings obtained by [PERSON] et al. [36]. Grasslands presented the lowest WUE in the study area due to their weak photosynthetic capacity and low water requirement. Northwestern Gansu Province is a representative arid and semi-arid region, where the soils are consistently dry and lacking in water, exhibiting a relatively hard texture. Precipitation is generally low and unevenly distributed, making efficient absorption difficult, which results in limited or even unchanged evapotranspiration rates. Furthermore, the combination of high temperatures and elevated VPD contributes to lower humidity in the region. Even with increased precipitation, vegetation tends to close its stomata under dry and hot conditions to minimize water loss, leading to a reduction in transpiration, which in turn constrains the potential increase in evapotranspiration. In most of the northwestern part of the study area, the interannual fluctuation in vegetation WUE was affected by precipitation to a large extent, which is consistent with previous results [37]. The positive response to precipitation in most areas of northwestern Gansu was attributed to the fact that while evapotranspiration remained constant, the increase in precipitation affected the WUE of vegetation by enhancing its GPP. However, this response was negative in the central irrigated farmland, which presents mainly cultivated vegetation and is strongly influenced by human activities, and where the effect of climatic factors on vegetation WUE may be weakened [38]. The positive correlation between vegetation WUE and air temperature was mainly distributed in the area where the mean annual air temperature is higher than 10 \({}^{\circ}\)C, and the mean annual air temperature is in the range of 0\(-\)\(-\)\(5\,^{\circ}\)C in southeastern Gansu. The main reason for this is that when the air temperature is lower than the optimal value for vegetation photosynthesis, an increase in temperature induces an increase in the plant photosynthesis rate and dry matter production, which in turn increases the efficiency of water utilization [39].