Abstract: 【Objective】Clarifying the spatiotemporal changes of water use efficiency (WUE) and its driving forces in the vegetation ecosystem over the Loess Plateau (LP) provide the basis for greater understanding the interaction of ecosystem and hydrology on the LP under the dual disturbance of ecological restoration and climate change.【Method】Using trend analysis and stepwise regression analysis, our study investigates spatiotemporal variation in WUE of vegetation ecosystem and its driving forces on the LP during the period 2000-2014. 【Result】The WUE reveals a significant increasing trend on the LP during 2000-2014 (linear tendency 0.02 gC·kg^-1 H₂O·a^-1, P<0.001). Intra-annual analysis shows that the WUE of different vegetation ecosystems mainly showed a bimodal pattern, which the two peaks appeared in April to May and September to October, respectively. Spatially, the WUE of the LP generally reveals an increasing trend, and the area of increasing trend and significantly increasing trend (P<0.05) accounting for 95.04% and 66.96% of the total study area, respectively, but the patterns of trend in different seasons vary significantly. The mean and trend of WUE among different vegetation ecosystems also vary significantly, among which the mean of WUE of the open shrublands and grasslands are lower than that of other vegetation ecosystems, while the trend of WUE of needleleaf forest decrease significantly. The results of slope statistic show that the WUE of vegetation ecosystem reveals a continuous increase trend in the range of 25°-50°. When annual evapotranspiration (ET) is below 3 700 mm, Gross Primary Productivity (GPP) is positive correlated with ET increase, while when annual ET above 3 700 mm, GPP decrease with the increase of ET. In addition, the WUE of the central and eastern part of the study area is mainly controlled by GPP (57.25%), while ET (42.75%) mainly affects the WUE of the western part of the study area. Moreover, WUE is significantly positive correlated with Leaf Area Index (LAI), indicating the increase of LAI will promote increase in WUE. Stepwise regression analysis shows that precipitation, sunshine duration, and relative humidity are the main climatic factors that lead to the change of WUE and its components in recently 15 years. 【Conclusion】Under the dual disturbance of climate change and anthropogenic activities, the WUE of the LP shows a significantly increasing trend in the past 15 years, and most of the vegetation types show a bimodal pattern within the year. Meanwhile, the ecological restoration project not only improve the vegetation coverage of the LP, but also significantly improve the WUE of vegetation ecosystem of the LP, becoming the main driving forces for explaining changes in WUE on the LP during recent 15 years.

Key words: vegetation ecosystem, water use efficiency, spatiotemporal variation, driving forces, the Loess Plateau