Regional variations in vegetation greening and climate change impacts on gross primary productivity and evapotranspiration in the Loess Plateau

The Loess Plateau is vital to China's grain and energy production. Understanding the spatio-temporal variations and driving factors of gross primary productivity (GPP) and evapotranspiration (ET) in this area is crucial for optimizing ecological restoration strategies and effectively managing water resources. However, the impacts of land use types and flux changes driven by land use transitions and ecological restoration policies across regions remain unclear. Using a model based on remotely sensed vegetation indices and multi-period land use data, we estimate the spatial distribution and trends of GPP and ET from 1982 to 2017. Factorial simulations quantified how climatic factors and vegetation changes directly affected GPP and ET, and assessed contributions from land use and flux changes. The results indicate that the average annual GPP and ET for the entire plateau are 430.5 ± 58.7 g C m^-2 yr^-1 and 359.2 ± 29.4 mm yr^-1, respectively, with soil evaporation accounting for the largest proportion of ET (42.7 %), particularly in Sandy land region (I) (58.9 %). Meanwhile, both GPP and ET have significantly increased at rates of 5.3 g C m^-2 and 2.3 mm annually. The increase in ET is mainly driven by vegetation transpiration, especially in Loess hilly and gully region (II) and River valley plain region (III), while soil evaporation has slightly decreased. Vegetation greening is the dominant factor influencing GPP and ET changes, particularly after 2000. Climate factors, including precipitation and temperature, also contribute to these increases, but their influence is comparatively smaller. Further analysis reveals that flux changes in grasslands and croplands are dominant drivers of increased total GPP and ET across the Loess Plateau, with regions I and II contributing the most to these increases, particularly in the recent decade. This research provides valuable insights into optimizing ecological restoration strategies and balancing carbon-water dynamics in arid and semi-arid regions.