Evaluation and trade-off and synergy relationship of ecosystem services in a typical small watershed in the Loess Area of Western Shanxi

The Loess Plateau is located in the heart of the Yellow River basin． Quantifying its ecosystem services is therefore essential for ecological protection and high-quality development in the region． This study examines the Caijiachuan watershed in the Western Shanxi Loess region, using land use, soil, topographic, and meteorological data to analyze land use changes since the Grain for Green Program was implemented． Using the InVEST model, we quantitatively evaluated four key ecosystem services—water yield, soil conservation, carbon sequestration, and habitat quality—from 1990 to 2020, and applied trade-off and synergy criteria and trade-off and synergy index to assess their spatiotemporal interactions． The results indicate that: 1) Forestland area has expanded consistently since the program’s launch, mainly through conversion of grassland and farmland． Its share of total area rose from 57.31% in 1990 to 73.67% in 2020, with converted areas accounting for approximately 14.65% of the watershed． 2) Water yield and soil conservation showed marked interannual variability, while carbon sequestration and habitat quality increased steadily． 3) A significant and spatially homogeneous trade-off and synergy relationship was observed between water yield and soil conservation, whereas interactions among other services exhibited strong regional heterogeneity． These results offer valuable insights for integrated watershed management and high-quality development in ecologically vulnerable areas of the Loess Plateau．