Spatiotemporal Changes in the Water Conservation Function of the Three-River Headwaters Region and Their Environmental Associations, Considering Frozen-Ground and Snow Processes

Water conservation is a key regulating service provided by terrestrial ecosystems, and its variation directly affects regional water security and ecosystem health. Existing studies of water conservation have focused largely on quantitative assessment, whereas its qualitative connotation remains unclear. The Three-River Headwaters Region (TRHR), which gives rise to the Yangtze, Yellow, and Lantsang–Mekong rivers, is critical to the water supply of several hundred million people downstream, so an accurate assessment of its water conservation function is essential. However, conventional assessment methods are systematically biased in cold regions because they do not adequately account for the low permeability of frozen ground or the seasonal storage and release of snow. Here we define the water conservation function of the TRHR in both quantitative and qualitative terms (water conservation volume and water conservation capacity). On this basis, a revised InVEST Seasonal Water Yield model was selected and its parameters localized, and the model was further validated using isotope-based runoff source analysis results. The results show that (1) from 1981 to 2020 the mean annual water conservation volume of the TRHR was 123.4 mm (451.6 × 108 m3), increasing at 0.6 mm/a overall, with areas of increasing and decreasing trends accounting for 63.5% and 36.5% of the region, respectively; (2) water conservation capacity (mean 0.79) first declined and then recovered, was lower in the north and higher in the south, and declined mainly in seasonally frozen ground; and (3) water conservation volume was positively regulated by precipitation and negatively by evapotranspiration. Water conservation capacity varied with air temperature (positive below 0 °C and negative above 0 °C), elevation (negative), NDVI (positive), and freezing depth (negative), and ranked by land cover as marsh > forest > high/medium/low-cover grassland > cropland > bare land > desert. This framework can serve as a reference for assessing the water conservation function of the TRHR.