Environmental and land-use data collection
The environmental factors, i.e., water temperature (T), electrical conductivity (Cond), dissolved oxygen (DO), pH, turbidity (Turb), total N (TN), NO3--N, O2--N, NH4+-N, and total phosphorus (P; TP), were determined according to the methods of Zhao et al. (2020). Sediment TP and TN were determined using a UV-2450 ultraviolet-visible spectrophotometer (Shimadzu, Kyoto, Japan), and sediment total organic carbon was determined using a TOC-L analyzer (Shimadzu).
Catchment-scale land-use variables, i.e., the area proportions of forest land, shrub land, grassland, agricultural land, urbanized land, open water, and “others”, were obtained at each sampling site using ArcGIS v10.3 (ESRI Inc., Redlands, CA, USA). Each site was delineated using the soil and water assessment tool (SWAT; Jiang et al., 2021), with a spatial resolution of 30 m. The outputs were converted to a basin polygon for each site, which included the entire drainage area upstream of the site. For each sub-watershed, the land-use data used included available Landsat, Sentinel 2, and ASTER images. The images were interpreted and expressed as area proportions of six major land-use types.