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.