A novel framework for tracking hydrological processes and identifying key factors in mountain-lowland mixed catchments: Implications of forty years of modeling for water management.

Quantitative analysis of runoff, total suspended solids, and total nitrogen dynamics, along with the identification of key factors within catchments, is essential for accurately addressing issues related to turbid and polluted water. Nevertheless, their implementation encounters significant challenges when applied to a mixed catchment containing mountain areas and lowland polder regions, due to the highly heterogeneous hydrological behaviors and consequently the lack of an appropriate approach. Faced with this problem, this study developed a framework by coupling the Soil and Water Assessment Tool (SWAT) and improved Polder Hydrology and Nitrogen modelling System (PHNS), and Random Forest analysis method to track the spatio-temporal changes in runoff, total suspended solids, and total nitrogen loading and identify their environmental determinants in a representative mountain-lowland mixed catchment, southeastern China. The coupled model performed very well for runoff (R≥0.90) and water quality variables (total suspended solids: R≥0.88; total nitrogen: R≥0.73) in both the calibration and validation periods, and showed improvements compared with standalone SWAT model. Forty years' modelling results indicated that the upstream subbasins 15 (32.86 tonnes/ha/yr), 14 (33.96 tonnes/ha/yr), and 11 (32.32 tonnes/ha/yr) were the critical source areas for total suspended solids and total nitrogen. However, the downstream polder subbasins functioned as a sink for runoff, total suspended solids, and total nitrogen, exporting lower loading intensities. Precipitation and the proportion of slope of 0 to 30° were identified as the critical factors influencing runoff, total suspended solids, and total nitrogen. The proportion of water area also significantly, negatively influenced runoff and total suspended solids. This study provided a feasible method to investigate runoff, total suspended solids, and total nitrogen processes and their environmental factors' impact, and thus identifying the critical source areas and targeted measures to control the non-point source pollution of mountain-lowland mixed catchments.