Coupling mountain and lowland watershed models to characterize nutrient loading: An eight-year investigation in Lake Chaohu Basin

Quantifying nitrogen (N) and phosphorus (P) sources and their contributions at a watershed scale is a primary step to design best management practices. This task is challenging for a mixed watershed with both mountain and lowland areas, because most existing watershed models were developed for mountain areas with clear water flow directions. To solve this challenge, we coupled the Soil and Water Assessment Tool (SWAT) for mountain watersheds, Phosphorus Dynamic model for lowland Polder systems (PDP) and Nitrogen Dynamic model for lowland Polder systems (NDP) for lowland artificial watersheds (i.e., polders) to characterize N and P loading for a typical mountain-lowland mixed watershed, Lake Chaohu Basin in China. The coupled model was calibrated based on eight-year (2011–2018) meteorological, hydrological and water quality datasets, and had a satisfactory performance for both hydrological and water quality variables (R2 > 0.6, NS > 0.6). Our eight-year investigation in the five watersheds of Lake Chaohu Basin revealed that 1) the estimated N and P loading had an annual averaged amount of 10,832 and 781 t/yr, and an intensity of 15.72 and 1.33 kg/ha/yr, respectively. Three watersheds (Nanfei, Pai and Baishitian River Watersheds) had relatively high loading intensity for N (35.16, 18.34 and 13.34 kg/ha/yr) and P (2.49, 1.25 and 2.72 kg/ha/yr), and contributed 67.6% (N) and 68.5% (P) to the loading of the five investigated watersheds. 2) Polders in Lake Chaohu Basin can be N and P sources or sinks for their surrounding rivers, depending on the polder-river interaction. Within Nanfei, Pai and Baishitian River Watersheds, N and P retention ability was higher in polders than that in mountain watersheds. 3) Population density, precipitation intensity and soil erosion were critical drivers of N and P loss and showed positive correlation with N and P loss intensity. This study demonstrated the high value model coupling to characterize N and P loading in a mountain-lowland mixed watershed system, and thus to improve our understanding on its N and P cycling.