Modelling the hydrological processes of a Chinese lowland polder and identifying the key factors using an improved PHPS model

Improved hydrological processes simulation and identification of determinants are urgently needed in risk management to reduce socio-economic losses resulting from polder water problems. This paper improved the lowland Polder Hydrology and Phosphorus modelling System (called PHPS2.0) by coupling the PHPS, Modified Universal Soil Loss Equation (MUSLE), and Integrated Catchments model of Phosphorus dynamics (INCA-P). Then, based on the PHPS2.0 and data collected from the Jianwei polder in southeastern China, a boosted regression tree (BRT) algorithm was used to explore the key factors determining the temporal variations in polder discharge and phosphorus export. The results showed that PHPS2.0 achieved excellent agreements between the simulated and observed discharge, surface water and phosphorus. The model performance was significantly improved because field-scale phosphorus dynamics were considered. The water and phosphorus balances exhibited substantial seasonal changes, especially in terms of fertilization, crop harvest, and discharge. The temporal change in catchment discharge mainly depended on precipitation and evaporation, which accounted for 63.7% of the discharge variations. In contrast, the temporal variation in phosphorus export was determined by the crop cover and fertilizer application, which accounted for 75.4% of the phosphorus variation. To mitigate the phosphorus loss to downstream freshwater, it is recommended to remain crop residues, reduce the overuse of phosphorus chemical fertilizers, and draft emergency plans. This study expands the knowledge of the relationship between polder environmental factors and hydrological processes and provides guidance for pollution controls.