Modelling the impact of runoff generation on agricultural and urban phosphorus loading of the subtropical Poyang Lake (China)

摘要

Water quality degradation and eutrophication in surface water bodies caused by excessive phosphorus loads from agriculture and urbanisation are widespread problems. Although phosphorus (P) is often the limiting factor of aquatic ecosystems, only few studies have characterised the primary factors that influence P export in subtropical monsoon catchments. This study aims to assess the temporal and spatial variations in P fluxes, and to investigate the factors controlling P export in a subtropical catchment. Runoff and P export in the Le An River catchment, which flow to Poyang Lake (China) were evaluated using the HYPE model. Sensitivity analysis and calibration of hydrological and P transport parameters were undertaken using PEST. Results show that: (a) HYPE reproduced sufficiently well (NSE ≥ 0.73, PBIAS ≤ 14.5) the stream flow dynamics for widely varying climatic conditions and across six sub-catchments of contrasting physiographic characteristics; (b) HYPE captured intra-annual patterns of total P (TP) loads, although with overestimation of annual TP loads by 8.5-47.0 across two monitoring sites; (c) TP loads had the ranges between 0.24 and 0.88 kg/ha/yr and 4.31-4.69 kg/ha/ yr from forest-dominant upstream areas and the downstream plains region, respectively, with the latter being more urbanised and having higher percentage of agriculture; (d) Soil erosion from surface runoff and adsorption/ desorption of soil P are the main factors controlled P export from the catchment; (e) most P export occurred during March-August when rainfall-runoff is highest and agricultural practices are most active; (f) stream TP concentrations were greatly affected by point source inputs, especially during low-flow conditions in downstream plains region. This study indicates that measures to reduce TP export to receiving water bodies in subtropical monsoon areas, like Poyang Lake, should focus on managing diffuse sources from soil erosion and surface runoff etc., whereas improvem