Total phosphorus retention characteristics and influencing factors in rivers and reservoirs of phosphorus-rich watersheds

Abstract

[Objective] Phosphorus retention has a significant impact on the environmental security of river and reservoir systems,especially in phosphorus-rich reservoir watersheds. [Methods] The Soil and Water Assessment Tool( SWAT) model was employed to analyze the spatiotemporal distribution patterns of river total phosphorus(TP) retention volumes and retention rates in the Xuanmiaoguan reservoir watershed, located in the upper reaches of the Huangbai River, from 2014 to 2018. Redundancy analysis(RDA) was conducted to perform qualitative analysis of the influencing factors. [Results] The result showed that:(1)the SWAT model effectively simulated TP loads in the phosphate mining area. It achieved Nash-Sutcliffe efficiency(NSE) values of 0. 86 and 0. 82, Percent Bias(PBIAS) of 12. 54% and 13. 42%, Coefficient of Determination(R²) values of 0. 88 and 0. 82,and Ratio of Root Mean Square Error to Standard Deviation(RSR) values of 0. 37 and 0. 42 during the calibration and validation periods, respectively.(2) On multi-year scales, the retention rate of the reservoir(53. 72%) was higher than that of the river(maximum 38. 11%), while the retention rate of tributaries(18. 88%) exceeded that of the main stream(8. 46%). However,at smaller time scales( annual or monthly), the retention process exhibited considerable variability and may even exhibit a “source” effect under certain conditions.(3) Both river retention rates and volumes showed significant positive correlations with slope length, while the monthly reservoir retention rates were positively correlated with inflow volumes and inflow-to-outflow ratios. [Conclusion] The result indicate that a large amount of phosphorus loss from phosphate mining watersheds tends to be retained within river and reservoir systems, which may serve as a new phosphorus source under specific conditions. Measures should be implemented to mitigate internal phosphorus pollution. These research findings provide valuable references for phosphorus pollution control in phosphorus-rich watersheds.

Keywords

phosphorus-rich watershed / phosphorus retention / phosphorus loss / reservoir watershed / SWAT model / influencing factors