Distribution Characteristics Of Nitrogen And Phosphorus And Estimation Of Output Flux In Tongzi River Small Watershed

The Tongzi River Basin is a typical karst plateau mountainous basin.The river is sensitive to changes in the ecosystem caused by human activities.At present,the urbanization process of riverside towns is accelerating,agricultural activities are expanding,and land use types have changed significantly,directly affecting the water body of Tongzi River.Therefore,understanding the nitrogen and phosphorus output characteristics of the water quality of the Tongzi River Basin is of great significance for preventing and controlling the deterioration of water quality.In this study,samples were collected and tested in the Tongzi River Basin during the wet and dry seasons from2019 to 2020,analyzed the temporal and spatial distribution characteristics of nitrogen and phosphorus in the Tongzi River water body,discussed the main factors affecting the changes in nitrogen and phosphorus concentration,and estimated the output flux of nitrogen and phosphorus in order to provide decision support for the water environment management of the Tongzi river basin.The following main results are obtained:（1）Compared with other major tributaries of the Chishui River,the TN concentration was relatively high,with an average of 3.27 mg/L,and the average TP concentration of 0.11 mg/L.The concentration of TN was the dry season>the wet season,and the TP was the dry season<the wet season.The TN concentration in the dry swim fluctuated from upstream to downstream during the wet season,while the TN concentration in the upper and middle reaches was higher than that in the downstream during the dry season.The overall trend of TP was that the upstream was higher than the middle and lower reaches.Some tributaries had relatively high nitrogen and phosphorus concentrations and poor water quality.（2）The analysis of the temporal and spatial dynamics of the nitrogen and phosphorus concentration and composition of the Tongzi River water body showed that the TN concentration was relatively low in the high water period,indicating that the TN of the Tongzi River was mainly controlled by dilution factors during the high water period,and the NH₄⁺-N/TN ratio significantly increased during the dry season,and TN was enriched during the dry season.The TP was higher in the wet season,PP/TP reached 87.11%in the wet season,and TP was mainly affected by the erosion effect.Concentrations of nitrogen and phosphorus appeared high in the upstream,and the dry season changed more drastically,mainly related to point source pollution such as upstream industrial wastewater and urban domestic sewage.The main tributaries in the middle reaches had a higher TN concentration during the wet season,and the downstream water bodies had increased DTP,which was mainly related to agricultural production activities such as fertilizer application,livestock and poultry breeding,and non-point source pollution caused by rural domestic sewage.After the Tongzi River flows through the hydropower station in the dry season,TN and TP had a downward trend,which was affected by the purification effect of water conservancy projects.（3）Estimated by the output flux formula,the output fluxes of TN and TP of the Tongzi River during the wet and dry seasons were significantly higher than those of other major tributaries of the Chishui River.The estimated annual flux of TN was5003.66 t/a and TP was 129.90 t/a.Through the establishment of the output coefficient model,it was found that the output load of non-point source TN and TP pollution in the Tongzi River Basin from 2000 to 2020 had been significantly reduced,and the cultivated land had the greatest impact on the output load of TN and TP.