Study Of Non-point Source Pollution In Weishui Watershed Based On HSPF Model

Water resources are the material basis for human survival and development and an important guarantee for sustainable development.Water pollution is the main reason for the shortage of available water resources.In recent years,with the new urbanization construction and water environmental protection awareness increasing,sewage treatment is standardized and point source pollution discharge is controlled to some extent.However,eutrophication and deterioration of water quality caused by non-point source pollution is becoming more and more prominent.Weishui watershed,located in Ningxiang City,is an important water source for agriculture,industry and drinking water.This paper applies the HSPF semi-distributed hydrological model to simulate the hydrology and water quality of Weishui basin,then verify the applicability of the model in this basin.This paper analyzes the spatial and temporal distribution characteristics of pollutants and the driving factors with the help of the model,further constructs three future scenarios based on the land use prediction model to analyze the changes of source pollution load under different scenarios,so as to provide references for the planning of the basin and the protection of important water sources.The main conclusions are as follows.(1)Based on the satellite remote sensing images to decode the information of land use types in different periods and analyze the change trends in the watershed.The results show that forest land is the main land use type in Weishui watershed,followed by cropland,the conversion of cropland,forest land and construction land is the most frequent,the conversion of cropland to construction land is the most intense,and the trend of construction land is growing continuously.The CA Markov land use prediction model of Weishui watershed is constructed and validated with Kappa=0.90,indicating that the model has high accuracy and can be further used for future land use scenarios.(2)The HSPF watershed hydrological model is constructed based on the hydro-meteorological data of Weishui watershed and the actual measured runoff data of Ningxiang control station(Ⅲ),and the applicability of the model is evaluated as well.The results show that the relative margin for error of total annual runoff rate period is 4.45%,and the relative margin for error of total annual runoff in the validation period is-8.14%.The rate period of NSE in month scale is 0.90 and R2 is 0.95,and the NSE of the validation period is 0.82 and R2 is 0.84.The parameters and simulation results meet the model accuracy requirements,indicating that the HSPF model can be used for the simulation of hydrology in the Weishui watershed.(3)The HSPF water quality model of the watershed was constructed and the results showed that the NSE and R2 of ammonia nitrogen and total phosphorus rates are all over or equal to 0.6 for both rate and validation periods,indicating that the HSPF model is suitable for hydrological water quality simulation in Weishui basin.The most remarkable loadings of ammonia nitrogen in Weishui basin in 2020 are sub-basins 5 and 38,with loads accounting for 19.58%of the whole basin,and the largest loadings of total phosphorus are in sub-basin 5,with loads accounting for 8.50%of the whole basin.Overall,the pollution loads and generation of sub-basin 5 and 38 are higher.(4)Based on the land use prediction model and water quality model,this paper simulates the conversions of pollution loads under the future land use scenarios in Weishui watershed in 2035 and 2050.Compared with 2020,due to the reduction of cropland area,the decrease rates of ammonia nitrogen rate and total phosphorus loadings in 2035 are 0.46%and 0.75%respectively,and the decrease rates of ammonia nitrogen and total phosphorus loadings in 2050 are 1.25%and 2.48%respectively.Besides,further build three cropland integration models to explore the changes of pollutant loadings in different spatial layouts of croplands.The results show that with the 2050 base case,the integration of cropland and concentrated farming in the downstream cultivation area of Weishui near the urban area will increase the total export pollutant loads,while the integration of cropland and concentrated farming in the upstream cultivation area of Weishui and the cultivation area of Wujiang,a tributary of Weishui where far from the urban area,will reduce the ammonia nitrogen and total phosphorus pollutant loads to a certain extent.The most prominent reduction of ammonia nitrogen and total phosphorus pollutant loads at the total outlet of the watershed is achieved by integrating scattered farming areas in the cultivation area of Wujiang River,a tributary of Weishui River,which is far from urban areas.