Simulation Of Runoff And Nitrogen And Phosphorus Pollution Load In Typical Headwaters Of Poyang Lake

Under the dual influence of climate change and human activities,China is faced with serious water pollution,and water environment has become a key factor affecting the stability of lake ecosystem.As one of the most important ecological sensitive areas in the world,Poyang Lake is facing the threat of eutrophication.It is of great practical significance to study the temporal and spatial distribution characteristics of nitrogen and phosphorus and predict the future trend of nitrogen and phosphorus in poyang Lake watershed.This paper takes the Lianhua River Basin located in the headwaters of the Poyang Lake Basin as the research object.Firstly,Mann Kendall method and wavelet analysis are used to analyze the change trend and oscillation period of climate and hydrology in the study area under long-time series,and the characteristics of nitrogen and phosphorus pollution load in the basin are analyzed based on the measured water quality data.Then,the SWAT model is used to simulate the watershed runoff,and the simulation results are combined with the improved output coefficient model to analyze the spatial and temporal distribution characteristics of nitrogen and phosphorus pollution in the watershed on the basis of verifying the feasibility of the output coefficient model.Based on the land use data of the simulated watershed by CA-Markov in 2030,the response law of total nitrogen and total phosphorus pollution load to climate and land use changes in the watershed was clarified.The main conclusions are as follows:(1)From 2008 to 2018,the runoff in this basin showed an upward trend,but the overall upward trend was not significant at theα=0.05 level.From 2012,the runoff entered a stage of gradual increase,and the oscillation period ranged from 256 days to512 days.(2)The average mass concentrations of total nitrogen and total phosphorus in the basin are 0.7 mg/L and 0.1 mg/L during September 2019 to August 2021,the daily precipitation has no obvious correlation with total nitrogen,but has a very significant positive correlation with total phosphorus.The total nitrogen content was low in autumn and high in spring,summer and winter;total phosphorus content was highest in summer,followed by spring,and lower in autumn and winter.The ratio of nitrogen to phosphorus(total nitrogen/total phosphorus)in the basin ranges from 0.2 to 150.0.Most of the time,the phosphorus load is too high,a few times are suitable for the normal growth of algae,and few times are phosphorus limited.The concentrations of total nitrogen and total phosphorus in the watershed were higher in the rainy period than in the non-rainfall period;the average concentrations of the two were relatively the lowest under the light rain event,but the average concentration of total nitrogen was the highest under the moderate rain event.The average concentration of total phosphorus was the highest under the rainstorm event.(3)The total nitrogen loads in the basin from 2018 to 2020 were 282.3,460.0,and394.7 t/a,respectively;the total phosphorus loads were 34.4,52.0,and 45.4 t/a,respectively.From 2018 to 2020,the average load intensity of total nitrogen and total phosphorus in each sub watershed of Lianhua basin was 5.6 kg/ha and 0.3 kg/ha respectively,which showed that the southeast of the basin was higher than that in the central and western regions The total nitrogen and total phosphorus output load of land use in the basin accounted for 53%and 29%of the total load of the basin respectively.The total nitrogen and total phosphorus load both showed cultivated land>forest land>construction land>grassland>water area.The total nitrogen load intensity is construction land>water area>cultivated land>grassland>forest land,and the total phosphorus load intensity is cultivated land>construction land>water area>grassland>forest land.The contribution rates of rural life to total nitrogen and total phosphorus were 22%and 19%respectively.The contribution rates of livestock and poultry breeding to total nitrogen and total phosphorus were 25%and 52%respectively.(4)Compared with the reference period(1961-1990),the annual average minimum and maximum temperature of the basin will increase by 1.4°C and 1.5°C in the future period(2021-2050),and the multi-year average rainfall in the simulation period is 3%higher than that in the reference period.The variation range of monthly rainfall is-16%～43%.By the middle of the century,the average annual runoff will increase by about 3%,with the maximum and minimum runoff occurring in 2040 and2027,respectively.(5)Compared with 2020,the area of forest land and water area in Lianhua basin will decrease in 2030,and the area of cultivated land,grassland and construction land will continue to increase,the transferred area mainly comes from forest land.The area of cultivated land,forest land,grassland,water area and construction land will reach152.1,331.6,26.8,1.4 and 31.9 km~2 respectively.Due to the dual impact of climate change and land use,the total nitrogen load of the basin will increase by 9%in 2030compared with 2020.Land use,rural sewage,livestock and poultry breeding account for 55%,20%and 25%respectively.The total phosphorus load will increase by 9%,and land use,rural sewage,livestock and poultry breeding account for 31%,17%and52%respectively.