Non-point Source Pollution Nitrogen And Phosphorus Load And Its Response To Land Use Change In Danjiang River Basin

The Danjiang River Basin is located at the junction of the three provinces of Shaanxi,Henan and Hubei,upstream of the Danjiangkou Reservoir.With the preliminary control of point source pollution in the river basin in recent years,the proportion of non-point source pollution in the total amount of pollutants in the river basin will gradually increase.Therefore,relevant research on the simulation of non-point source nitrogen and phosphorus pollution loads and their sources is of great importance and significance to the water environment pollution control of the entire river basin and even the Danjiangkou Reservoir.In order to clarify the sources and loads of non-point source nitrogen and phosphorus pollution in the Danjiang River Basin,this paper divides the main pollution sources in the river basin into three categories:land use,rural population and animal husbandry.The Johnes classic output coefficient model was used in the Danjiang River Basin in 2007,2012 and 2017,the TN and TP pollution loads were estimated,combined with land use change amplitude and speed analysis,dynamic analysis,state index analysis and other methods,the trend of land use change in the basin from 2007 to 2017 was analyzed and contributed.Indicators such as the rate and unit contribution rate index further analyze the impact of each land use type on the overall TN and TP.The main results and conclusions of this study are as follows:(1)The non-point source TN and TP pollution loads in the watershed showed a gradual decreasing trend from 2007 to 2017.In 2007,the output of TN and TP in the watershed were 9800.47t and 1301.15t,and in 2012 they decreased to 8584.14t and1203.67t.In 2017,it was reduced to 7828.49t and 1158.96t,and the TN pollution load was about 7 times that of the TP pollution load.(2)According to the actual agricultural farming situation in the watershed,eight land use types including dry land,paddy field,grassland,forest land,water area,unused land,garden land,and construction land in the watershed were selected for analysis,and the TN and TP pollutant loads were simulated.The changes in the period from 2007 to 2017 show that:The TN and TP pollutant loads of non-point sources of different land-use types in the watershed have gradually decreased.The TN load decreased from 4546.08t to 4202.05t,and the TP load decreased from 571.7t to542.12t;the order of TN and TP loads for different land use types is dry land>forest land>paddy field>grassland>water area>construction land>unused land>garden land.The TN and TP load of dry land and paddy field shows a gradually decreasing trend However,the degree of change in pollutant load on forest land and grassland is not large;it shows that cultivated land(dry land,paddy field)is the largest pollution source among the eight land use types,and it can also be seen that the non-point source TN and TP pollution caused by agricultural production is the most serious.(3)The change law of non-point source TN and TP pollution load in the river basin from 2007 to 2017 is a gradual increase.The TN load increases from 1078.21t to 1307.84t,and the TP load increases from 319.64t to 389.71t.The load of TN and TP pollutants produced by various livestock and poultry is ranked as follows:pig>cattle>poultry>sheep,in which the load of pig output accounts for about 50%of the total of these four livestock and poultry species,indicating that pigs are the most pollutant source of TN and TP pollutants among livestock and poultry species.(4)The TN and TP contribution rates of non-point sources in the watershed are:land use>agricultural population>livestock breeding,with contribution rates of49.16%,35.30%,and 15.54%respectively,indicating that land use is the most important cause of non-point source pollution in the DJRB.In summary,cultivated land and pigs are the key sources of TN and TP pollution.(5)Considering that different land use is the main source of non-point source nitrogen and phosphorus pollution load,the mutual conversion between different land use types between 2007,2012 and 2017 is selected.The research results show that during 2007 to 2017,there was a gradual decrease in dry land,from 560.41km2 to520.3km~2,and paddy fields also showed a gradual decrease trend,from 357.76km~2 to308.6km2.Grassland decreased by 170.39km2 in 11 years.Woodland can reduce the loss of nitrogen and phosphorus,thus reducing the non-point source nitrogen and phosphorus pollution load in the watershed.The unit contribution rate index(UCI)can more objectively reflect the impact of land use on TN and TP than the contribution rate.Among them,dry land and paddy fields have a higher impact on non-point source pollution,and other land use types have a lower impact and gap is not big.