Land-use Change And Response Of Agricultural Non-point Source Pollution In The Upper Reaches Of Nanliu River Basin Based On SWAT Model

In recent years,the problem of water environmental non-point source pollution（NPS）has become increasingly serious.Guangxi has begun to carry out comprehensive ecological improvement in the major basins,among which Nanliu River Basin is an important part of deepening the comprehensive ecological environment control in Beibu Gulf.Due to the rapid development of urbanization in Yulin section of the upper reaches of Nanliu River Basin,irrational land use has intensified agricultural non-point source pollution,which has greatly threatened the ecological environment security of the basin.Based on this,the Yulin section of the upper reaches of the Nanliu River Basin was taken as the research area where combined with GIS technology to analyze the land-use change.By the way,established a SWAT semi-distributed hydrological model suitable for the research area to explore the response mechanism of agricultural non-point source pollution in different land-use types,and the spatial and temporal distribution characteristics of nitrogen and phosphorus pollution in the Nanliu River Basin,which has important reference value for land-use planning and sustainable ecological development in the basin.It also provided an theoretical basis and suggestions for the follow-up treatment of non-point source pollution in the basin.The main results as followsing:1.Based on the analysis of land-use change in Yulin during recent 10 years（2010-2020）,it could be seen that:From the perspective of land use area,the area of forest-land remained the largest,with an average area and proportion of8028.54 km²,63.87%;followed by farmland,with an average area and proportion of 3928.98 km²,31.86%respectively.From the perspective of increase-decrease rate in the past ten years,the growth rate of construction land was relatively obvious,with an annual change rate of 13.51%;the decrease of farmland was the largest,with a reduction area of 341.27 km²,and the annual change rate was-0.83%.The changes of forest-land and farmland were more drastic,whose dynamic degree was 4.00%and 2.10%,respectively.2.SWAT model was constructed in the research area,according to the result of model calibration and verification which based on monthly measured datas of runoff,sediment load,the concentration of ammonia nitrogen and total phosphorus（TP）of Hengjiang and Bobai hydrological sites,it could be seen that:the correlation coefficient（R²）and Nash efficiency coefficient(E_ns)were above0.6,it had a good simulation precision.the SWAT model could be used in the research area and subsequent simulation research.3.Based on the simulation results of SWAT model,the spatial and temporal distribution of nitrogen and phosphorus pollution input in the basin,it could be seen that:In terms of time,the pollution loads showed seasonal fluctuations.The input of nitrogen and phosphorus increased sharply in spring and summer（March to May）each year,and the input in autumn and winter decreased significantly,with large differences in annual load.In terms of spatial distribution,nitrogen and phosphorus pollution input were serious from the middle reaches of the basin to the lower reaches of the river.Sub-basins 17,20,23,25 and 33,located near the intersection of Yulin city and Bobai County,as well as most areas of Bobai County,were the key sources of nitrogen and phosphorus pollution.The input of nitrogen and phosphorus pollution in the upper reaches of the river was less and the pollution was lighter.4.According to the SWAT model,the key land-use types of watershed pollution were identified and it could be seen that:From 2010 to 2020,farmland and construction land in the Yulin section of the upper reaches of the Nanliu River Basin were the main sources of non-point source pollution in the basin,and the contribution rate of total nitrogen（TN）in farmland and total phosphorus in construction land reached more than 60%.The average total amount of total nitrogen in the entire study area was 385.10 t/a,and the total phosphorus was1.68 t/a;the average unit area generation intensity of total nitrogen was 50.60kg/（km²·a）,and the total phosphorus generation intensity per unit area was 0.22kg/（km²·a）.The higher output of total nitrogen load was concentrated in the northeastern and central regions of the basin,and the higher output of total phosphorus was in individual sub-basins at the central boundary of the basin.From 2010 to 2020,the non-point source pollution in the whole basin generally showed a decreasing trend.Compared with the total phosphorus pollution,the total nitrogen pollution occurred in a more intensive and wider range.Based on the occurrence regulation of non-point source pollution in this research basin,the agricultural non-point source pollution control direction in the basin in future was clarified.It was suggested that the prevention of non-point source pollution in the basin should focus on the prevention at the source,especially focus on the changes of nitrogen and phosphorus content during spring and summer as well as the content changes in the water of Yulin City-Bobai County in the middle and lower reaches of the basin,strengthen the control of agricultural source pollution discharge and the rational application of chemical fertilizers and pesticides to promote ecological sustainability of land-use development.