Study On Non-point Source Pollution In The Western And Northern Regions Of Erhai Basin Using SWAT Model

Non-point source pollution plays an important role on lake eutrophication in China.Erhai Lake is on the early eutrophication state and non-point source pollution is main pollution source.The landform of Erhai Lake basin is very complicated.There are large mountains arround and several narrow or small plains between the mountains and Lake Erhai.Many short and small rivers and ditches which connet to reservoirs or irrigation facilities flow through the plains.To study the nitrogen and phosphorus pollution characteristics of surface runoff and the land use impacts on the runoff water quality,the nitrogen and phosphorus morphological characteristics of surface runoff in rainy season at western Erhai Lake basin was investigated,and the relationship between the land use patterns and water quality was also studied from the catchment and buffer scales.Hydrological and water quality in north and west watersheds were studied applied to the SWAT model,and temporal and spatial distribution of non-point source pollution and land use impacts were also analyzed.The main results were shown as follows:(1)The average concentration of ammonia-nitrogen in the dry season was higher than that in the rainy season.The average concentrations of total nitrogen and total phosphorus were not significantly different in the dry and rainy seasons.The average order of total nitrogen concentration was in the order of eastern>west>north.The 88.17% of the total phosphorus load was concentrated in the western basin in the rainy season.(2)Urban land use at the catchment scale in the western basin has a prominent effect on the total phosphorus concentration.Highly fertilized farmland has a great effect on total nitrogen and nitrate nitrogen.With increasing distance from the lake,the impact of land use types on surface runoff water quality decreases.(3)In 2015,the load of total nitrogen and total phosphorus into lake from the west were 419.26 t/a and 44.03 t/a,respectively,which were 1.38 times and 2.14 times from the north.The coincidence of chemical fertilizer application period and rainfall period will cause heary nitrogen and phosphorus pollution.The highly nitrogen and phosphorus produced area in the west is in Xizhou Town and Dali Town.The highly nitrogen and phosphorus produced area in the north is in plains beside the rivers,such as Sanying Town,Yousuo Town and Dengchuan Town.(4)In 2015,the contribution of farmland is highest to total nitrogen load(43.02%).The contribution of construction land(rural settlements and urban land)is highest to total phosphorus load(47.58%).The contribution of fertilizer application to the total nitrogen and total phosphorus into the lake reached more than 30% both in the west and north.In western basin,the rural domestic pollution occupied larger percentage of total nitrogen load(17.69%)than other pollution sources.However,in northern basin,livestock and poultry pollution occupied larger percentage of total phosphorus load(21.94%)than other pollution sources.