Temporal And Spatial Distribution And Loss Characteristics Of Nitrogen And Phosphorus In Small Watershed In The Middle Route Of The South-to-North Water Transfer Project

The Dan River Basin is an important water conservation area for the Middle Route of the South-to-North Water Diversion Project,and its water environment quality is of great significance to the water supply safety of the Middle Route of the South-to-North Water Diversion Project.Based on the long-term monitoring of water and sediment nutrients and the collection and analysis of soil samples in minjiahe watershed,a tributary of Danjiang River,the distribution characteristics and loss rules of nitrogen and phosphorus in soil,surface runoff and sediment in minjiahe watershed were studied by using geostatistics,output coefficient model and hydrology methods,The main conclusions are as follows:(1)The average contents of total nitrogen and total phosphorus were 2.42 g/kg and 0.77 g/kg in minjiahe watershed.Soil total nitrogen(phosphorus)was mainly concentrated in 2-5 mm aggregates,and less in 0.25-0.5 mm aggregates.The average concentration of total nitrogen in surface runoff was 3.5 mg/L,and the range of total phosphorus concentration was 0.02-0.19 mg/L.The content of total nitrogen and total phosphorus in sediment mostly concentrated in<0.25 mm size fraction.The total nitrogen concentration of surface runoff was higher in the watershed and the downstream human activity intensive area.From upstream to downstream,the total phosphorus content in sediment increases,and the total nitrogen content first increases and then decreases.(2)Compared with farmland,forest and grassland significantly reduced the proportion of<0.25 mm aggregates,increased the proportion of>5 mm and 2-5 mm aggregates,and promoted the accumulation of total nitrogen and phosphorus.Land use type has a great influence on the output load of nitrogen and phosphorus.The load of nitrogen and phosphorus mainly comes from farmland.The output load of total nitrogen and total phosphorus accounts for 34.82%and 39.41%of the total output respectively.The increase of forest area can reduce the total nitrogen concentration of surface runoff and the total phosphorus content of sediment(3)There were 66 rainfall events in the whole study period,including 26 erosive rainfall events.The rainfall events can be divided into three types:heavy rainfall short duration rainfall(type Ⅰ),small rainfall medium duration rainfall(type Ⅱ)and heavy rainfall long duration rainfall(type Ⅲ).The nitrogen loss in minjiahe river basin is mainly in dissolved form,while the phosphorus loss is mainly in granular form.And the difference between the concentration of pollutants carried by rainfall runoff and base flow pollutants is the main reason for the impact of rainfall runoff on the water quality of MinJia River Basin in flood season.