Spatio-temporal Variation Of Net Anthropogenic Nitrogen Input In Watersheds And The Impact On River Water Quality

Nitrogen inputs to the watersheds are increasing due to the vatious human activities,and the nitrogen input increasing have caused many water environmental problems.This research selected the upper basin of Xin'anjiang River and the upper basin of Yangtze River as the study areas,and analyzed spatio-temporal variation of the net anthropogenic nitrogen input(NANI).A geomorphological based nonpoint source pollution(GBNP)model was employed to simulate the nitrogen input into the river and transport along the river network.Based on the simulation results,it analyzed the characteristics of hillslope nitrogen loading and the impact on river water quality.Firstly,the net anthropogenic nitrogen input was calculated in the upper basin of Xin'anjiang River and the upper basin of Yangtze River based on the social and economic statistic data.Then GBNP models were built in the two study basins using NANI as the nitrogen inputs.The calibration and validation results demonstrated that the GBNP model performed well in the two study basins.Fanally,based on the simulation results,the spatial and temporal characteristics of hillslope nitrogen loading and river water quality were discussed.In the upper basin of Xin'anjiang River,the impact factors of river nitrogen retention along the river network were investigated.In the upper basin of Yangtze River,the impact of human activities on hillslope nitrogen loading and impacts on river water quality has analyzed.The results obtained in the upper basin of Xin'anjiang River illustrated that the main nitrogen sources were fertilizer application and atmospheric nitrogen deposition.The higher values of NANI were concentrated in Tunxi,Shexian and Jixi counties.Urban area and farmland area had significant positive correlation with NANI,and forest area had a negative correlation with NANI.The simulated results showed that annual total nitrogen loading was 1.16 ton/km2.In the rainy season,the amount of nitrogen loading took 57.7% of annual total nitrogen loading,and the export coefficient of nitrogen was 0.16 in the upper basin of Xin'anjiang River.The temporal variation of hillslope nitrogen loading was influenced by annual precipitation and rainfall intensity,and the spatial variation of hillslope nitrogen loading was correlated with landuse types.The concentration of total nitrogen was higher in dry season,and the nitrogen retention coefficient in rivers was 80%.The higher the stream order(in Hortan-Strahler scheme),the smaller the nitrogen retention ratio;the higher nitrogen input into the river,the higher the nitrogen retention ratio in the river.The results obatained in the upper basin Yangtze River demonstrated that the main nitrogen sources was fertilizer application and atmospheric nitrogen deposition,and food/feed nitrogen input was also high.The high values of NANI was concentrated in Chengdu Plain.Increasing urban area would change way of farming and cultivation in the city and around the city,and increase the total amount of NANI.The simulated results showed that annual total nitrogen loading was 1.50 ton/km2.The amount of nitrogen loading in July and August took more than 65% of the annual total nitrogen loading,and the export coefficient of nitrogen was 0.26 in the upper basin Yangtze River(at Yichang station).The export coefficient was influenced by the supply of nitrogen and the processes of rainfall-runoff.The concentration of total nitrogen was higher in rainy season,and the stream nitrogen retention ratio was 87%.Scenario analysis showed that conversion of cropland to forest and reduction of fertilizer could effectively reduce the total nitrogen load into rivers as well as the export of nitrogen from the basin.