Study On Characteristics Of Non-point Source Pollution In A Typical Small Watershed Of Three Gorges Reservoir Area

Non-point source pollution is a major environmental problem currently facing of Three Gorges reservoir area.After the water storage of Three Gorges Reservoir,A large number of cultivated land flooded.The contradiction between man and land is more prominent.Adjustment of Agricultural Planting Structure,Food production pressure is increased,it is objectively promote the fertilizer,pesticides and other applications increased.Extensive sources of pollutants,larger potential pollution load,more condition of occurrence which provides favorable conditions for the occurrence of serious agricultural non-point source pollution in the reservoir area.This is a huge threat to the water environment in the reservoir area.This paper takes the typical small watershed of the Three Gorges Reservoir area--Shenjiahe small watershed of study.Relying on Three Gorges reservoir area soil and water conservation and environmental research pilot station of the Chinese Academy of Sciences,using the complete erosion sediment runoff observation area.small watershed and sediment observation weir.According to the invention patent construction can simultaneously determine the Overland flow and inter flow in the runoff area.Conduct field in situ observation experiment.The study:(1)Characteristics,pathways and fluxes of runoff,sediment and nutrient loss of different land use types such as woodland,village,slope farmland and paddy field;(2)Characteristics of runoff loss in catchments area and small watershed,loss of sediment and nitrogen and phosphorus nutrients and flux.The results and conclusions were listed as follows:(1)The TN flux of sloping land is 42.30kg·hm-2,The flux of TP in sloping land is 1.99 kg·hm-2.The TN and TP flux of citrus grove is 7.95 and 2.60kg·hm-2.The flux of TN in natural grove land is 4.29kg·hm-2,and the TP flux is 0.405kg·hm-2.The TN flux for rural settlements is 23.31kg·hm-2,and The TP flux is 3.85kg·hm-2.The fluxes of TN and TP in paddy field were 3.04kg·hm-2 and 0.62 kg·hm-2,respectively.It can be seen that the contribution of slope farmland to non-point source TN and TP fluxes in small watershed was 52.3%,21%;Citrus grove TN and TP contribution were 9.8% and 27.5%;The contribution of settlements to TN and TP fluxes of small watersheds were 28.8% and 40.7%.Therefore,slope farmland,citrus grove and residential area were the main pollution sources in the typical small watershed of the Three Gorges reservoir area.(2)The runoff of sloping land increased with the increase of rainfall,with the increase of runoff,TN and PN increased rapidly during the first time indicating that the accumulation of nitrogen in the surface soil was loss rapidly.At the time of 45 min,TN and PN reached the highest,which were 7.26mg·L-1 and 6.03mg·L-1.The peak appears before the runoff peak.Followed by increased runoff,that TN and PN content decreased,but the change is small.The process of nitrogen transport in the process of inter flow is actually the process of nitrate nitrogen migration.NN with the runoff process changes that the first rapid rise,reached the peak at 110 min,and then slowly reduced,stabilized,and continued for a long time.In the typical rainfall time,the NN concentration is higher,which more than 10 mg·L-1 of drinking water safety standards.During the rainfall,the TN flux was 1.02,3.10kg·L-1,respectively,with Overland flow and inter flow.The time-concentration curve of total phosphorus(TP)and particulate phosphorus(PP)in the process of runoff was always close,There was a significant positive correlation between TP and PP in the process of runoff(r=0.968**,N=12),The results show that the combination of sediment is the most important form of runoff migration on the purple soil slope land.At the final stages of runoff,the time-concentration curve of total nitrogen(DTP)dissolved in runoff is almost coincident with TP,Indicating that the late rainfall reduction,runoff has not enough capacity to carry sand,runoff in the dissolved state of phosphorus.During the rain,the average TP of Overland flow and inter flow loss was 0.069,0.004kg·L-1,respectively.(3)The concentration of nitrogen and phosphorus in the main pollutants decreased first and then decreased with the runoff,but the peak of the concentration appeared before the peak of the flow rate.The process of runoff and pollutant concentration showed a single trend.The average nitrogen concentration of TN was 8.42mg·L-1,the soluble nitrogen(DN)was 5.61 mg·L-1and particulate nitrogen was 2.81mg·L-1,the average concentration of TP was 1.23mg·L-1,the average content of soluble total phosphorus(DTP)was 0.37mg·L-1 and particulate phosphorus was 0.86mg·L-1.Particulate phosphorus accounted for more than 60% of total phosphorus pollutants,and the concentration variation is more stable.(4)The nutrient flux of dry sloping land is about 13.48kg·N·hm-2and 1.58kg·P·hm-2,The nutrient flux in the catchments is 13.11kg·N·hm-2 and 2.02kg·P·hm-2,the nutrient fluxes of integrated small watersheds were 9.89kg·N·hm-2 and 0.71kg·P·hm-2.The differences in the ecological function of the land forms in the small watershed resulted in differences in nutrient flux between different scales,Paddy field is the sink of dry land nutrient with the runoff loss into the lower reaches of the small watershed,while the ditch vegetation on the upper reaches of the dry slope of the nutrients are also retained and absorbed.So the nutrient load of small watershed is lower than that of dry slope.The nutrient load of the small watershed is mainly composed of the slope nutrient output in the heavy rain process,and also includes the contribution of some urban domestic sewage and irrigation water output.It also shows that non-point source pollution in larger scale watershed is more susceptible to a variety of factors.