Characteristics Of Soil,Water And Nutrient Loss In Lanling Xi Watershed In The Three Gorges Reservoir Area

In recent years,the eutrophication of the Three Gorges reservoir area is serious,and agricultural non-point source pollution is an important cause of the decline in water environment quality.It is of great practical significance to strengthen the control of agricultural non-point source pollution in the reservoir area.The Lanlingxi watershed in Zigui is a national technical demonstration site for returning farmland to forests and also a typical small agro-forestry complex basin.Pre-research and monitoring displayed that the nitrogen（N）and phosphorus（P）in the watershed of the watershed had continued to exceed the standard,and the eutrophication in the catchment area had been aggravated.This study took the Lanlingxi watershed as the research object,systematicly observed and studied it in runoff plots,catchments and watershed scales from May to October in 2016 and 2017,comprehensively used the method of cross-section monitoring,comprehensive water quality assessment,basic flow segmentation,and GIS spatial analysis to analyze the nitrogen and phosphorus output,comprehensive water quality and the proportions of land use types in catchment areas.It revealed the distribution of soil erosion and nutrient loss in small watersheds,and analyzed the characteristics and pathways of soil nutrient output in runoff and sediment.Nutrient loss pathways were revealed,and the key period of N and P output was identified,and the response of runoff N and P output to typical rainfall was explored.This provided a scientific basis for decision making to develop agricultural surface Source pollution prevention.Based on the monitoring and research in 2017,the main results are as follows:（1）Characteristics of runoff.The rainfall in the watershed was 1022.6 mm from May to October in 2017,and total runoff depth was 682.6 mm,including base flow（125.3 mm）and surface runoff（557.4 mm）,which contributed 18.35% and 81.65%,respectively.Surface runoff,total runoff,and rainfall were significantly correlated（P<0.01）.In general,the total runoff coefficient in the small watershed was 66.8%,of which the surface runoff coefficient was 54.5%,making it a high runoff conversion rate of the watershed.（2）Soil erosion.From May to October 2017,a total of 481.81 tons of sediment was discharged from the small watershed,and the erosion modulus of the sediment was 335.28 t·km^-2·a^-1,which was a slight erosion.Sediment output was significantly correlated with total runoff and surface runoff（P<0.01）.The sub-heavy rain events had an important impact on soil erosion.The sub-storm event（08/28）produced sediment loss（50.02 t）accounting for 49.9% of the total sediment loss in August.Sand was the main form of Erosion output sediment（runoff plot,flow meter）,accounting for more than 80% of the particle morphology.The content of clay and silt in sediment was significantly positively correlated with the fractal dimension of sand particle size（P<0.01）,and the sand content was significantly negatively correlated with the fractal dimension（P<0.05）.Sediment particle size can be used as a “fingerprint factor” to trace sediment sources in small watershed.（3）N and P nutrient output in catchments.The watershed was divided into seven catchment areas.N and P nutrient concentrations in the runoff increased from top to bottom.The total nitrogen（TN）,total phosphorus（TP）,nitrate nitrogen（NN）,and ammonium nitrogen（AN）concentrations ranged from 2.14²2.37 mg·L^-1,0.01⁰.10 mg·L^-1,1.08²0.86 mg·L^-1,0.03⁰.17 mg·L^-1.Concentration of N and P in each catchment area was quite different,and the comprehensive water quality of them belonged to Grade III^V（Integrated water quality index index method）.Among them,two catchment areas did not meet the standard of surface water function area,whose land use structure was 69.04% forest land,0.85% cultivated land,20.91% garden land,6.49% residential land,1.77% unused land,0.94% water area;and 48.96% forest land,3.92% cultivated land,17.90% garden land,20.78% residential land,4.64% unused land,3.80% water area.TN was the over-standard project in all catchment areas,among which nitrate nitrogen accounted for about 80%,which is the key factor for the failure of comprehensive water quality.The comprehensive water quality of runoff in the catchment area was significantly negatively correlated with the proportion of forest land,and it was significantly positively correlated with the ratio of residential land and garden land.Compared with other catchment areas,output of N and P in the forest catchment was lower,and the comprehensive water quality was more stable.（4）Runoff nutrient output concentration in small watershed.The daily average concentrations of TN,TP,NN,and AN in the small watershed in 2017 were 14.752,0.039,10.594,and 0.150 mg·L^-1,respectively.Among them,the TN concentration exceeded the Grade V standard of surface water,AN and TP belonged to Grade I and II standards of surface water.The N-P ratio（378.3）was much higher than the standard for algae growth in the reservoir area,and the water body was nitrogen-restricted.（5）Small watershed runoff nutrient loss load.Runoff nutrient loss pathway was divided into surface runoff and base flow.In the runoff nutrient loss load,TN,TP,NN,and AN output totaled 8.926 t·km^-2,0.039 t·km^-2,5.990 t·km^-2,and 0.115 t·km^-2.The output through surface runoff accounted for 77.81%,76.12%,82.71% and 88.88% of the total runoff.The peak period of nutrient output in the base stream was from June to August,and the peak period of nutrient output from surface runoff was from August to October.（6）N and P Output response to rainfall and runoff in the small watershed.Concentration of NN was negatively correlated with runoff and its initial effect was not significant.Concentration of AN and TP were significantly positively correlated with rainfall（P<0.05）,and their initial effects were significant.The order of the initial effects of N and P nutrients in rainfall in the small watershed was in order of TP>AN>TN>NN.（7）Small watershed silt nutrient loss load.TN,TP,NN,and AN output a total of 0.109 t·km^-2,0.328 t·km^-2,0.010 t·km^-2,and 0.001 t·km^-2.The peak period of N and P output of sediments was from July to October,TN and TP respectively accounted for 70.94% and 75.80% of the TN and TP loss in sediment.The content of N in the sediment was positively correlated with the percentage of silt particles and clay particles（P<0.05）.The TP content and particle size did not reach significant levels,and the content of N and P nutrients were negatively correlated with the percentage of sand particles.Sand was the main particle form of sediment nutrient output,and the nutrient load output by sand grains exceeds 65% of the total sediment nutrient load.（8）Characteristics of watershed nutrient loss.In total,TN,TP,NN,and AN were lost by 9.934 t·km^-2,0.367 t·km^-2,6.016 t·km^-2 and 0.117 t·km^-2,and the peak period of nutrient loss was from August to October.The runoff contributed 98.8%,10.6%,99.8% and 98.9% of the total load,and the sediment loss load accounted for 1.2%,89.4%,0.2% and 1.1% of the total load.（9）The loss load of TN,TP,NN and AN in the small watershed of Lingxi had a good linear regression relationship with the base flow,surface runoff and sediment.Estimated nutrient annual loss in small watersheds using the regression model was TN 14.551 t·km^-2·a^-1,TP 0.397 t·km^-2·a^-1,NN 9.671 t·km^-2·a^-1 and AN 0.217 t·km^-2·a^-1.The annual loss rate of nitrogen fertilizer in the small watershed was 20.3%,far higher than the average loss rate of nitrogen fertilizer in the reservoir area of 9.0%;the loss rate of phosphate fertilizer was 5.4%,and it is equivalent to the average loss rate of the reservoir area（5.7%）.