| As the last pass of the ecological barrier in the upper reaches of the Yangtze River,the Three Gorges Reservoir area had an important strategic position in the national ecological spatial pattern.Therefore,studying the soil erosion and non-point source pollution caused by soil and water loss in the Three Gorges Reservoir area and exploring effective soil and water conservation measures and management mechanisms are of great significance to control soil and water loss in the reservoir area and ensure the development of ecological civilization in the reservoir area.Taking the catchment area（CT）under the traditional farming mode and the agroforestry complex catchment area（FH）equipped with soil and water conservation measures in shipanqiu small watershed,a typical small watershed in the Three Gorges Reservoir area,as the research area,the soil erosion status in the watershed was described by the soil erosion modulus,and the water environment status in the watershed is described by the load of runoff nitrogen and phosphorus in the process of secondary rainfall.Based on the landscape quantitative characteristics and landscape pattern characteristics of the watershed,Based on the theory of landscape pattern,this paper discusses its impact on soil erosion modulus and non-point source pollution at the scale of small watershed,so as to clarify the current situation and driving factors of soil and water loss in small watershed,and provide scientific basis for optimizing landscape pattern and preventing soil and water loss in small watershed.The main results were as follows:（1）The study on the landscape pattern of shipanqiu small watershed shows that the landscape area ratio of"source"and"sink"in CT catchment area was 7:3,mainly"source"landscape,while the landscape area ratio of"source"and"sink"in FH catchment area was about 4:5,and the layout was balanced;In the distribution of landscape quantity with slope,the number of landscapes with CT distribution below 5°was twice that of FH,while the number of landscapes with FH above 25°was six times that of CT.In the distribution of landscape quantity with relative elevation,the number of landscapes with FH above 50m relative elevation is six times that of CT.FH catchment area occupies most of the high and steep slope areas in the basin;In terms of the characteristics of landscape index in the study area,under the landscape level index,the differences between CT catchment and FH catchment were mainly reflected in patch density（PD）,edge density（ED）and shape index（LSI）,while the differences between Contag and Shannon diversity index（SHDI）were not significant;Under the type level index,the PD of CT catchment area in the"source"landscape was less than FH catchment area,while the ED was greater than FH catchment area.The PD of CT catchment area in the"sink"landscape was greater than FH catchment area,and the ED was less than FH catchment area.（2）Through the calculation of soil erosion modulus of shipanqiu small watershed by RUSLE model,both CT and FH catchments show the erosion pattern with micro erosion as the matrix.The erosion modulus of the two catchments were 66.66 t·km-2·a-1and 65.54 t·km-2·a-1 respectively,there was no significant difference,but there was a strong erosion area with erosion modulus of 5530.02 t·km-2·a-1 in CT catchments;Drylands and orchards were the landscape types with high erosion modulus in the"source"landscape,while other woodlands and other grasslands were the landscape types with high soil erosion modulus in the"sink"landscape.（3）The analysis results of various forms of nitrogen in the runoff of four erosive rainfall events in the study area in August 2021 show that the average EMC of TN,DN,NN and an in the runoff of CT catchment area were 3.93 mg·L-1,2.32 mg·L-1,1.26mg·L-1and 0.13 mg·L-1 respectively,and the average output load was 104.30g,80.34g,41.37g and 5.76g respectively;The average EMC of TN,DN,NN and an in the runoff of FH catchment area were 1.96 mg·L-1,1.57 mg·L-1,0.79 mg·L-1 and 0.10 mg·L-1respectively,and the average output load was 61.87g,50.02g,30.86g and 3.31g respectively.NN was the main loss form of dissolved nitrogen in the runoff of the two catchment areas.The analysis results of various forms of phosphorus in the runoff of four erosive rainfall events in the study area in August 2021 show that the average EMC of TP,TDP and DIP in the runoff of CT catchment area are 0.68 mg·L-1,0.43 mg·L-1and 0.11 mg·L-1 respectively,and the average output load was 32.86g,17.10g and 4.76g respectively;The average EMC of TP,TDP and DIP in the runoff of FH catchment area were 0.53 mg·L-1,0.36 mg·L-1 and 0.11 mg·L-1respectively,and the average output load is 15.29g,8.60g and 3.20g respectively.DIP was an important loss form of dissolved phosphorus;In August 2021,the monthly losses of TN,DN,NN and an in CT catchments were 6.13kg·m-1,3.62 kg·m-1,1.97 kg·m-1 and 0.20 kg·m-1 respectively.Compared with CT catchments,FH catchments reduced the proportion of various forms of nitrogen by 59.22%,47.75%,48.73%and 35.00%respectively;The monthly losses of TP,TDP and DIP in CT catchments were 1.06 kg·m-1,0.67 kg·m-1 and 0.17 kg·m-1respectively.The reduction proportion of various forms of phosphorus in FH catchments reaches 35.85%,31.34%and 17.65%respectively.The concentrations of total nitrogen and total phosphorus in the two catchments exceed the national class V water standard,but FH catchments greatly reduce the output load of various forms of nitrogen and phosphorus in runoff and play a positive role in the prevention and control of surface source pollution.（4）The correlation analysis between landscape pattern and erosion modulus in the study area shows that PD,ED and LSI were significantly positively correlated with soil erosion in the two catchments,and the percentage of"source"and"sink"area also had a significant impact on soil erosion modulus;In the multi factor regression analysis of erosion modulus and landscape pattern index,the complex correlation coefficients were greater than 0.8,indicating that the comprehensive effect of each factor is highly correlated with soil erosion;In the redundancy analysis of landscape pattern and runoff nitrogen and phosphorus nutrient concentration in the study area,the juxtaposition and dispersion index（IJI）and shape index（LSI）had the highest interpretation rate of runoff nutrient concentration in CT,while the contribution rate of pit and pond surface,orchard and dry land to runoff nutrient concentration in CT catchment area was the highest;In FH catchment area,the landscape indexes with high nutrient decomposition and release rate of runoff were LSI and PD respectively,while the landscapes with high nutrient contribution rate of runoff were terrace and forest land respectively.（5）The risk of soil and water loss in shipanqiu small watershed is high.Dry land and orchard were the most widely distributed high-risk landscape types,while other forest lands and other grasslands were the main landscapes that play a buffer role in soil and water loss in the watershed.The transformation from"source"landscape to"sink"landscape,avoiding centralized and continuous development,the layout of soil and water conservation measures on high and steep slopes and the establishment of water outlet buffer zone were the main ways to adjust the landscape pattern of the basin. |
