| With the improvement of the degree of agricultural intensification,the non-point source pollution in the watershed has increased significantly,and the river water quality has been degraded.Changes in watershed landscape type composition and spatial layout can affect ecological processes to varying degrees,thereby changing the generation and output of non-point source pollution.Therefore,it is of great significance to explore the impact of changes in watershed landscape characteristics on runoff water quality.This paper selects the Zhangjiashan small watershed in the Danjiangkou reservoir area as the research object.Based on the catchments division of the small watershed,and the sampling and analysis of the runoff water quality in the rainy and dry seasons,the landscape pattern indexs and the location-weighted landscape contrast indexs of the catchment area are calculated.Using the multivariate statistical method combining Spearman correlation analysis and the redundancy analysis(RDA),the effects of landscape characteristics on runoff water quality in small watershed are discussed from two aspects of landscape quantity characteristics and landscape pattern characteristics,in order to improve water purification of small watershed.The main findings are as follows:(1)Based on the DEM,the Zhangjiashan small watershed is divided into 22catchments by means of ArcGIS hydrological analysis tools.The average area of the catchment is 1.43 km~2,the smallest area is 0.12 km~2 and the maximum is 7.22 km~2.(2)Forest and orchard are the main types of land use in Zhangjiashan watershed,accounting for 77.89%of the area;The area of flowland and fallow land is second,accounting for 9.28%and 7.04%of the total area of the small watershed;the construction and water area account for the smallest proportion,accounting for only 1.56%and 0.78%of the total area of the small watershed.The forest is mostly distributed in the upper and lower area of the small watershed.The orchard,flowland and construction are mainly distributed in the middle reaches of the small watershed.The proportion of forest and fallow land area increases with the increase of slope and elevation;orchard,flowland and construction are mainly distributed in gentle slopes of 300m-500m and steep slopes.(3)Based on the“source-sink”theory,the results of the location-weighted landscape contrast indexs(LCI)of the small watershed and each catchment show that the overall LCI of the Zhangjiashan small watershed is 0.70,which is less than the threshold of 1.0,indicating that the overall landscape spatial pattern of the small watershed is well and the possibility of non-point source pollution is small.From the upper to the lower aera of the small watershed,LCI showed a trend of increasing first and then decreasing,and the risk of non-point source pollution was the highest in the central area.(4)The overall runoff water quality of the Zhangjiashan small watershed is characterized by high nitrogen,high organic and low phosphorus.The total phosphorus in each catchment has reached the national Class II water standard in different periods;however,it is polluted by nitrogen and organic.The total nitrogen and chemical oxygen demand exceeds the standard rate in the rainy season by more than 80%,and some catchments exceed the national Class V water Standard.Nitrogen pollution is mainly composed of nitrate nitrogen,and ammonia nitrogen can basically meet the national III water standard.(5)Based on the cluster analysis of the runoff water quality indexs concentration in the catchments,the 22 catchments in the Zhangjiashan small watershed are divided into 4categories.The variance analysis of the water quality indicators in the four categories of catchment shows that,in addition to total phosphorus and nitrate nitrogen,total Nitrogen,ammonia nitrogen and chemical oxygen demand all showed obvious spatial and temporal changes.On the time scale,the water quality indexs showed a trend of rainy season>dry season.On the spatial scale,the water quality indexs of the small watershed increased from the upper to the lower.The trend of reduction is that the runoff water quality in the upper area of the small watershed is the best,followed by the runoff water quality in the left and right tributaries of the small watershed,and the middle and lower area are poor.(6)Spearman correlation analysis and redundancy analysis showed that the percentage of forest was the most significant landscape variable for runoff water quality,followed by the percentage of orchard.The percentage of forest area and nitrogen indexs showed a significant negative correlation or a significant negative correlation,indicating that the increase of forest area can effectively enhance the reduction of nitrogen pollution.The area percentage of other landscape types except forest was negatively correlated with water quality indicators.Orchard was one of the main sources of nitrogen pollution,and construction and total phosphorus were closely related.The percentage of the total area of the orchard,cultivation,construction and fallow land is the most important indicator of runoff water quality in the dry season,with a contribution rate of72.7%.Among them,the percentage of orchard area plays a leading role.In addition,forest land also plays a role in reducing pollution and organic pollution during the dry season.(7)Speramn correlation analysis showed that the location-weighted landscape contrast indexs(LCI)showed a significant or extremely significant positive correlation with all water quality indicators,indicating that the landscape spatial pattern of the small watershed and catchments can significantly affect runoff water quality.The RDA ranking shows that the interpretation rate and contribution rate of runoff water quality change in different periods are ranked in the previous position,indicating that the location-weighted landscape contrast indexs can reflect the distribution characteristics of non-point source pollution in small watershed and catchments.Landscape patch density(PD)and contagion(CONTAG)can also explain the temporal and spatial variability of runoff water quality to a certain extent,indicating that the fragmentation degree and dominance of landscape patches have a greater impact on runoff water quality.(8)Based on the relationship between the landscape characteristics of Zhangjiashan small watershed and the quality of runoff water quality,the regulation and optimization scheme of small watershed landscape pattern based on water environment protection can be proposed.In terms of the number of landscapes,the proportion of forest in the catchments of No.9,10,and 11 with the highest risk of non-point source pollution should be increased to more than 50%.In the characteristics of landscape pattern,the landscapes with or without slopes greater than 25°and elevations less than 200 m and within 50 m of water bodies can be converted into forest land;and the scattered forest in the lower area of the small watershed is connected into complete plaques.The wasteland is converted into forest,which enhances the landscape superiority of the forest.For key areas,rainwater regulation tanks or small artificial wetlands can be embedded in large-scale orchards,and narrow orchards that transmit pollutants can be converted into forest,or a certain width of forest land can be added at the downhill position.In addition,setting a certain width of vegetation buffer zone or artificial wetland between orchards,constructions,cultivation with high surface pollution load and receiving water body is also an effective means to reduce the threat of non-point source pollution to water bodies. |
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