| It is of great significance to study the temporal and spatial variation of pollutants and water quality in drainage ponds in plain river network area for the control of agricultural nonpoint source pollutants,it is necessary to explore the key factors affecting water quality changes for the prevention and control of non point source pollution.Taking the drainage pond of the Yangzhou section of the Grand Canal as an example,this paper monitored the changes of water quality in the study area during 2018-2022.Based on the monthly observation results of 19 monitoring points in 3 channels,the water quality data were divided into wet season(July-August)and dry season(November-December),and the variance analysis was used to study the variation rules of nitrate nitrogen(NO3--N),ammonia nitrogen(NH4+-N)and total phosphorus(TP)in the system.The single factor water quality index method and comprehensive water quality index method were used to classify and evaluate the water quality of the ditch.The changes of landscape pattern and gully pond in the study area were described according to the land use type,landscape index and gully pond dynamic index of each buffer zone in different radii(50 m,75 m,100 m,125 m)outside the monitoring site.Finally,Spearman correlation analysis and random forest classification model were used to analyze the changes of landscape pattern and gully pond.100 m buffer zone),landscape index(6,100 m buffer zone)and gully pond dynamic index(4)were used to analyze the key factors affecting the change of gully pond water quality.The main research results are as follows:(1)The changes of water quality in the study area during 2018-2022 were as follows:the maximum concentration of NO3--N occurred in June and July,and the minimum concentration occurred in March and May;NH4+-N concentration increased in April and July,but was lower in other months.TP concentration had little change,and increased slightly from June to July.The monitoring concentration in 2019-2022 was lower than that in 2018,but the trend was the same.(2)The average concentrations of NO3--N,NH4+-N and TP in wet season were 125.7%,138.6%and 123.9%of those in dry season,respectively.There are spatial differences in the water quality of ditches and ponds in the study area.There is no significant difference between the concentration of NO3--N and NH4+-N in each channel,but there is a significant difference with the concentration of TP.Although the drainage pond has a certain capacity of water purification,it is not enough to reduce the nitrogen and phosphorus pollutants exported from paddy fields in wet season.(3)Among the four types of land use in the study area,farmland area accounts for the largest proportion,followed by ditch area,residential area and road area.No correlation was found between NO3--N and the proportion of landscape type,while NH4+-N had little correlation with the proportion of landscape type,and the correlation increased slightly with the increase of buffer scale.TP concentration was strongly correlated with landscape type proportion,and increased gradually with the increase of buffer scale.With the increase of buffer radius,the correlation between TP and Edge Density(ED),Interspersion Juxtaposition Index(IJI)and Shannon’s Diversity Index(SHDI)gradually increased,but no significant correlation was found between NO3--N and NH4+-N and landscape index.The size index,discharge index and matching index were negatively correlated with water quality change,while the initial concentration index was positively correlated with water quality change,but the correlation was not significant.(4)The results of random forest importance analysis of water quality change and influencing factors show that the influence of dynamic indexes on water quality change is higher than other indexes.The gully can be regulated according to the importance of different gully characteristics on water quality change.Among the other indexes,farmland proportion,Shannon’s Diversity Index and Largest Patch Index(LPI)had a high degree of influence on water quality,which reflected the influence of human activities on water quality in gully ponds. |
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