Non-Point Source Pollution Risk In A Typical Semi-Urbanized Small Basin Based On "Source-Sink" Landscape

The water environment quality of Houxi Basin in Xiamen city is directly related to regional ecological environment security.In recent years,with the continuous expansion of urbanization,the landscape dynamics of Houxi Bain which located in typical semi-urbanized areas,have changed dramatically because of an increasing risk of non-point source pollution.In order to clarify the change of non-point source pollution and control its development trend,this paper studied the landscape dynamics and non-point source pollution risk of Houxi Basin in Xiamen city under the influence of urbanization expansion in recent ten years,and put forward landscape regulation countermeasures.Firstly,the landscape pattern index was calculated using Google image and DEM data to analyze the landscape dynamic characteristics of the Houxi Basin.Secondly,the non-point source pollution risk assessment index was constructed based on the"source-sink"theory.The pollution weights of different landscapes were calculated according to the regional background,and the grid cell pollution index(GPI)was proposed to evaluate the risk composition and degree of regional non-point source pollution.Finally,the control theory of landscape pattern was used to design optimization regulation policy of different scenarios.Taking Houxi Basin in Xiamen as an example,we estimated the non-point source pollution risk in 2020,performed the landscape pattern control,and compared the risk area and the risk value of change,to achieve the purpose of effective risk control agricultural non-point source pollution with less land use changes.The main findings of the study are as follows:(1)Landscape dynamic characteristics in Houxi BasinThe spatial distribution of land use in the research area shows that the landscape type of land use is dominated by the construction in the south and woodland in the north.From2010 to 2020,the landscape pattern of the Houxi Basin changed significantly,and the area of construction land increased by 16.06 km~2,and the dynamic attitude of land use was4.32%;The area of cultivated land decreased the most,from 20.32 km~2 to 6.68 km~2,and the dynamic attitude of land use was-6.71%;The trend of land use transfer from woodland,arable land,bare land and other land resources that are easy to develop to construction land is gradually shifting.(2)Non-point source pollution risk assessment in the Houxi BasinThe distribution of non-point source pollution risk areas in the research area is similar to that of land use.The average GPI values of non-point source pollution risk over the years are 0.24,0.25 and 0.27,respectively,showing an upward trend.In the last decade,under the continuous influence of urbanization,the decrease of cultivated land area and its pollution weight are the main reasons for the decrease in high-risk areas and its risk value.However,the area of construction land keeps expanding,and 27.31 km~2 of other land use types are transferred to construction land,which increases the proportion of construction land in the risk area from 35%to 69%,and then leads to the increase of average GPI in the medium and low risk areas.Further analysis of the risk composition can be found that the distribution,proportion,combination form of landscape types and their own sewage capacity determine the occurrence of regional non-point source pollution risks,and urbanization has an impact on the key source areas of non-point source pollution risks to a large extent.(3)Landscape regulation of non-point source pollution in the Houxi BasinLandscape ecology theory and"source-sink"theory were used as the theoretical basis of regulation.Based on the landscape types,geographical factors and landscape spatial structure,the optimization framework of non-point source pollution risk was constructed,and three control strategies for high,middle and low levels were proposed for different urbanization stages.Based on the land use and non-point source pollution risk data of Houxi Basin in Xiamen city in 2020,the optimal regulation and control of risk areas were carried out.The risk areas were divided into five regions from R-1 to R-5,and the landscape pattern land use regulation was carried out for the grid of risk areas in the five regions,respectively.Only through 3.46 km~2 land use regulation,the"sink"landscape area of the whole study area increased by 7.73%compared with the original.The area of medium and high risk areas was reduced by 38.96%,which realized the goal of controlling the risk of non-point source pollution by minimizing land use change as much as possible.