流域景观结构的城市化影响与生态风险评价

以南京市九乡河流域为研究区域，以2003、2009年两期遥感影像数据为基本信息，在构建生态风险指数的基础上，利用ARCGIS的空间分析功能，揭示了城市化对研究区景观结构的影响，生态风险的时空变化以及城市化与生态风险之间的关系。结果表明：2003-2009年，城市化过程使流域的景观结构发生较大变化，建设用地大幅增加，分离度降低，而耕地则大幅减少，耕地、水域、草地等景观类型的分离度、破碎度增加；流域生态风险程度从中度/低风险向中度/较高风险转变，生态风险有增加的趋势；生态风险的空间差异明显，生态风险较低的区域主要集中在流域南部青龙山一带，较高的区域主要集中在西北部地区，中度风险区集中在流域中部的平原农业区；城市化对生态风险的正效应明显，生态风险随着城市化水平的增加而升高。

The urbanization effects on watershed landscape structure and their ecological risk assessment

Regional ecological risk assessment is an intricate, challenging task. Methods of ecological risk estimation have been recognized in recent years. But most research methods focused on empirical and statistical approaches, which can't exhibit the special differences of ecological risk levels in regional scales. The intrinsic ability of GIS to store, analyze and display large amounts of spatial data enables it to make a significant contribution to ecological risk assessment. The purpose of this paper is to develop an integration methodology for regional ecological risk assessment, and to help managers to analyze and visualize spatial differences in ecological risk levels in the Jiuxiang River watershed for effective eco-environmental protection.Jiuxiang River watershed in Nanjing city, as a case study area, its two landscape classification maps was conducted from 2003 and 2009 by remote sensing images interpretation, which were used as the main data source for assessment of ecological risk levels. Based on landscape ecology, an ecological risk assessment index was firstly constructed, and further developed by embedding the index into the GIS system. In addition, the spatial distribution map of ecological risk level was created by using the spatial sample method and the ordinary Kriging interpolator method, which could effectively exhibit the spatial-temporal variation characteristics of watershed ecological risks in 2003 and 2009. Finally, the relationship between watershed urbanization and watershed ecological risks were analyzed. the conclusions included: (1) There were clear changes in watershed landscape structure as a result of urbanization, in which the patch numbers of open waters sharply declined, and the cultivated land, grassland, building land increased. The area of building lands increased rapidly, and its separation decreased in landscape scale during period of 2003 and 2009. With the area of cultivated land declined substantially, the landscape fragmentation in cultivated lands, open waters and grasslands increased. As results,the dominant landscape type in watershed transformed from woodlands/ cultivated lands to woodland/ building lands. (2) the ecological risk assessment showed that the level of ecological risk in watershed developed from the medium/lowest-risk to medium /higher-risk during the period of 2003-2009.The peak value of risk level increased from 0.1827 to 0.2309, and the mean value increased from 0.1137 to 0.1204. The ecological risk levels of watershed was showing a tendency of increasing. (3) Spatial differences of ecological risk levels were also significant in the watershed. The regions with low ecological risk levels were mainly distributed in the southern Qinlong mountain areas, and regions with high risk levels were mainly distributed in the northern areas, and the regions with medium risk levels were mainly located at the middle areas of watershed where were the farming areas during period of 2003-2009.(4) Urbanization had a positive effect on ecological risk levels, which showed the levels of ecological risk increased with urbanization. So, we should establish a watershed ecological planning to reduce the ecological risk level by protecting forestlands and croplands, and controlling the density of building lands strictly in Jiuxiang River watershed.