城市景观组分影响水质退化的阈值研究

运用景观格局与水质监测方法评价城市景观变化对河流水质的影响,是当前景观格局-效应研究的热点问题.为实现城市发展目标与水环境保护目标的统一,需要科学判断城市景观变化对水质的影响程度与范围,特别是以城市不透水表面为代表的景观组分变化,是目前水质退化研究中的核心对象,而对水质退化的景观阈值研究目前尚存争论.基于截面数据进行统计分析,构建阈值判定方法,选择深圳市为案例研究区,研究快速城市化地区的河流水质退化的景观阈值水平.结果表明,在深圳市,河流缓冲区宽度为100-200 m时,景观变化对水质显著性影响最高(P＜0.001).缓冲区内,景观变化与耗氧、营养盐等类指标呈指数关系,具有显著性,是这类指标变化的最主要影响因素；同时,景观变化与有毒物质及重金属等类指标呈指数关系,具有显著性,但并非这类指标变化的最主要因素.影响水质退化的不透水表面比例阈值水平介于38.2％-50％之间,最小阈值水平为38.2％,即当流域缓冲区内不透水表面百分比超过38.2％时,河流水质显著退化.

The degradation threshold of water quality associated with urban landscape component

It is one of the key topics in landscape pattern and ecological process analysis to combine landscape pattern analysis and water quality monitoring, so as to assess the impact of urban landscape change on water quality. Moreover, to realize the harmonization between urban development and water quality protection, it is in great need for the quantitative analysis of the relationship between urban landscape change and water quality degradation. Unfortunately, there is no enough scientifically understanding on the very relationship so far. One of the solutions is to quantify the degradation threshold of urban landscape change impact on water quality. In this research, nonlinear regression was used to analyze the relationship between urban landscape change and water quality degradation, and a new method was developed to estimate the threshold based on cross-section statistics. Impervious Surface Area (ISA) was conducted to monitor the urbanization degree in the watershed, and chemical indicators were used to analyze the water quality. All of 31 sample sites and watersheds were selected in Shenzhen. Watershed and buffer zone scale of urban landscape change was quantified with the application of Linear Spectral Mixture Method using Landsat TM images, and water quality was monitored 12 times a year using 15 water quality indicators. Both were collected in 2005. The results indicated that, when stream buffer zone was set at 100-200m, there was the highest significance of the impact of urban landscape change on water quality, with the P value less than 0.001. Furthermore, the exponential relationships between such water quality indicators as DO, S^2-, CGB and F^- and urban landscape change were significant when the flow path distance was accepted, with the significantly exponential relationships for COD_Mn, BOD₅, NH₃-N, TP, TN, Zn, Pb, VP, Oils, ANC and SO₄^2- in Euclidean distance. The exponential relationships between oxygen consumption, eutrophication of water quality and impervious component indicated that urban landscape change was the most important influencing factor, which interpreted about 50-60% of water quality degradation. However, for other water quality indicators, such as heavy metals and toxicity, their changes could only be interpreted by urban landscape change for 30%-40%, which meant urban landscape change was no longer a significant influencing factor compared with other factors such as point source pollution input to stream ecosystem. These results agreed with that urbanization degree of riparian was of great significance to chemical water quality of stream. Furthermore, with the exception of Zn, F^-, Pb and Oils, the degradation threshold level of water quality associated with landscape change was between 38.2% and 50%. The minimum value of 38.2% meant if the ratio of watershed imperviousness area reached this level, water quality degradation would began and the water quality would be prone to be hardly recovered. The results showed that if the target of watershed management was set as to control stream water quality, it is in great need to scientifically plan the impervious surface area in stream buffer zone.