Identification of critical source areas: an efficient way for agricultural non-point source pollution control
Identification of critical source areas: an efficient way for agricultural non-point source pollution control
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作者：
Zhou Hui;Zhu Xiao
农业面源污染是河流和湖泊富营养化蔓延趋势上升的主要因素，在中国的许多地区和世界上，NPS污染已经成为一个紧迫的环境问题。从农业用地排放的NPS污染物通常没有明确或可识别的排放点，并在大量的点上少量进入水环境系统。因此，通过技术措施来控制它们是极其困难的。如果没有足够的时间和资金来实施所有推荐的最佳管理实践(BMP)，缓解NPS污染造成的水质问题就更加复杂。为了在有限资源的情况下最大限度地改善受损的水环境，需要采取创新措施，按照优先顺序处理污染源，以加快修复进程。非点源污染在空间上并不均匀分布。(许多污染源微不足道，而其他污染源则对水资源损害有很大影响。)总体而言，地表径流和地下径流中的几乎所有非点源污染物在几次相对较大的风暴事件期间主要来自农业流域内的小区域，因为气候、地形、水文、土壤和农业因素往往结合在一起，使一些非点源比另一些更不利于水资源的有益利用。这些小部分土地包括特定的、可识别的、最容易受到污染物损失的关键源区(CSA)。识别CSA并将管理工作集中在这些最小的土地部分是比在大范围内实施一般战略更好的措施，本文综述了国内外与CSA识别相关的研究进展，旨在促进这一有效的水质管理策略的应用。NPS关键区域的定义包括识别主要污染源和评估从源头到受控水的水文传输系统。向水道贡献大量污染物的源区往往会对水资源产生重大影响，而不考虑水文系统的效率。而水文传输系统的效率差异使得污染物从一个源比从另一个源向水环境输送的可能性更大。因此，在确定CSA时，必须考虑源和水文传输系统的大小，以及这两组因素的一致性。在不同的NPS污染物中，磷(P)被认为是富营养化的限制营养物质，因此在淡水系统中尤其令人担忧。发达国家已经提出了几种策略，以确定景观中易受P损失影响的区域。近似方法，如出口系数，多准则分析和详细的数学模型都被用来确定农田和汇水区的土壤磷素损失。由于特定场地对污染物输出的脆弱性取决于众多因素，利用易于获得的信息并具有更大的修改灵活性的多标准分析方法被最常用作得出磷损失风险评估的手段。典型的土壤磷素损失风险评估框架如下：(1)收集背景信息，建立指标体系，然后确定相关的权重和数值。(2)将研究区域划分为小单元，(3)建立风险评价指数模型，计算各单元内的风险指数。(4)对风险指数进行降序排序，确定指标图中的临界区。最后，讨论了各种方法的优缺点及其在不同尺度和不同地理环境下的适用性，并提出了改进中国社区安全评估识别工作的建议。
Agricultural non-point source(NPS) pollution,a major factor of the upward trend in the spread of eutrophication of rivers and lakes,has become a pressing environmental concern in many regions of China as well as of the world.NPS pollutants discharged from agricultural land usually have no defined or identifiable point of outfall and enter the aquatic environment system in small quantities at a large number of points.It is therefore extremely difficult to control them by technical measures.Mitigating water quality problems caused by NPS pollution is further complicated if sufficient time and funding are not available to implement all the recommended best management practices(BMPs).In order to achieve the greatest improvement of impaired aquatic environments with limited resources,innovative measures are needed to speed up the restoration processes by treating the pollution sources in order of priority.The non-point source pollution is not spatially evenly distributed.(Many sources are insignificant,while others contribute substantially to water resource impairment.) In general,almost all NPS pollutants in both surface runoff and subsurface flow orginated primarily from only small areas within agricultural watersheds during a few relatively large storm events,because climatic,topographic,hydrologic,pedologic,and agronomic factors often combine to make some non-point sources more detrimental than others to the beneficial use of water resources.These small portions of land comprise the critical source areas(CSAs) that are specific,identifiable,and most vulnerable to pollutants loss.Identifying CSAs and concentrating management efforts on these minimal portions of land are better measures than implementing general strategies over a broad area,since they help ensure that limited available resources are appropriated as efficiently as possible.In this paper,current research progresses related to CSAs identification,both in China and in the world,were reviewed with a purpose to boom the application of this efficient water quality management strategy.Defining critical NPS areas involves identifying the major pollutant sources and assessing the hydrologic transport system from the sources to the receiving water of concern.A source area that contributed large amounts of pollutants to a waterway often has a significant impact on water resources regardless of the efficiency of the hydrologic system,while the difference in the efficiency of the hydrologic transport system made the delivery of pollutants to aquatic environments from one source much more likely than from the other.Thus,to determine the CSAs,the magnitude of the source and the hydrologic transport system,as well as the coincidence of the two sets of factors must be considered.Among varies of NPS pollutants,phosphorus(P) is of particular concern in freshwater systems because it is thought to be the limiting nutrient for eutrophication.Several strategies have been proposed in developed countries as a means of identifying areas within a landscape that are vulnerable to P loss.Approaches such as 'export coefficient','multi-criteria analysis' and 'detailed mathematical models' were all adopted to identify CSAs at field and catchment.As the vulnerability of a particular site to pollutants exportation is dependent on numerous factors,multi-criteria analysis approaches designed for application to both field and catchment,utilizing readily accessible information and having also more flexibility for modifications,have been most frequently used as a means of deriving a P loss risk assessment.A typical framework for CSAs identification is as follows:(1) Collecting background information and developing an indicator system followed by determining the related weights and values.(2) Discretizing the study area to small units,within which indicators could be properly quantified.(3) Developing risk assessment index models and calculating the risk indices within each unit.(4) Sorting the risk indices with descending order according to which to determin the critical areas from the index map.Finally,the advantages and disadvantages of those approaches were discussed together with their applicability at different scales and to different geographical context.Some suggestions were also put forward to improve CSAs identification work in China.