Abstract：
Freshwater systems in cold regions, including the Laurentian Great Lakes, are threatened by both eutrophication and salinization, due to excess nitrogen (N), phosphorus (P) and chloride (Cl-) delivered in agricultural and urban runoff. However, identifying the relative contribution of urban vs. agricultural development to water quality impairment is challenging in watersheds with mixed land cover, which typify most developed regions. In this study, a self-organizing map (SOM) analysis was used to evaluate the contributions of various forms of land cover to water quality impairment in southern Ontario, a population-dense, yet highly agricultural region in the Laurentian Great Lakes basin where urban expansion and agricultural intensification have been associated with continued water quality impairment. Watersheds were classified into eight spatial clusters, representing four categories of agriculture, one urban, one natural, and two mixed land use clusters. All four agricultural clusters had high nitrate-N concentrations, but levels were especially high in watersheds with extensive corn and soybean cultivation, where exceedances of the 3 mg L-1 water quality objective dramatically increased above a threshold of ∼30 % watershed row crop cover. Maximum P concentrations also occurred in the most heavily tile-drained cash crop watersheds, but associations between P and land use were not as clear as for N. The most urbanized watersheds had the highest Cl- concentrations and expansions in urban area were mostly at the expense of surrounding agricultural land cover, which may drive intensification of remaining agricultural lands. Expansions in tile-drained corn and soybean area, often at the expense of mixed, lower intensity agriculture are not unique to this area and suggest that river nitrate-N levels will continue to increase in the future. The SOM approach provides a powerful means of simplifying heterogeneous land cover characteristics that can be associated with water quality patterns and identify problem areas to target management.
摘要：
寒冷地区的淡水系统，包括劳伦森五大湖,受到富营养化和盐渍化的威胁，由于过量的氮（N），农业和城市径流中输送的磷(P)和氯(Cl-)。然而,确定城市与城市的相对贡献在土地覆盖混合的流域，农业发展对水质损害具有挑战性，这是最发达地区的典型代表。在这项研究中,自组织图（SOM）分析用于评估安大略省南部各种形式的土地覆盖对水质损害的贡献，人口稠密，然而，劳伦蒂大湖流域的高农业区，城市扩张和农业集约化与持续的水质受损有关。流域分为八个空间集群，代表四类农业，一个城市,一个自然,和两个混合土地利用集群。所有四个农业集群都有很高的硝酸盐氮浓度，但是在广泛种植玉米和大豆的流域中，水平特别高，其中3mgL-1水质目标的超标量急剧增加，高于至少30％分水岭行作物覆盖率的阈值。最大磷浓度也发生在排水最严重的经济作物流域，但是P与土地利用之间的联系并不像N那样清晰。城市化程度最高的流域的Cl-浓度最高，城市地区的扩张主要是以牺牲周围的农业用地覆盖为代价的。这可能会推动剩余农业用地的集约化。瓷砖排水的玉米和大豆地区的扩张，往往以混合为代价，低强度农业并不是该地区独有的，这表明河流硝酸盐氮水平将在未来继续增加。SOM方法提供了一种强大的方法，可以简化与水质模式相关的异质土地覆盖特征，并确定目标管理的问题区域。
