Relationships between borders, management agencies, and the likelihood of watershed impairment

In the United States, the Clean Water Act (CWA) establishes water quality standards important for maintaining healthy freshwater ecosystems. Within the CWA framework, states define their own water quality criteria, leading to a potential fragmentation of standards between states. This fragmentation can influence the management of shared water resources and produce spillover effects of pollutants crossing state lines and other political boundaries. We used numerical simulations to test the null prediction of no difference in impairment between watersheds that cross political boundaries (i.e. state lines, national or coastal borders, hereafter termed “transboundary”) and watersheds that cross no boundaries (hereafter “internal”). We found that transboundary watersheds are more likely to be impaired than internal watersheds. Further, we examined possible causes for this relationship based on both geographic and sociopolitical drivers. Though geographic variables such as human-modified land cover and the amount of upstream catchment area are associated with watershed impairment, the number and type of agencies managing land within a watershed better explained the different impairment levels between transboundary and internal watersheds. Watersheds primarily consisting of public lands are less impaired than watersheds consisting of private lands. Similarly, watersheds primarily managed by federal agencies are less impaired than state-managed watersheds. Our results highlight the importance of considering Integrated Watershed Management strategies for water resources within a fragmented policy framework.

在美国，《清洁水法》（Clean Water Act, CWA）确立了对维持健康淡水生态系统至关重要的水质标准。在该法的框架下，各州自行制定各自的水质标准，这可能导致各州之间的标准出现碎片化问题。这种标准碎片化现象会影响共享水资源的管理，并引发污染物跨越州界及其他政治边界的溢出效应。我们通过数值模拟检验了一项零假设预测：跨越政治边界（即州界、国家边界或海岸边界，以下简称“跨界流域”）的流域与未跨越任何边界的流域（以下简称“内部流域”）在水体受损程度上不存在差异。研究结果显示，跨界流域的水体受损概率高于内部流域。此外，我们基于地理与社会政治驱动因素，探究了该关联背后的潜在成因。尽管人工改造土地覆被、上游集水区面积等地理变量与流域水体受损存在关联，但流域内土地管理机构的数量与类型，能更好地解释跨界流域与内部流域之间的受损程度差异。以公共土地为主的流域，其水体受损程度低于以私有土地为主的流域。同样，主要由联邦机构管理的流域，其受损程度也低于州级机构管理的流域。本研究结果凸显了在政策碎片化的框架下，针对水资源采用流域综合管理策略的重要性。