The Need to Define Hydrologic Equivalence at the Landscape Scale for Freshwater Wetland Mitigation

Attempts to replace wetlands or define hydrologic equivalence for wetland mitigation must be based on an understanding of the complexity of wetland hydrology and of the relationship of individual wetlands to the landscape. Because mitigation has the potential to re‐configure the kinds and spatial distribution of wetland ecosystems over large geographic areas, I advocate a landscape approach to defining hydrologic equivalence. This approach does not depend on specification of hydroperiod or other hydrologic variables for individual wetlands. It relies instead on knowledge of landscape properties that control wetland hydrology and water chemistry. In this paper I develop the conceptual framework for defining hydrologic equivalence for wetland mitigation viewed as a de facto landscape management policy with the potential to reduce the diversity of wetland types within regions. I review modern hydrogeological understanding of where wetlands form in the landscape and identify key hydrologic variables responsible for the formation of specific wetland types. I also review existing evaluations of mitigation projects in several states. On the basis of these reviews, I argue that, in setting regulatory criteria for judging hydrologic equivalence, the scale must be enlarged from the individual wetland project to include the broader landscape. Only this broader view can provide the context within which decision‐makers can evaluate the potential cumulative effects of individual mitigation decisions on broad‐scale patterns of wetland diversity. The landscape approach to defining hydrologic equivalence that I advocate is based on the concept of templates for wetland development. These templates are the diversity of settings created in specific landscapes by the complex interactions of hydrogeologic factors and climate. These interactions, in turn, control key hydrologic variables and hydrologically influenced chemical variables that cause specific wetland types to form. Hydrologic equivalence then can be defined at the scale of landscapes in terms of the kinds, numbers, relative abundances, and spatial distribution of wetland templates. The approach can be implemented through the identification of landscape goals and profiles based on knowledge of these templates. The profiles would catalog and map the diversity of wetland templates and the diversity of existing wetland types within a given landscape, focusing the attention of decision‐makers on broad‐scale patterns of loss in wetland types and providing a context within which individual mitigation projects could be evaluated. Landscape goals for maintaining a diversity of wetland templates are suggested.