Overview of the HGM Approach
HGM is divided into two connected parts. It was originally developed as a classification of wetlands based on hydrologic regime and geomorphic position (link to Brinson 1997) and later it was developed as an approach for assessing wetland functions (see G-G documents link).
The HGM Approach for assessing wetland functions, developed by scientists at the Engineer Research and Development Center (ERDC), Environmental Laboratory, is a procedure for assessing the capacity of an ecosystem to perform functions. It is designed to assess wetland and aquatic ecosystems, which are normally characterized in terms of their structural components and the processes that link these components. Structural components of the ecosystem and the surrounding landscape, such as plants, soils, hydrology, and animals, interact with a variety of physical, chemical, and biological processes. Understanding the interactions of the structural components of the ecosystem with surrounding landscape features is the basis for assessing ecosystem functions, and it is the foundation of the HGM Approach.
Wetland functions are the normal or characteristic activities that take place in wetland ecosystems. Wetlands perform a wide variety of functions. However, not all ecosystems perform the same functions nor do similar ecosystems perform the same function to the same level of performance. The ability to perform a function is influenced by the characteristics of the ecosystem and the physical, chemical, and biological processes within the ecosystem.
In the last few years HGM has been expanded to include other ecosystems including streams and other aquatic ecosystems. The HGM Approach assesses ecosystem function(s) based on deviation of measured or estimated variables from reference standard sites. The assessment is designed to be applied rapidly, provide repeatable results, and conducted nearly any time of the year in keeping with cost, manpower, and time constraints, without extensive specialized training or equipment of the agencies charged with assessing permit actions and project impacts. Therefore, actual measurements of ecosystem function are beyond the scope of the assessment procedure (though methods to make such measurements are identified in the guidebook for use in potential independent validation). Instead of directly measuring ecosystem function, the approach employs indicators of community structure and process, aggregated into simple logic models, to indirectly reflect functionality. This is not the same thing as a “condition assessment,” though the structural measures reflect condition due to the clear and easily measured disturbance regimes typically observed within the ecosystem. In this regard, the HGM assessment approach is completely consistent with other ecosystem assessment tools that use indirect indicators, including HEP and various IBI approaches.
HGM guidebooks are procedural document designed to provide the user with the necessary information and data forms to complete an assessment for the ecosystem subclass(s) and geographic region identified in the guidebook.