Long Term Monitoring

Rapid Assessment



Reference Site & Habitat Class Analogy
Reference Conditions
Classification of Habitats
Index of Biological Integrity
Hydrogeomorphic Model
Indicator Statistical Methods
Indicator Results





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CRAMP Rapid Assessment. Hydrogeomorphic Model (HGM)

The HGM approach, using function, geomorphic position, and hydrology to assess wetlands, recognizes the influence of hydrology and geomorphology on biology. It was developed by scientists at the US Army Corps of Engineers (Smith et al. 1995) to measure the ability of a wetland to perform critical functions. Its functional capacity index, which estimates the operational ability of wetland processes, ranges from 0 to 1. It was originally developed to assist in permit review and mitigation, by identifying and assessing environmental impacts (Magee 1996). HGM involves classification, function definition, and reference development. The general HGM principles are similar to the IBI approach. Both models initially develop an environmental classification scheme and depend on reference conditions to define system integrity. They differ in that HGM is based on functional rather than environmental classification and attributes are not solely biological.

There has been recent interest in applying a hydrogeomorphic model (HGM) classification approach to Hawaiian coral reefs (USACE Coral Reef Functional Assessment Workshop 2004). This model has been applied widely to wetlands and places emphasis on abiotic features with three components: (a) geomorphic setting, (b) water source and its transport, and (c) hydrodynamics (Brinson 1993; Brinson and Rheinhardt 1996; Magee 1996). Geomorphic setting is the topographic location of the wetland within the surrounding landscape. The types of water sources can be simplified to precipitation, surface or near-surface flow, and groundwater discharge. The third component (hydrodynamics) refers to the direction of flow and strength of water movement within the wetland. These components are responsible for maintaining many of the functional aspects of wetland ecosystems.

The Coral Reef Functional Assessment Workshop was held at the University of Hawai‘i from 31 Aug to 2 Sept 2004 under the auspices of the U. S. Army Corps of Engineers (USACE) with participation by EPA, Hawaii DOH, NOAA, CZM and a wide range of research units. The main purpose was to explore the possibility of using the HGM model in developing reference sites for the Hawaiian Islands.

The analysis of the Rapid Assessment data demonstrated that when identifying major factors, the composition of biological communities is controlled by the physical factors of wave energy and depth zone, which define broad ecological habitats. This result suggested an approach similar to the broad HGM classifications for the first tier, in which geomorphology and hydrodynamic characteristics (depth, degree of wave shelter, wave regime) define the major habitat classes. Further, it is necessary to make reef condition comparisons only within each major habitat. For example, low coral coverage may be more indicative of wave regimes and depth than of deteriorated conditions and coral cover was found to be statistically significant between depths. Thus in defining the habitat classifications, the HGM model provided a valid first approximation for separating out natural from anthropogenic effects.

The Hydrogeomorphic approach considers geomorphological and hydrodynamic characteristics such as wave energy regimes.


Brinson, M. M. 1993. A hydrogeomorphic classification for wetlands, Technical Report WRP-DE-4, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. NTIS No. AD A270 053.

Brinson, M.M. and R.D. Rheinhardt. 1996. The role of reference wetlands in functional assessment and mitigation. Ecological Applications 16:69-76.

Magee, D.W. 1996. The Hydrogeomorphic Approach: A Different Perspective. Bulletin of the Society of Wetland Scientists 13(2): 12-14

Smith, D. R., Ammann, A., Bartoldus, C., and Brinson, M. M. 1995. An approach for assessing wetland functions using hydrogeomorphic classification, reference wetlands, and functional indices. Technical Report WRP-DE-9, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. NTIS No. AD A307 121.


Last Update: 04/21/2008

By: Lea Hollingsworth

Hawai‘i Coral Reef Assessment & Monitoring Program

Hawai‘i Institute of Marine Biology

P.O. Box 1346

Kāne‘ohe, HI 96744

808-236-7440 phone

808-236-7443 fax