**The Hydrogeomorphic Approach to Functional Assessment for Piedmont
Slope Wetlands**

Bruce Vasilas^1^, Lenore Vasilas^2^ and Michael Wilson^2^

^1^Department of Plant and Soil Sciences, University of Delaware

^2^USDA Natural Resources Conservation Service

***Abstract***

Wetland functions are the characteristic activities (biological,
chemical, and physical processes) that occur in these ecosystems.
Functional assessment models are used to quantify the functional
capacity of individual wetlands. The Hydrogeomorphic (HGM) Approach to
functional assessment of wetlands classifies wetlands to the regional
subclass level and then creates a model for an individual subclass in a
given geographic area. An HGM model is essentially a functional profile
that describes the physical, chemical, and biological characteristics of
wetlands in the regional subclass, identifies those functions that are
most likely to be performed on a sustained basis, and addresses
landscape and ecosystem attributes that can be expected to influence
each targeted function. The HGM Approach does not assign absolute values
to functional capacity. Instead it rates functional capacity for a given
wetland relative to reference standards-the highest level of functional
capacity exhibited by wetlands in the regional subclass. Reference
standards are established by assessment of wetlands that exhibit those
attributes consistent with maximum sustained functional capacity. The
HGM Approach is presently being applied to Piedmont slope wetlands in
the Mid-Atlantic Region.

***Introduction***

The purpose of this paper is to introduce the reader to (1) the
Hydrogeomorphic (HGM) Approach to functional assessment of wetlands and
(2) an HGM model developed for Piedmont slope wetlands in the
Mid-Atlantic Region which is in the developmental stage. Our intent is
not to familiarize the reader to the intricacies of HGM. That is beyond
this paper's scope and would require an extensive time commitment.
Rather, our objective is to summarize one of the functional assessment
methods available in hope that this paper will help individuals decide
whether the HGM Approach has merit pursuing for their particular
purposes. For greater detail on the subject the reader is encouraged to
read Brinson (1993, 1995, 1996), Brinson et al. (1995), and Smith et al.
(1995).

Wetland functions are the characteristic activities (biological,
chemical, and physical processes) that occur in these ecosystems. Some
examples of functions are nitrogen removal through denitrification,
interception of storm water, and soil organic matter accretion. Two
wetlands may not have the same capacity for any given function because
of inherent differences in size, hydrology, water chemistry, plant
community composition, etc., or because of varying degree of
anthropogenic impacts. A number of attempts have been made to develop
functional assessment procedures to quantify the functional capacity of
individual wetlands for purposes of mitigation (compensation "in kind"),
selection of sites for wetland enhancement, identification of
environmentally critical areas, or evaluation of the health of wetlands
in a given geographic area.

***Discussion***

**Overview of the HGM Approach**

In 1991 the Army Corps of Engineers initiated efforts to develop the

Hydrogeomorphic (HGM) Approach to functional assessment of wetlands.
Basic concepts of the HGM Approach were first published in 1995 (Smith
et al. 1995).

The HGM Approach considers structural components of the wetland and
surrounding landscape such as plants, animals, hydrology, and soils;
biological, chemical, and physical processes; and the interaction of
these components and processes. Surrounding land use is addressed
because it impacts structural components and processes in the wetland.
For example, surrounding land use can significantly affect hydrologic,
nutrient, and sediment loading into a wetland.

The HGM Approach has four primary elements that are used to assess the
functions of wetlands (Brinson 1993, Brinson 1995, Brinson 1996): (1)
the HGM classification, (2) identification, definition, and description
of subclass functions and selection of reference wetlands, (3)
development of assessment model/functional indices, and (4)
establishment of assessment protocols. This paper will address elements
1-3.

The following terminology is critical to the HGM Approach and will be
used throughout the course of this discussion.

1.  Functional capacity: The degree to which a particular wetland
    performs a particular function. It does not refer to performance
    across multiple functions.

> 2\. Functional capacity index: Ratio of functional capacity of a
> wetland under
>
> existing conditions to the functional capacity of a wetland exhibiting
> reference
>
> standards.

3\. Reference domain: All wetlands of a single HGM subclass in a defined

geographic area.

4\. Reference standards: Highest level of sustainable functional
capacity exhibited

within a HGM regional subclass.

5\. Reference wetlands: Sites within the reference domain that encompass
the full

range in functional capacity for that subclass.

6\. Reference standard wetlands: Subset of reference wetlands that is
comprised of

those wetlands that exhibit the attributes consistent with maximum
sustained

functional capacity. In general, these are wetlands that exhibit the
least

anthropogenic disturbance in landscapes that exhibit the least
anthropogenic

disturbance.

7.  Subindex: Rating assigned to a model variable based on its
    relationship to functional capacity.

Below is a list of basic concepts/procedures inherent to the HGM
Approach. The list is not meant to be all-inclusive. Rather it is meant
as a template for a more in-depth discussion which follows.

1.  The wetlands of interest are classified to a regional subclass
    level. This classification serves to minimize some of the
    variability associated with wetland types, and, therefore, increases
    model resolution.

2.  Reference wetlands are selected for data collection.

3.  The surrounding landscape is also considered in the process as
    surrounding land

> use impacts structural components and processes in the wetland.

4.  A subset of the reference wetlands is chosen to represent those
    wetlands exhibiting the highest sustainable level of functional
    capacity (reference standards). This subset (reference standard
    wetlands) usually represents wetlands that exhibit the least
    anthropogenic disturbance in landscapes that exhibit the least
    anthropogenic disturbance.

5.  A suite of functions is selected based on perceived significance to
    that subclass.

6.  Each function is evaluated individually.

7.  Simple aggregation models (assessment models) are constructed to
    show the relationship between a given function and measurable
    wetland attributes (model variables).

8.  Model variables are assigned numerical values (subindices) to
    represent functional capacity. The value range is 0.0-1.0. Reference
    standards are assigned a value of 1.0.

9.  Each assessment model estimates functional capacity for one
    function.

10. The HGM model is comprised of the entire suite of functional
    assessment models.

11. Inherent to the HGM Approach is that the initial model may be
    revised during

> model calibration, verification, field testing and validation.

**HGM Classification**

The HGM Approach uses a hierarchial classification with seven current
hydrogeomorphic classes: depressions, slope, riverine, organic soil
flats, mineral soil flats, estuarine fringe, and lacustrine fringe
Table 1. The purpose of the classification is to minimize functional
variability inherent to major differences in wetland types. The
hydrogeomorphic designation is based on three primary characteristics
that distinguish wetland types and significantly influence functional
capacity: geomorphic setting (topographic position in the surrounding
landscape), primary hydrologic input (precipitation, ground water
discharge, surface or near-surface flow), and hydrodynamics (water
movement in a wetland). Hydrodynamics are characterized by both the
direction(s) and strength of water movement. For example, mineral soil
flats have low energy, vertical water table fluctuations; riverine
wetlands have high energy, unidirectional lateral surface water flows
associated with overbank flooding. Each class may be further divided
into subclasses for a specific geographic region, e.g. Delmarva Bays
(depressions on the Delmarva Penninsula). It is important to note that
hydroperiod-the seasonal pattern of the water level in a wetland-is not
a core component although it significantly impacts functions associated
with water storage, biogeochemical cycling, and wildlife habitat.
Diversity in hydroperiods is common within a class. For example,
depressions may be seasonally saturated, permanently saturated,
seasonally inundated, or permanently inundated. This classification
system also differs from many previous systems in that it is independent
of vegetation. Instead, it addresses abiotic factors that are known to
impact plant community composition and, in turn, wildlife habitat.

Table 1. Hydrogeomorphic classes of wetland showing dominant water
sources, hydrodynamics, and examples of subclasses (Smith et al., 1995).

+---------------------+--------------+---------------+-----------------+
| Hydrogeomorphic     | Water Source | Hydrodynamics | Example of      |
| Class               |              |               | Regional        |
|                     | (dominant)   | (dominant)    | Subclass        |
| (geomorphic         |              |               |                 |
| setting)            |              |               |                 |
+---------------------+--------------+---------------+-----------------+
| Riverine            | Overbank     | U             | Bottomland      |
|                     | flow from    | nidirectional | hardwood        |
|                     | channel      | and           | forests         |
|                     |              | horizontal    |                 |
+---------------------+--------------+---------------+-----------------+
| Depression          | Return flow  | Vertical      | Delmarva Bay    |
|                     | from         |               |                 |
|                     | groundwater  |               |                 |
|                     | and          |               |                 |
|                     | interflow    |               |                 |
+---------------------+--------------+---------------+-----------------+
| Slope               | Return flow  | Un            | Sideslope Seeps |
|                     | from         | idirectional, |                 |
|                     | groundwater  | horizontal    |                 |
+---------------------+--------------+---------------+-----------------+
| Mineral soil flats  | P            | Vertical      | Wet pine        |
|                     | recipitation |               | flatwoods       |
+---------------------+--------------+---------------+-----------------+
| Organic soil flats  | P            | Vertical      | Peat bog        |
|                     | recipitation |               |                 |
+---------------------+--------------+---------------+-----------------+
| Estuarine fringe    | Overbank     | B             | Chesapeake Bay  |
|                     | flow from    | idirectional, | marsh           |
|                     | estuary      | horizontal    |                 |
+---------------------+--------------+---------------+-----------------+
| Lacustrine fringe   | Overbank     | B             | Great Lakes     |
|                     | flow from    | idirectional, | marsh           |
|                     | lake         | horizontal    |                 |
+---------------------+--------------+---------------+-----------------+

**Reference Wetlands and Standards**

An HGM model is essentially a functional profile that describes the
physical, chemical, and biological characteristics of wetlands in the
regional subclass, identifies those functions that are most likely to be
performed on a sustained basis, and addresses landscape and ecosystem
attributes that can be expected to influence each targeted function.

Development of HGM models is based on data collection from reference
wetlands within a given regional subclass. Wetlands within a subclass
exhibit diversity in functional capacity because of inherent differences
in natural processes or degree of anthropogenic disturbance. Therefore,
the reference wetlands should span the range of variability exhibited in
that regional subclass. Reference wetlands are selected from a defined
geographic area or 'reference domain'. The reference domain selection
should consider project objectives and priorities. The reference domain
may be relatively small, such as a localized watershed, or as large as a
physiographic region. It should be realized that a large reference
domain will exhibit a greater range in subclass diversity, requiring a
greater sample size, and lower assessment resolution.

The HGM Approach does not assign absolute values to functional capacity.
Instead it rates functional capacity for a given wetland relative to
reference standards-the highest level of functional capacity exhibited
by wetlands in the regional subclass. Reference standards are
established by assessment of wetlands that exhibit those attributes
consistent with maximum sustained functional capacity. These reference
standard wetlands are a subset of the reference wetlands.

Inherent to the HGM Approach is the assumption that the highest
sustainable, functional capacity occurs in wetlands and surrounding
landscapes that have not been subject to long-term anthropogenic
disturbance. Therefore, the reference standard wetlands are usually
selected on that basis. From a conceptual standpoint this assumption is
open to criticism. For example, constructed wetlands are created for
their capacity to improve water quality. Most natural wetlands have a
lower functional capacity for sedimentation and denitrification. From a
practical standpoint, in areas like the Mid-Atlantic Coastal Plain
anthropogenic disturbance is near ubiquitous. This forces the
practitioner to define a working "pristine' condition for purposes of
selecting reference standard wetlands.

**Assessment Models/Variables**

Each HGM model is a series of simple multivariate formulas (assessment
models) that express the relationship between characteristics of the
wetland and surrounding landscape and the capacity of a wetland to
perform a specific function. Each assessment model addresses one
function. Assessment model variables represent attributes that influence
function, for example soil organic matter (denitrification), surface
roughness (energy dissipation), and microtopography (surface water
storage). A given attribute may impact multiple functions. Therefore, a
given variable may be used in multiple assessment models. The assessment
models may be simple or relatively complex. Below are several examples
of assessment models adopted from regional guidebooks.

1\. Function: groundwater discharge (depressions)

Index of function=V~GNDWTR~

Where: V~GNDWTR~=evidence of groundwater discharge

2\. Function: long term surface water storage (low energy riverine
systems)

Index of function=(V~SURWAT~+ V~MACRO~+V~MICRO~)/3

> Where: V~SURWAT~=presence of surface water; V~MACRO~= macrotopographic
> relief; and V~MICRO~=microtopographic complexity

3\. Function: dynamic surface water storage (riverine)

> Index of function=\[(V~FREQ~+V~WETUSE~)/2
> x(V~INUND~+V~MACRO~+V~WROUGH~+V~HROUGH~+V~CWD~)/5\]^1/2^
>
> Where: V~FREQ~=frequency of overbank flow; V~WETUSE~=wetland land use
> (based on agricultural impacts); V~INUND~=average depth of inundation;
> V~MACRO~=macrotopographic relief; V~WROUGH~=woody vegetation
> roughness; V~HROUGH~=herbaceous vegetation roughness; and
> V~CWD~=coarse woody debris.

Model variables are assigned a subindex on the basis of the relationship
between the variable and functional capacity. Variables are assigned a
subindex based on qualitative (nominal or ordinal) or quantitative
(interval or ratio) scale data collected from the reference wetlands.
The subindex range is 0.0 to 1.0 where 1.0 represents a condition
similar to the reference standards. Therefore, attribute data must be
transformed to fit the allowable range. An example is presented below
(Adopted from Smith et al, 1995):

Variable: Frequency of overbank flooding

1.  Stage data indicate a return interval of less than 2 years:
    subindex=1.0

2.  Stage data indicate a return interval of 2-5 years: subindex=0.5

3.  Stage data indicate a return interval of greater than 5 years; no
    long-term alteration of hydrology is evident: subindex=0.1

4.  Stage data indicate a return interval of greater than 5 years;
    long-term alteration of hydrology is obvious: subindex=0.0

In data collection for model development and testing, complex variables
such as water chemistry or variables which are temporally dependent such
as soil Eh may be considered. However, for a model to be of practical
use, simple (and, preferably, visual) indicators of such variables may
be used to generate a subindex. Indicators are readily- measured (or
observed) characteristics that correlate to a quantitative measure of a
variable. Examples of possible surrogates include woody debris for
biogeochemical cycling of nutrients and microtopography for water
storage in slope wetlands.

Because model variable data is scaled to reflect the relationship to
reference standards, the assessment models generate functional capacity
indices (FCI). The possible range in FCI is 0.0 to 1.0. A value of 1.0
indicates that the wetland is performing the function at a level
equivalent to the reference standard, or more accurately has the
necessary attributes consistent with that of the reference standard
wetlands. A value less than 1.0 indicates that the wetland has a
capacity for that particular function below that of the reference
standard wetlands. A value of 0.0 represents an unrecoverable loss of
function. Variables are mathematically arranged in the model to reflect
the perceived interaction between attributes and to prevent the FCI from
exceeding a value of 1.0.

**Piedmont Slope Wetland Model**

A HGM model was recently created for Piedmont slope wetlands in
Pennsylvania, Delaware, and Maryland. The model was created from data
collected from twenty six sites over a five year period. The primary
source of hydrology is low energy ground water discharge from seeps.
Some are side-slope discharges; others are toe-slope discharges. Most of
the sites are driven by side-slope seeps and permanently saturated.
Those sites driven by toe-slope seeps exhibit dry intervals and are more
dependent on precipitation to maintain saturation. Several of the sites
are floodplains in that, at times, flowing surface water is present. In
some of the side-slope sites surface water becomes channelized
throughout much of the wetland and contributes to first order streams.
The vegetation is mixed hardwood forest, scrub-shrub, or emergent
(herbaceous). The emergent sites in general are the wettest. All sites
have mineral soils although histic epipedons (organic surface horizons
at least 20 cm thick) are present in several sites.

Development of this HGM model was close to completion at the time this
paper was submitted. However, the assessment models were not finalized.
All data have been collected. The functions and assessment model
variables have been chosen. Some of this information is presented below
to assist those individuals who may consider creating their own slope
model.

The following functions were selected for assessment:

1\. Surface and shallow subsurface water storage

2\. Particulate retention (physical processes)

3\. Organic carbon export

4\. Cycling of nutrients

5\. Removal of elements and compounds (biogeochemical processes)

6\. Maintenance of characteristic plant communities

7.  Maintenance of characteristic wildlife habitat

Reference data were collected for the following categories:

1\. Vegetation: species composition, percent cover, number of strata,
presence of

alien species

2\. Soils: soil classification, textural class, presence/depth O and A
horizons, percent

organic matter, hydric soil indicators

3\. Hydrology: water table depth, presence of surface outflow channel,
presence of

sub-surface outflow, aquic moisture regime

4\. Wetland assessment area: slope, aspect, physical dimensions, surface
roughness

5\. Landscape/wetland complex: landscape disturbance level, landscape

connectivity, landscape complexity, wetland proximity, buffer condition

Selected model variables include the following:

Vegetation

V~PSCOMP~ \~ Plant species composition

V~VSTRATA~ \~ Vegetation strata index

V~TBIOMASS~ \~ Tree biomass

V~SBIOMASS~ \~ Shrub biomass

V~GVBIOMASS~ \~ Ground vegetation biomass

V~WDBIOMASS~ \~ Woody debris biomass

V~DETRITUS~ \~ Detritus biomass

Soils

V~SOM~ \~ Soil organic matter

V~CLAY~ \~ Clay content

V~AQUIC~ \~ Aquic moisture regime

> V~SINFILT~ \~ Water infiltration downward from the soil surface

## Hydrogeomorphic

V~SOURCE~ \~ Water source area condition

V~SURSLOPE~ \~ Wetland surface slope

V~WTDEPTH~ \~ Water table depth

V~SUBOUT~ **\~** Subsurface outflow

V~SROUGH~ \~ Surface roughness or micro-topography of the wetland
surface plane

V~WFCONNECT~ \~ Surface water outflow connectivity

## Landscape

V~LDISTURB~ \~ Landscape level disturbance

V~LCONNECT~ \~ Landscape level connectivity

V~LCOMPLEX~ \~ Landscape level complexity

V~WPROXIMITY~ \~ Proximity of nearest suitable wetland

V~BCONDITION~ **\~** Condition of the buffer adjacent to the wetland

***Literature Cited***

Brinson, M.M. (1993). *A hydrogeomorphic classification of wetlands*.
Technical Report.

WRP-DE-4. U.S. Army Engineer Waterways Experiment Station, Vicksburg,
MS.

Brinson, M.M. (1995*). The HGM approach explained*. National Wetlands
Newsletter.

17(6):7-13.

Brinson, M.M. (1996). *Assessing wetland functions using HGM*. National
Wetlands

Newsletter. 18(1):10-16.

Brinson, M.M., R. Hauer, L.C. Lee, W.L. Nutter, R. Rheinhardt, R.D.
Smith, and D.F.

Whigham.(1995). *Guidebook for application of hydrogeomorphic
assessments to*

*riverine wetlands*. Technical Report. WRP-DE-11. U.S. Army Engineer

Waterways Experiment Station, Vicksburg, MS.

Smith, R.D., A. Amman, C. Bartoldus, and M.M. Brinson. (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.