Functional Assessment of Wetlands
Since wetlands were first subject to federal regulation in the 1970s, federal agencies have been attempting to develop techniques for assessing wetland functions. The motivations for assessing functions have been primarily the need to predict the effects of wetland alteration and to set appropriate requirements for mitigation. More recently, assessment of functions has been used to rank or categorize wetlands, which might ensure that wetlands with highly valued functions receive greater protection than wetlands in general. Assessing functions currently presents many challenges in methodology, but the problem is a subject of active research and conceptual development.
FUNCTION AND VALUES OF WETLANDS
Wetland functions are the physical, chemical, and biological processes that characterize wetland ecosystems, such as flooding, denitrification, provision of habitat for organisms, and support of aquatic life (see Chapter 2). Objective measurement of wetland functions falls within the realm of the natural sciences and, barring changes in the ecosystem being measured, is repeatable over time. Many wetland functions are considered useful or important by society. For example, inundation of wetlands can prevent flood damage elsewhere, denitrification can improve water quality, wetland habitat can help maintain waterfowl populations, and anaerobiosis can influence the development of unique plant communities that contribute to the conservation of biodiversity.
The value of a wetland is a measure of its importance to society. Wetland functions are valued to various degrees by society, but there is no precise, general relationship between wetland functions and the value of wetlands to society, and values can be difficult to determine objectively. Wetlands also have aesthetic values. A wetland's value can be weighed directly or relative to other uses that could be made of the site; thus, the location of a wetland affects its value to society. For example, wetlands in urban settings might have higher value for recreation and education or for alternative uses of the site than wetlands in undeveloped lands or far from population centers. Assessing the value of wetlands involves methods of social science, especially economics.
Decisions about whether to protect wetlands or how much to spend on wetland protection are policy decisions based in part on the value society places on wetlands. Part of that value depends on a sound scientific knowledge of what wetlands do, i.e., a knowledge of wetland functions. Indeed, some groups have suggested the creation of a national scheme that would designate wetlands of high, medium, and low value based on some general assessment of wetland functions that does not require field evaluation. Examples of categories that have been proposed as having low value include wetlands of under 10 acres (4 ha) or of some other specific size; fastlands, which are wetlands maintained behind dikes or levees; wetlands within industrial complexes or in intensely developed areas; wetlands affected by anthropogenic disturbance; artificial wetlands; frequently farmed wetlands; and regionally abundant types of wetland. As described above, however, it is usually not possible to relate such categories in a reliable way to objective measures of wetland functions, in part because the relationships between them are variable and in part because we do not have enough scientific knowledge. Wetlands of any of the categories can have a variety of wetland functions based on objective measurements (Bostwick, 1992). Examples include the prairie potholes of the upper midwest, which are isolated wetlands generally smaller than 10 acres (4 ha), but which provide vital wildlife habitat; and the wetlands of the Lake Calumet area in heavily industrialized southeast Chicago, which provide habitat for more than 170 bird species, of which 11 are listed by the state of Illinois as threatened or endangered (Kendall, 1990). The remainder of this chapter deals with the scientific assessment of wetland functions and its usefulness in planning.
GENERAL REQUIREMENTS FOR FUNCTIONAL ASSESSMENT
Important concepts in functional assessment of wetlands include functional capacity, predictors of function (indicators), and thresholds for functions. Functional capacity is the ability of an ecosystem to perform a function. An example would be kilograms of sediment removal per hectare per year. A predictor is an observable condition, the state of which is related to the capacity or ability of a wetland to perform a function. For example, morphometry of a wetland might
serve as a predictor of sediment retention. A threshold is a functional discontinuity across which a wetland changes qualitatively. For example, a nutrient- or sediment-loading threshold is a condition above which a wetland's functional state would change. Thresholds apply to anthropogenic as well as natural conditions.
Use of functional assessment in conjunction with a Clean Water Act (CWA) Section 404 permit requires quick, simple, repeatable, and objective methods that are applicable or adaptable to a wide range of wetlands. Regulatory functional assessment should particularly facilitate analysis of wetland functions directly relevant to CWA's objectives.
METHODS OF FUNCTIONAL ASSESSMENT
Scientists and government agencies have been developing methods of functional assessment for wetlands over the last two decades. Measurement of wetland functions is fundamentally different from wetland identification or delineation. The system developed by Cowardin et al. (1979) for the National Wetlands Inventory, for example, was designed to describe classes of wetlands, arrange them in a system useful to resource managers, and provide a standard set of concepts and terms. The needs of federal agencies, however, often extend well beyond classification or delineation to planning and site selection, regulatory action, assessment of anthropogenic effects, management, mitigation, and acquisition of property. These have been the primary factors supporting the development of functional assessments (Lonard et al., 1981). Functional assessment allows comparison of a wetland to other wetlands or to other potential uses of a wetland site, and it shows the extent of compensatory mitigation that might be necessary if a wetland were altered.
The first attempt at evaluating wetland functions for regulatory purposes emerged in the USACE permit regulations of 1975, which were revised in 1977 and 1986. These regulations list wetlands that provide functions important to the public interest, including support of food chains and wildlife habitat, education and recreation, prevention of erosion, reduction of storm or flood damage, ground water discharge and recharge, water purification, and maintenance of biological diversity (33 CFR 320.4).
USACE (1979) developed a manual entitled ''Wetland Values: Concepts and Methods for Wetlands Evaluation.'' Preparation of this manual was devised specifically to assist USACE field staff in making permit decisions. Technical guidelines in the manual that encompass physical, biological, and cultural factors permit at least qualitative approximation of functional efficiency (Reppert, 1981). The manual was developed from a large amount of information on the biological and ecological characteristics of wetlands. Information on some important wetland functions, including water quality improvement, shoreline protection, ground water recharge, and flood water storage, was sparse, however (Reppert, 1981).
In 1981 the U.S. Water Resources Council published an analysis of 20 functional assessment methods used by the USACE Waterways Experiment Station. USACE concluded that few methods could assess all functions of wetlands, and that some functions were assessed very subjectively by most methods. Habitat evaluation methods were the most common and the most elaborate. Hydrologic values were poorly integrated into evaluation methods, and there were few techniques for assessment of functions associated with recreation, heritage, or agricultural potential (Balco, 1981; Lonard et al., 1981). All of the methods that were reviewed included qualitative judgments by a resource manager or an interdisciplinary team.
In 1980, the U.S. Fish and Wildlife Service (FWS) published its own wetland evaluation method, the Habitat Evaluation Procedure (HEP), to replace its previous wetland valuation system, which was based on estimates of direct public use. HEP was designed primarily to evaluate the effects of proposed projects on fish and wildlife resources. The method is based on a numerical analysis of habitat quality and quantity, and measures functional capacity. The numerical analysis of HEP permits alternative plans to be compared. The procedure is done by a team of biologists who use file data, field observations, experience, statistical analysis, and simulation modeling to develop a habitat suitability model that expresses the quality of habitat through use of a habitat suitability index for individual species on a scale of 0-1. The scale is directly related to carrying capacity or abundance of a species per unit area. For instance, habitat units might be expressed as squirrels per acre, or coveys of quail or broods of ducks per acre (FWS, 1980). HEP is used extensively by FWS for impact assessment, but it is not used regularly in the wetlands regulatory programs of USACE or NRCS. Many regulators consider the method too time consuming, and regulators and wetland scientists consider it too narrowly focused on fish and wildlife to be used in routine regulatory assessments of wetlands.
A national assessment methodology was developed for the Federal Highway Administration (FHWA), as reported by Adamus and Stockwell (1983) and Adamus (1983). The Method for Wetland Functional Assessment—or the FHWA method, as it became known—uses an extensive literature review, including evaluation of a large volume of quantitative data, to build a series of evaluation algorithms that represent the functions of wetlands. The algorithms are sequential, dichotomous decision trees that use thresholds or binary responses to rank a wetland high, moderate, or low for a specific function or value. The FHWA method was designed specifically for use by state and federal highway departments, but was revised so that it could be used more broadly. The USACE concluded that the revised FHWA method might be useful for the CWA Section 404 permit program. A modified version, the Wetland Evaluation Technique (Adamus, 1987; Adamus et al., 1991) incorporates an interactive computer analysis program.
The FHWA method was intended to provide a means of using field observa-
tions to categorize the functions of wetlands relevant to the section 404 permit review process. The method has three separate procedures: threshold analysis of single wetlands, comparative analysis of two or more wetlands, and mitigation analysis for comparison of mitigation alternatives.
The FHWA algorithms incorporate the concepts of effectiveness, opportunity, and significance. A wetland could be potentially effective in performing a particular function, for example, but there might be no opportunity for it to do so, or the function might not be significant to society. These distinctions are particularly important for functions that have high value in modified or developed landscapes, such as flood water storage, sediment retention, or pollution abatement. Wetlands receive low ratings for such functions in undeveloped landscapes where the opportunity for the function does not exist. As a drainage is developed, its ranking for such functions increases, even though the wetland itself might become environmentally degraded. Similarly, the significance of the function—the degree to which a function is valued, used, or needed by society—is likely to increase as a landscape is developed.
Temporal changes in wetland contexts expose a failing of the FHWA and other methods that use the concepts of opportunity and significance. Regulatory or management decisions based on current contexts discount future wetland values. From an ecological perspective, such management is short-sighted for systems that could persist for hundreds or thousands of years.
The FHWA method recognizes that CWA objectives are not necessarily compatible with one another. As a wetland performs some functions to a higher percentage of its potential capacity, the capacity for other functions might decrease. For example, pollution abatement by a wetland could decrease its capacity to sustain wildlife. The original FHWA method ranks relative values of wetlands after consideration of the potential incompatibilities of functions. The second generation of the FHWA method (WET) uses a slightly different approach in that it ranks the probability that a wetland will perform a given function, and the significance of the function to society, rather than directly ranking the value of the wetland.
Another recent approach to wetland evaluation is the Hollands-Magee method (Hollands and Magee, 1986), which derives a numerical index for each function; it was developed for northeastern wetlands. Other examples include the Connecticut and New Hampshire methods (Amman et al., 1986; Amman and Stone, 1991), which use observable indexes of conditions and functions to derive rankings and can be used by nonspecialists; the Wisconsin method (Reed, 1986), which is a modification of the FHWA method; the New Jersey Department of Transportation modification of the FHWA method (McColligan, 1986); the Virginia Institute of Marine Science method for nontidal wetlands (Bradshaw, 1992); and the Evaluation of Planned Wetlands method (Bartoldus et al., 1994), which emphasizes mitigation design.
USACE published WET 2.0, a revised version of WET, in 1991. WET 2.0
uses a series of word models to evaluate a wetland relative to functions and values, such as ground water recharge, ground water discharge, flood flow alteration, shoreline and sediment stabilization, sediment and toxicant retention, nutrient removal and transformation, food chain support and export of production, wildlife diversity and abundance, diversity and abundance of aquatic life, uniqueness and heritage significance, and recreational uses. The results of the WET 2.0 evaluation are qualitative rankings of the probability that a wetland performs a given function (effectiveness), that its position in the landscape allows it to perform the function (opportunity), and that the function offers societal benefits (social significance) (Adamus et al., 1991).
WET 2.0 was the first widely used methodology to encompass all wetland functions, but it does not specifically account for regional variation of wetlands. The developers of WET 2.0 used an extensive literature review, including quantitative data, to build a series of independent evaluation models for the functions of wetlands.
The FHWA method, WET, and other methods that use rank present problems in the determination of compensation ratios for wetland mitigation because they do not give quantitative estimates of the functional capacities of altered wetlands and mitigated wetlands. In general, methods for wetland evaluation do not provide a means for direct comparison of wetlands on an areal basis, do not provide a basis for estimating mitigation ratios on an areal basis, are not readily adaptable to a variety of wetland types, and have data requirements that are too cumbersome for routine field application.
FUTURE METHODS OF FUNCTIONAL ASSESSMENT
Assessment Based on Hydrogeomorphic Classification and Reference Wetlands
A new procedure for assessment of wetland functions is being developed at the USACE Waterways Experiment Station (WES). The procedure differs from WET and some other assessment procedures in two important ways. First, it recognizes that wetlands exist under a wide range of climatic, geomorphic, and hydrologic conditions that can cause variation in functions among wetlands. Second, it uses functional indexes that can be quantified on a scale that is developed from reference wetlands. The functional indexes account for the need for functional assessment to strike a balance involving consistency, reliability, and scope of coverage under the constraints of time and resources that are available to regulatory programs.
The WES procedure uses the hydrogeomorphic wetland classification developed by Brinson (1993a). This system identifies five broad groups of wetlands on a national level. Wetlands within each group show similarities in function because of similarities in geomorphic setting, water source, and hydrodynamic
features. Each of the groups is divided into regional subgroups. Subgroup divisions are based on landscape position and ecosystem type for wetlands of a given group. The functions that are most likely to be associated with a particular regional subgroup are assessed by the use of functional models that describe and define the relationship between wetland functions, the wetland ecosystem, and the landscape within which the wetland is located.
Reference wetlands in several regions are used in establishing relationships between hydrogeomorphic type, landscape, and functional capacity. These relationships form the basis for functional models that provide a scale for the functional capacity of individual wetlands across the range for a given hydrogeomorphic subgroup. The index values that result from the application of these models can be used in the evaluation of wetland alteration or to mitigation of wetland alteration.
The hydrogeomorphic approach is likely to improve the precision, consistency, reliability, and timeliness of functional assessment. Even so, it is subject to many of the same limitations that affect other procedures. Limitations involve quality and amount of background information, difficulty of incorporating a landscape perspective into the assessment, and difficulty in establishing relationships between functions and societal values.
The quality and amount of information relevant to functional assessment vary greatly among wetland types, wetland functions, and geographic regions. In general, physical functions are understood poorly by comparison with biological functions, and wetlands in the western United States have been studied less thoroughly than have been those in the eastern United States. Functional assessment models have of necessity drawn extensively on the technical literature and expert opinion. Primary research on functions within reference wetlands has been supported insufficiently in view of its relevance to quantification of functions through the use of functional indexes. The functional indexes represent only a qualitative approximation of the functional capacity of wetlands.
The hydrogeomorphic approach recognizes the reciprocal interactions of wetland ecosystems with the landscape. The functional analysis itself, however, is limited to functions that occur within the wetland. This reflects the difficulty in collecting data that will be relevant to an analysis of the relationship between the wetland and the entire landscape.
The hydrogeomorphic approach provides a measure of the ability of a wetland to perform a function, but it does not assign a measure of the importance or societal value to specific functions. Societal importance or value can be measured only through the analysis of a set of factors that are different from those that are considered in the assessment of wetland functions. Procedures for conversion of functions to societal value may become important, however, in establishing the relative importance of wetlands in the landscape.
Wetland Evaluation Under the National Food Security Act Manual
The Food Security Act of 1985 (P.L. 99-198), which establishes penalties for conversion of wetlands (Chapters 2 and 8), exempts some agricultural activities as having "minimal effect." The National Food Security Act Manual as revised in 1989 contains a procedure for determining minimal effects from a rudimentary wetland evaluation. Minimal effects include the conversion of less than 2% of a wetland smaller than 100 acres (40 ha), or 1 acre (0.4 ha) of a wetland larger than 100 acres. Actions that have a measurable effect on the hydrologic and biological functions of the remaining wetland are, however, not considered minimal, regardless of other considerations. Also, wetland conversions that exceed the size thresholds are not considered minimal, regardless of effects, unless accompanied by compensatory mitigation. The manual states that "[m]inimal effect determinations involving restoration must be supported by an assessment that indicates that wetland values lost as a result of the conversion have been fully replaced by restoration." These regulations (7 CFR § 12.5(b)(6)(F)) require the Natural Resources Conservation Service state conservationist, in consultation with FWS, to determine the mitigation acreage necessary to replace wetland functions or values.
The NFSAM procedure for a minimal-effect determination under the terms of FSA contains a checklist of 13 wetland functions that are to be designated as either present or absent. NRCS revised the determination procedure in March 1994 and incorporated a more elaborate wetland evaluation method. The new procedure, which appears in Section 527.6 of the third edition of NFSAM (NFSAM, 1994) includes a much broader range of wetland functions and requires that site evaluations include records of indicators of the presence or absence of specific functions. This procedure is modeled after the hydrogeomorphic classification system, and was it promulgated in anticipation of the adoption of a wetland evaluation procedure derived from this system by the federal regulatory agencies.
Relevance of Hydrologic Factors to Functional Assessment
The degree, frequency, and seasonality of inundation vary widely among wetland types (Chapters 2 and 7). Although hydrology is the most important factor explaining the development and maintenance of a wetland, other factors influence wetland functions. For example, the position of the wetland in the landscape, the uses of land in the surrounding watershed, the density of vegetation in the wetland, the soils and geologic features, the source of water, and the size of a wetland greatly influence function. The functional capacity of wetlands cannot be predicted from the frequency or duration of inundation alone, or from any other single characteristic.
RELEVANCE OF WETLAND ASSESSMENT TO 404 PERMIT APPLICATIONS
Assessment of wetland functions is required for a CWA Section 404 permit application, but not as part of the identification and delineation of a wetland. The first decision in the review of a permit application is whether a site that would be affected by the applicant contains wetlands or other waters of the United States subject to regulatory jurisdiction. if jurisdictional wetlands will be affected, the next step is to determine the location of the wetland boundary. Assessing wetland function is not necessary for either of these steps. Wetland evaluation is relevant to the issuance of a permit, however, because review of a permit application involves evaluation of probable effects and of reasonable ("practicable") alternatives for the proposed project. Corps regulations require that benefits of the proposed activity be balanced against its foreseeable detriments (33 CFR § 320.4(2)(i)). The U.S. Environmental Protection Agency's Section 404(b)(1) guidelines allow regulators to consider the relative functional capacity of a wetland when determining the environmental costs of a proposed project as compared with alternatives (R. Wayland and M. Davis).
Assessments of function also are used in determining mitigation requirements. The CWA Section 404 program requires that adverse effects be avoided, minimized, or compensated for through mitigation as a condition for issuance of a permit. Compensatory mitigation is determined in part by functional impairment of a wetland. According to USACE and EPA, the objective of compensatory mitigation is to provide, at a minimum, full replacement of wetland value (USACE and EPA, 1990). Replacement of value requires replacement of underlying wetland functions.
The degree to which a wetland performs a specific function of value to society does not influence regulatory jurisdiction. Conversely, many activities that can decrease the functional capacity of a jurisdictional wetland are not regulated. These activities include diversion of water from a wetland, flooding, diversion of sediment, shading, change of nutrient concentrations, indirect introduction of toxic substances, grazing, disruption of natural populations, and alteration of adjacent uplands.
USE OF FUNCTIONAL ASSESSMENT IN WATERSHED PLANNING
Local or regional planning at the watershed level can provide a scientifically sound framework for consideration of variations in wetland functions. This would require evaluation of all wetlands within the boundaries of a particular watershed or planning area, accompanied by mapping of probable development patterns. Some wetlands could be identified as deserving stringent protection, while wetlands of lower significance could be identified as appropriate for gen-
eral permits or other regulatory flexibility, although such a ranking system is not required under current law. Ranking or classification based on value is presently accomplished outside the regulatory framework by two processes: advanced identification programs (ADIDs) and special area management plans (SAMPs).
Advanced Identification (ADID)
Section 230.80 of EPA's 404(b)(1) guidelines (40 CFR § 230), allow EPA and the CWA Section 404 permitting authority to identify sites that are suitable or unsuitable to receive discharges of fill. The results of such an ADID wetland classification are advisory, not regulatory. Through ADID, developers and landowners benefit from predictability and consistency, and conservation interests can review and use information on wetland functions and values (EPA, 1988b).
ADID has been applied to areas ranging from 14 mi.2 (36.4 km2) (Joliet, Illinois) to 4,200 mi.2 (10,920 km2) (Rainwater Basin, south central Nebraska). As of December 1992, 35 ADID processes had been completed and 36 were in progress throughout the United States (EPA, 1992).
Special Area Management Plans
SAMPs were established under the Coastal Zone Management Act (CZMA) amendments of 1980 (16 U.S.C. § 1452). A SAMP is a plan for natural resource protection consistent with economic growth. It consists of policies, standards, and criteria for use of public and private lands and waters. Grants issued to states and communities for implementation of CZMA can be used for SAMPs in coastal zones.
USACE has adopted the SAMP concept for inland areas, as well as coastal areas. Its program is funded through a special studies segment of the USACE regulatory budget. The program is defined administratively, rather than legislatively, through a regulatory guidance letter originally issued in October 1986. According to the letter, SAMPs, which are labor intensive, can be justified only when an area is environmentally sensitive and under strong pressure for development, the plan is sponsored by a local agency, the public is fully involved, and all parties are willing to conclude the process with regulatory products. Ideal regulatory products include local or state approvals and a USACE general permit for specific activities and local, state, or EPA restrictions for undesirable activities (USACE, 1986). The identification of wetlands is a valuable component of many SAMPs. SAMPs generally go beyond ADIDs, however, in the numbers of interests and authorities involved and in the development of regulatory products.
Even within a given watershed, ranking systems and predetermined permit decisions are subject to a variety of criticisms. Prohibitive designations could raise legal questions about the confiscation of property without due process or just compensation, although policies such as transfer of development rights could
reduce this concern. At the other end of the scale, the designation of some wetlands as having low value would seem to sanction their conversion to other uses. The role of a wetland in a region and the cumulative functioning of wetlands in a watershed or flyway also must be evaluated; this is the landscape perspective.
The National Wetlands Policy Forum of 1987 (Conservation Foundation, 1988) made several recommendations about wetland categorization (ranking). One was that agencies consider establishing regional general permits that would allow the conversion of a limited amount of wetland of low value in exchange for full compensatory mitigation. These permits would be issued only within the framework of an EPA-approved state wetland conservation plan. The compensation requirement would be consistent with the cost of acquiring, restoring, and managing wetlands in a location and of a type similar to those covered under the permit. The forum also recommended the establishment of wetland mitigation banks, consistent with state wetlands conservation plans, through which permit recipients could satisfy compensation requirements (Conservation Foundation, 1988). The regulated community has endorsed national policies that allow market mechanisms of this type to operate while protecting wetland functions and values (Hackman, 1993; Hahn, 1993; Marshall, 1993; Wennberg, 1993). The environmental community will expect programs that provide flexibility in the management of lower quality wetlands to also provide stringent protection for areas determined to be of high quality.
In addition to the flexibility provided in current policy to scale regulatory responses to the effects of specific projects and the functional capacity of specific wetlands, the regulatory program includes provisions for landscape-level planning processes that evaluate the relative value of wetlands. These processes, such as ADID programs and SAMPs, allow state and local agencies to work with federal regulatory agencies in surveying wetlands and ranking them according to their relative functional capacity or value. Through these landscape level planning processes, authorities and other interested parties can be integrated with the wetland regulatory program to achieve a mix of objectives within a specific geographic area.
Although it is possible to evaluate the functions of wetlands, the precision is low for some types of functions and in some regions. Progress is being made on the scientific basis for wetland evaluation. Functional assessment of wetlands is most useful in the context of watershed or landscape planning. This approach facilitates consideration of the interaction between the wetland and surrounding landscape features, as well as the location of the wetland in the watershed. Landscape-level planning provides a framework for incorporation of the interests of
all affected parties. The creation of such a framework increases the likelihood that regulatory actions will be acceptable to all parties.
Analysis of wetland functions should be extended and refined, with emphasis on interactions between wetlands and their surroundings and on various hydrogeomorphic classes of wetlands in specific regions.
The procedures for identification and delineation of wetlands must be kept separate from the analysis of wetland functions.