Desktop Assessment of Wetland Function

This step of the WARPT is designed to make a preliminary estimate of wetland functions remotely through a desktop GIS analysis. While field assessments of individual wetlands are necessary to more accurately evaluate wetland functions, a remote sensing approach to estimate wetland function provides a cost-effective way to rapidly identify priority sites for conservation and/or restoration. This screening process can also reduce a large number of wetland sites to a manageable level that can then be assessed in the field.  In this step, hydrogeomorphic descriptors interpreted using GIS are added to wetland inventory data and are then correlated to wetland functions based on best professional judgment of various specialists. For a technical review of wetland functions, see Mitsch and Gosselink (2000).

Assigning functions to wetlands provides some basis for determining which ones have the most value, in terms of the wetland functions and services (Table 3a.1) that are of most importance to the community, or by assigning economic value to wetland functions.  Tiner (2003b) has identified ten major functions provided by wetlands.  Not all wetlands perform all functions nor do they perform all functions equally well. Factors that may affect wetland functions include geographic location, location within a watershed, climatic conditions, quantity and quality of water entering the wetlands, and disturbances or alteration within the wetland or surrounding ecosystem. This information can be useful to guide conservation efforts towards those wetlands providing the functions of interest.  Communities can go one step further and evaluate vulnerability of wetlands to future land use changes and quantify the associated loss of function to help make the case for protecting wetlands through conservation, changes to development codes or comprehensive plans, or adoption of more stringent wetland protection measures.

Identifying the specific functions that individual wetlands are providing can offer more regulatory ‘teeth’ to protect wetlands from direct impacts. For example, if certain wetlands are designated as being critical to water quality, then states can use this information to support conditioning or denying permits that would impact these wetlands under CWA Section 401 water quality certification.  Communities can also provide their results to the local Army Corps district office to aid in making CWA Section 404 permitting decisions that require information on wetland functions. This information can be particularly useful in watersheds with very high wetland density where most development approvals require making decisions about which wetland impacts will be allowed. 

The process of estimating wetland functions in a watershed can also be applied to historic wetland data to determine what functions have been lost over time and identify candidate wetland restoration sites based on potential to provide certain functions.

There are two potential options for assigning preliminary functions to wetlands in your community:

1. Hire the U.S. Fish and Wildlife Service (FWS) or its primary cooperator, Virginia Tech’s Conservation Management Institute, to do it for you. This service generates an historical assessment of pre-settlement wetland types, acreage, functions and general trends; a watershed characterization of current wetland status and functions; and an identification of potential wetland restoration sites.  Costs for these services vary with the type and density of wetlands in a geographic area, the amount of historic loss, the recency of the NWI data, and the availability of digital data sources (e.g., land use/cover and soils). 
2. Follow the "Watershed-based Preliminary Assessment of Wetland Functions" approach developed by the U.S. FWS.