Watershed-Based Preliminary Assessment of Wetland Function (W-PAWF)

This approach, developed by the U.S. Fish and Wildlife Service (FWS), applies general knowledge about wetlands and their functions to develop a watershed overview that highlights possible wetlands of significance in terms of performance of various functions. The U.S. FWS determined that watershed managers could make better use of National Wetland Inventory (NWI) data if additional information to describe wetland properties not currently addressed in the NWI wetland classification system (based on Cowardin et al. 1979) were added to the current NWI database. The enhanced NWI digital data, called “NWIPlus” database (Tiner, 2010a), could then be used to predict the likely functions of individual wetlands, estimate the capacity of an entire suite of wetlands to perform certain functions in a watershed, or generate information of interest to policymakers and others (e.g., how many and how much of the wetland resource is isolated or connected to waters of the US).

The two major steps of this approach are:

1. Assign Landscape, Landform, Water Flow path, and Waterbody type (LLWW) descriptors to NWI Data
2. Correlate the LLWW descriptors and NWI types (NWIPlus data) with wetland functions

Step 1. Assign Landscape Position, Landform, Water Flow Path, and Waterbody Type Descriptors to NWI Data

The FWS identified four major types of attributes to add to the NWI to enhance its usefulness. These include landscape position, landform, water flow path, and waterbody type (collectively referred to as LLWW) and are described in Table 3a.2 and generally depicted for nontidal wetlands in Figure 3a.1. Note that the waterbody type descriptor applies only to open water habitats and not to wetlands.

Figure 3a.1. Examples of nontidal wetlands classified by LLWW descriptors (Tiner, 2010a).

The NWI database is currently being expanded in selected areas to include LLWW descriptors for mapped wetlands. As of December 2008, these descriptors had been added to the NWI for portions of 16 states and may be included as part of future updates to the NWI depending on funding and priorities.  Consult the applicable Regional Wetland Coordinator to determine the status of enhancing the NWI with LLWW descriptors for your region. If this update has already been completed for your community, you can move on to the second step of this approach to correlate the LLWW descriptors with wetland functions. If the LLWW descriptors are not already included in the NWI for your community, you will need to add them to your updated wetland maps using the GIS method described below. Additional details on the methodology are provided in Tiner (2003a) and Tiner (2008). 

GIS layers required for this analysis include topography, hydrology, soils, and aerial photos. In general, more detailed and accurate data is available from local and state governments compared to data derived at a national level.  Potential sources of data are described below.

• Soils: The NRCS Soil Data Mart provides state-wide or county-wide soil survey maps that designate hydric soils and inclusions (patches of hydric soil too small to map) for download. When soil data is not available digitally, NRCS and/or soil and water conservation districts often provide paper maps.
• Topography: If detailed topographic data are not available from the state or municipality, the best national source is the USGS National Elevation Dataset (NED), a seamless raster dataset of Digital Elevation Models (DEMs).  The NED is predominantly available at a 30 m resolution, with some regions available at 10 and 3 m resolution. Higher resolution types of elevation data, such as Light Detection and Ranging (LiDAR) and Interferometric Synthetic Aperture Radar (IFSAR) are recommended when available to generate DEMs.  Places to search for LiDAR or IFSAR include your state natural resources agency, local Universities, or NOAA Coastal Services Center.
• Hydrology: The National Hydrography Dataset (NHD) is digital data for the United States that includes lakes, ponds, streams, rivers, canals, and oceans.  If using this data, it is recommended that you download the high resolution (1:24,000 scale) NHD. Larger scales, such as 1:4,800, are becoming available in select areas. State and local sources of hydrography data may also be available from your local planning agency or state natural resources department.
• Aerial photos:  Aerial photography is available in most areas and may be obtained from government mapping agencies, university geography departments, or non-profit organizations. Potential sources include the USGS Earth Resources Observation and Science (EROS) Orthoimagery.  EROS provides high resolution ortho-imagery (1/3 m resolution), Landsat 7 mosaics (30 m resolution), and seamless digital orthophoto quarter quadrangles (DOQQ) (1 m resolution).  Another potential data source is the USDA Farm Service Agency (FSA), which produces 1 m resolution county mosaics of digital orthophoto quarter quadrangles (DOQQs) through the National Digital Ortho Photo Program.

By overlaying and/or intersecting the data described above with the wetland inventory data, specific hydrogeomorphic attributes can be characterized and added to the attribute table for each mapped wetland in your community.  Tiner (2003a, 2011a) provides a set of dichotomous keys and a coding system for assigning the LLWW descriptors to individual wetlands in the NWI. Simplified keys from Tiner that are current as of January 2010 are available here. The basic process is listed below.

1. Divide the Cowardin (1979) code into its individual components, which progresses from systems and subsystems to classes, subclasses, and dominance types. Click here for a diagram of the Cowardin code and its components, as well as a code interpreter from the U.S. Fish and Wildlife Service. The system and water regime components allow for the immediate assignment of several Landscape classifications (lake, estuary, pond, etc) and tidal (BT) Water Flow. Each wetland can also be identified from the class component as vegetated, open water, or nonvegetated.
2. Isolated wetlands are identified from a hydrology layer (typically from the NHD). Non-isolated wetlands are manually assigned a water flow.
3. The landscapes assigned in Step 1 above (lakes, rivers, ponds, etc) are then used to find adjoining wetlands. These are given the appropriate Landform or modifier. The remaining unclassified wetlands should only need a terrene or lotic Landscape classification. Landform is largely derived from the water regime. Drier climates are classified as flats, wetter ones as basins, and semipermanently adjoin a waterbody as fringe. Classifications, such as floodplain, are done manually.

Since the methods are revised periodically based on applications to other geographic areas, be sure to contact Ralph Tiner for the latest information about enhancing wetland inventory data for the preliminary assessment of wetland function, refer to Tiner (2002) or contact him directly at This e-mail address is being protected from spambots. You need JavaScript enabled to view it or  This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Step 2. Correlate the LLWW descriptors with wetland functions

After you have assigned the LLWW descriptors to your wetland inventory data, the next step is to correlate the data to specific wetland functions. Ten wetland functions may be evaluated based on the methodology described in Tiner (2003b, 2011b). A more recent correlation list between functions and wetland types from Tiner can be found here and is current as of September 2010. Recently another function, carbon sequestration, has been added to the list and the conservation of biodiversity function has been revised to a function called provision of habitat for unique, uncommon, or highly diverse wetland plant communities (R. Tiner, pers. comm. 2010).  Table 3a.4 lists the major wetland functions and provides examples (but not a comprehensive list) of wetland types (described by the Cowardin classification and/or LLWW descriptors) that have high potential to provide these functions.

The correlations described in Tiner (2003b) were developed for the Northeast, and are being modified based on wider applications, although many are universally applicable. The correlation between LLWW descriptors and wetland functions is currently a work in progress. These techniques are being tested and applied in pilot study areas across the Nation (e.g., Anchorage Alaska, California’s Ventura River watershed, Corpus Christi area of Texas, South Carolina’s Horry and Jasper Counties, the Mississippi Coast, and Wyoming’s Shirley basin). The results are anticipated to be published in late 2011 (Tiner, 2009).

Limitations of this Approach

The results of the W-PAWF provide an initial screening of wetlands that have a significant potential to perform various functions. It does not take into account land use practices that influence the ability of a wetland to perform a certain function. In addition, it does not consider the condition of the adjacent upland (e.g., level of disturbance) or the water quality of the associated waterbody, which are important for determining the health and functional capacity of individual wetlands. The W-PAWF also does not identify differences among wetlands of similar type and function, which is important for making decisions about wetland conservation and restoration. For these purposes, field assessments of wetland function are needed to verify and/or refine the results of the preliminary assessments.

Next Steps

The preliminary assessment of wetland functions provides some basis for determining which ones have the most value, in terms of the wetland functions and services that are of most importance to your community. Field assessments of wetland function should be conducted next to verify and refine the results of the preliminary assessment. Potential next steps include assigning economic value to wetland functions and/or evaluating vulnerability of wetlands to future land use changes to make the case for wetland protection.