Wetland Change Analysis: Ecology's Wetland Status and Trends Inventory

What is the Wetland Change Analysis?

The Wetland Change Analysis project developed a method for more accurately mapping wetlands. The resulting wetland maps will be used as a wetlands status and trends inventory to help determine if the goal of No Net Loss of wetlands is being achieved in Washington State.

>View the Wetland Inventory Map

The Wetland Change Analysis project partnered Ecology with the National Oceanic and Atmospheric Administration’s Coastal Services Center (NOAA-CSC) and the Washington Department of Fish and Wildlife (WDFW). The Environmental Protection Agency (EPA) provided funding for this project through the Puget Sound EPA Scientific Studies and Technical Evaluations Grant Program. Work on the project began in the fall of 2010 and concluded in September of 2013.

What was the need for the Wetland Change Analysis?

Ecology along with Stakeholders of the Mitigation that Works Forum wanted to know if the goal of No Net Loss of Wetlands was being achieved in the state of Washington. More specifically, we wanted to know if the goal of no net loss was being achieved in Puget Sound, given the interest in the recovery of this region. Wetlands play a major role in maintaining water quality at a watershed scale.

A major hurdle to answering this question was the lack of a state-wide or Puget Sound wide inventory of wetlands, other than the National Wetland Inventory (NWI). Though still in use, NWI data were produced by interpreting aerial photographs from the 1970s and early 1980s. NWI mapping in Washington State is nearly 40 years old. Much has changed, particularly in Western Washington, since these data were created. In addition, the NWI maps that were produced did not adequately identify forested and slope wetlands. In addition, the US Fish and Wildlife Service, the federal agency that conducted NWI mapping, decided to exclude certain types of “farmed wetlands”.

NOAA-CSC has been mapping land uses (including wetlands) for a number of years. Their data provide another option for analyzing the status and trends in wetland acreage through the Coastal Change Analysis Program (C-CAP). The C-CAP program uses 30-meter resolution LANDSAT imagery to map various land cover categories. This process is repeated every five years, and any land use changes that have occurred in the intervening years are mapped and analyzed. However, Ecology Wetland Specialists noticed that the C-CAP wetland layer did not accurately identify many large pasture wetlands, and it missed some forested and slope wetlands as well.

How was the Wetland Change Analysis conducted?

In an effort to improve C-CAP mapping and provide a wetland inventory that could be used for status and trends analyses, Ecology partnered with NOAA-CSC to identify the best methods and existing data sources for a consistent and repeatable process. Data sources, where available, included:

  • NWI
  • SSURGO Soils data (hydric soils and hydric soil inclusions)
  • NAIP aerial ortho-imagery
  • Elevation data, including LiDAR where available
  • Landsat Thematic Mapper imagery – multiple dates
  • Local wetland data layers.

Concurrently, WDFW applied a method they developed, using high (1-meter) resolution aerial imagery, to detect changes in land use from vegetated to unvegetated. WDFW compared the results of their High Resolution Change Detection (HRCD) with the results of NOAA-CSC’s C-CAP land cover changes to evaluate the feasibility of using HRCD to refine the medium resolution satellite imagery for future status and trends analyses.

What are the results of the Wetland Change Analysis?

The Wetland Change Analysis project provided three primary products (links to the products will be added when available):

  1. A Wetland Inventory for Western Washington modeled from the C-CAP data
    • It provides approximate locations for most wetlands larger than one acre
    • It includes a "Potentially Disturbed Wetlands" category for areas that have a high potential to be wetland, but have an observed land cover of "pasture/hay" or "cultivated"
    • It is more accurate than NWI for wetlands larger than one acre, especially in agricultural/pasture, forests, and stream corridors
    • It can be used for planning purposes and for initial permit review
    • It can NOT be used to determine regulatory boundaries.

    >View the Wetland Inventory Map or Download raster files
     

  2. Improved Land Cover Change Analysis
    • NOAA-CSC applied the improved wetland layer to previous C-CAP analyses, including 2011, 2006, 2001, 1996, and 1992
    • C-CAP will use the improved wetland layer for land cover categories and analyses, starting with 2016 and then every 5 years

    >View the Assessment Report: "Assessment Report of Wetland Mapping Improvements to NOAA's Coastal Change Analysis Program (C-Cap) Land Cover in Western Washington State", February 2013.
     

  3. High Resolution Change Detection (HRCD) completed for all 19 WRIAs that flow into Puget Sound
    • The Wetland Change Analysis Project initially helped to fund HRDC in a few WRIAs, but thanks to additional funding, HRDC has been completed for all 19 WRIAs that flow into Puget Sound.
    • High Resolution Change Detection (HRCD) works very well and can detect small scale changes, but must be overlaid with a wetland layer from appropriate year to detect wetland change
    • Based on HRCD comparison with the Wetland Inventory for Western Washington, more than three-quarters of change occurring in wetlands was less than 1 acre in size
    • C-CAP could be used to broadly identify areas where changes have occurred and then do finer scale analysis with HRCD to detect small scale wetland changes/loss.

    >View the High Resolution Change Summary Report (February 2013)
    >Go to WDFW's HRCD Project Web Page
     

    For questions about the High Resolution Change Detection, contact:

    Kenneth B. Pierce
    Washington Department of Fish and Wildlife
    (360) 902-2564
    piercekbp@dfw.wa.gov

What are the caveats on the use of Wetland Change Analysis data?

The Wetland Inventory for Western Washington CANNOT be used to determine regulatory boundaries of wetlands for two main reasons:

  1. The locations of mapped wetlands are approximate due to the medium resolution scale – each pixel equals about ¼ acre. Therefore, wetland boundaries are accurate only within 100 ft.
  2. Just because a wetland is mapped in a particular location does not guarantee that it is wetland. The wetland inventory was created by modeling existing geospatial data to identify areas that have a high potential to be wetland.

The Wetland Inventory is at a medium resolution scale. Wetlands less than 1-acre were likely not mapped. Therefore:

  • Absence of a wetland on the map does not mean absence of wetland in the field
  • Small wetlands could still be present.

Questions or Comments?

If you identify any errors with the inventory, either an area that was mapped as wetland but is confirmed to be non-wetland, or an area that was not mapped but is confirmed to be wetland, please let us know so that we may improve the accuracy in the future.

Patricia Johnson
Washington State Department of Ecology
P.O. Box 47600
Olympia WA 98504
(360) 407-6140
patricia.johnson@ecy.wa.gov

 

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