Wisconsin Department of Natural Resources

Potentially restorable wetlands

Source: Wisconsin DNR

Date: Last updated 7/2017

Summary: The Potentially Restorable Wetland (PRW) Layer identifies areas across the state of Wisconsin that could potentially be restored to wetland. PRWs are areas that are not currently mapped as wetland, but soil and water pooling data indicate it may be possible to restore them to wetland. The processes and data sets used to identify PRWs are described below. Because it utilizes existing data sets of varying currency and quality, field verification of mapped PRWs is highly recommended.

Synopsis: This polygon data set is derived, in part, from the following GIS layers: the federal Natural Resources Conservation Service Soil Survey Geographic Database (SSURGO), a Compound Topographic Index (CTI) – a surface water pooling layer derived from the statewide 10-meter digital elevation model, Wisconsin’s 1:24,000-scale hydrography, the Wisconsin Wetlands Inventory (WWI), the Wetland Restoration Tracking Database (WRTD), the Wisconsin land cover version 2 (WISCLAND 2), Wisconsin’s Ecological Landscapes, and Wisconsin Department of Transportation (WisDOT). All existing wetlands mapped on the WWI, recently restored wetlands from the WRTD, urban areas from WISCLAND 2, and roads and active railroad corridors from WisDOT are eliminated from consideration. Two hydric soil fields in the SSURGO database (% Hydric and Potential Wetland Soil Landscape) are used to identify whole soil map units as potentially restorable. If the unit is >75% hydric, the whole unit is considered PRW, while map units with 1% -74% hydric inclusions are investigated further using PWSL values and the CTI raster layer. Polygons with PWSL values between 1 and 80 or "null" are investigated using the CTI raster data. CTI dimensionless flow accumulation values of 10 or greater* are used to identify additional areas as potentially restorable. Further refinements based on size and landscape position are made to identify additional polygons as "contributing areas" and "PRW less than 0.5 acres."

Creation Process: The selecting fields for soil map units are Percent Hydric and PWSL. All polygons that were greater or equal to 75% hydric were automatically selected for consideration and named "Hydric Polygons." Additionally, soil polygons that had a Percent Hydric value from 1% - 74% were also selected, and all polygons that were not in these two sets but had a PWSL value between 1 - 80 or "null" were also selected and named "Pre-CTI" to be used as the selection layer for the CTI process. All duplicate polygons were then removed, leaving behind unique soil units for consideration.

"Pre-CTI" cells (10m) with CTI values of 10 or greater* were selected and converted to polygons. Then the polygons were simplified and a minimum area threshold of .09 acres was chosen to create the "CTI Polygon" layer. This output was then intersected with the selected Soil Units to reattach the tabular attributes.

The "Hydric Polygon" and the "CTI Polygon" layers were merged to create the "Pre-PRW" layer. From this layer the existing wetlands (the WWI), other surface waters (the 24K Hydrolayer) and recently restored wetlands (the WRTD) were erased and polygons less than 0.1 A were discarded. The result is the final "PRW Layer." Within this layer two additional classes of PRW were created. "Urbanized" was created by extracting urban areas (WISCLAND 2), roads, and active railroad corridors (WisDOT), labelling them as "Urbanized" and reinserting them into the PRW layer. Polygons less than 0.5 A were labelled as "Less than 0.5 A."

A separate class, now labelled as "Contributing Area,"** was created by selecting completely non-hydric (0%) soil units and further selecting areas immediately adjacent to existing wetlands or PRWs that have CTI values 10 or greater*.

* Where the CTI values are concerned it was found, independent of confirmation by efforts in Texas and Minnesota, that value threshold is somewhere between 9.7 and 9.9 depending on topography, soil composition, and exposed bedrock. Where the CTI is used in the process the value of 9.9 was reclassified into the value of 10. In addition it was found that a value of 12 was needed to more accurately reflect current water conditions where sand was the chief component of the Ecological Landscape. In addition it was found that a CTI threshold value of 12 where soils were predominantly sandy more accurately reflects water pooling conditions. Therefore a threshold of 12 was used for Wisconsin’s Ecological Landscapes with "Sand" in their name: Northwest Sands, Northeast Sands, Central Sands and Central Sand Hills.

** This term is currently being revisited, and while technically correct, interferes with a current legal definition.

Interpreting the identify report

MUKEY: The NRCS SSURGO soil mapping unit key. See the NRCS Wisconsin Soils or Wetland Indicator Soils layers for soil unit name and description.

PRW code: Shows the determination of the PRW. This determination is based on the best available data as of 2015. Polygons smaller than 0.5 acres will be "Less than 0.5 Acres". The remainder will be "PRW".

Acreage: The acreage of the polygon.

Related or Program Links:

For more information on the creation of this data layer, please visit the report at Assessment reports and maps