A REGIONAL WETNESS INDEX MODEL, WITH APPLICATION TO MAPPING AVIAN HABITAT 
ASSOCIATIONS.

Tim J. Fox, Wayne E. Thogmartin, and Melinda G. Knutson.
U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, 
WI 54602.

Vernal pools, ephemeral wetlands, sedge and fresh meadows, and wet prairies 
contain water for only part of the year, and are among the most threatened 
wetlands. These wetlands were once common in the prairie biome, but many have 
now been drained for farmland or filled for development. They support a wide 
variety of flowering plants, crustaceans, insects, reptiles, amphibians, and 
mammals, and are important food- and rest-stops for migrating birds.  Regional 
maps of ephemeral wetlands are not available primarily because they are 
difficult to identify using standard remote sensing technology.  While wetlands 
maps are available by state, each state uses somewhat different approaches to 
mapping wetlands. To provide a consistent and seamless surrogate to ephemeral 
wetlands at the regional geographic scale, we mapped a static wetness index for 
the Upper Midwest. Topographic indices such as the static wetness index have 
been widely applied in hydrology and were originally developed to predict zones 
of surface saturation, patterns of soil moisture, and saturation deficit.  There 
are various measures of static wetness, but the measure we used is ln(Catchment 
Area/tangent of the slope angle).  The catchment area is the upslope area 
draining into the location of interest.  Shallow areas are more likely to catch 
water than steep areas, and ridge tops have less upslope contributing area than 
valley bottoms.  We provide the hierarchy of geographic information system 
processes we used to produce this index using only digital elevation models and 
hydrological units as boundings.  We show how the location of wet meadows and 
seasonally-inundated woodlands can be predicted with our model.  We applied the 
wetness index in a habitat model for the Cerulean Warbler (Dendroica cerulea).   
We found that the wetness index was an important predictor of Cerulean Warbler 
abundance within the Prairie-Hardwood Transition Ecoregion (BCR 23).  Because 
ephemeral wetlands are important habitats for a wide variety of wildlife, 
further model development of wetness potential will have wide application.

Keywords: static wetness index, hydrological units, digital elevation models, 
National Wetland Inventory, bird habitat model