Topographic Wetness Index
The TWI is a physically based index or indicator of the effect of local topography on runoff flow direction and accumulation. The index is a function of both the slope and the upstream contributing area.The computation of TWI is performed using both geographic information systems (GIS) and Python, a programing software used to enhance computing capabilities. The indices help identify rainfall runoff patterns, areas of potential increased soil moisture, and ponding areas.
With support from FEMA, Illinois State Water Survey generated the index for the Illinois counties of DuPage and Cook (see report), and with the support of NRCS has subsequently generated the index for most Illinois counties as high resolution DEMs derived from LiDAR have become available. While TWI is usually viewed in raster format, ISWS is distributing a version in polygon format for easier access and use. Because TWI was computed using a bare earth DEM, TWI polygons mistakenly overlapped building footprints. These areas were subsequently removed during post processing using building footprints. This first release version is a direct product of computer processing without manual examination of areas across the state and may therefore inaccurately display areas such as lakes, rivers, and areas where road culverts were missing from the LiDAR.
Select a county of interest to download the shapefile. Please see our disclaimer.