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Topographic Wetness Index (TWI)
Susquehanna River Basin Commission SRBC
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GIS raster datasets displaying Topographic Wetness Index (TWI) for Adams, Cumberland, Dauphin, Franklin, Lancaster, and York Counties, PA. The TWI rasters were derived from 2016 LiDAR for Dauphin County, 2015 LiDAR for Lancaster and York Counties, and 2006-08 LiDAR for Adams, Cumberland, and Franklin Counties. The TWI rasters were derived from 2015 LiDAR for Lancaster and York Counties and 2006-08 LiDAR for Adams and Franklin Counties. The TauDEM extension (D-Infinity tools) for ArcMap was used to create flow direction, slope, and contributing area rasters. TWI was then calculated using the following equation: Ln (Contributing Area/Slope). The methodology was described by Cody Fink in his 2013 thesis entitled Dynamic Soil Property Change in Response to Natural Gas Development in Pennsylvania. TWI results in a dimensionless raster and should be displayed using a red (low values representing no flow) to blue (high, representing high probability flowpaths) color gradient. TWI results vary depending on raster size and analysis options so value thresholds for probability-based overland flowpaths for water should be field verified.