Concave, low gradient areas will gather water (low WI values), whereas steep, convex areas will shed water (high WI values). The Hillslope Wetness index, also called Topographic Wetness Index (TWI) uses Flow Direction and Flow Accumulation rasters as inputs. Create the first from a DEM, the second from the first. WI values will vary by landscape and DEM, but they typically range from less than 1 (dry cells) to greater than 20 (wet cells). Threshold values are applied to the output raster, via classification, based on local knowledge, field characteristics, and observations of the local terrain’s response to heavy precipitation and overland flow. Customized classifications and weighting schemes can be devised to better represent relative wetness of hillslopes in your study area. Predictive soil mapping (RASP model), wildfire hazard risk (Chuvieco and Congalton, 1989; Chuvieco and Salas, 1996; Lein and Stump, 2009), and landslide studies involving logistic regression (Chang et al., 2007) have employed this index as a model input.
WI = ln(As/tanB)
WI = wetness index
As = local upslope contributing area (m2) from flow accumulation raster
B = local slope angle (degrees). Conversion needed: Radians = (degrees)(∏/180)
In Spatial Analyst > Map Algebra > Raster Calculator, enter the following equation:
Ln((“FLOWACC”*900) / Tan(“SLOPE”))
WI = wetness index output raster that will be created
a = area of each pixel in m2 if 30m pixels are used (30m x 30m = 900m2)
FLOWACC = name of flow accumulation raster
SLOPE = name of slope raster
Refs:
Speight (1974)
Bevan and Kirkby (1979)
Moore et al. (1991)
Moore et al. (1993)
Gessler et al. (1995)
Western et al. (1999)
Toby Rodgers/RASP Model (c.2005, WSU MS thesis)
Bohner and Selige (2006)
Phil Roberts (c.2008 WSU MS thesis)