Hillslope Wetness Index

Concave, low gradient areas will gather water (wet = high WI values), whereas steep, convex areas will shed water (dry = low WI values). The Hillslope Wetness index 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 (unitless relative values)
As = “specific upslope contributing area” (from flow acc raster) divided by the length of the the grid cell, thus the units for As are lengths (meters).

B = local slope angle (degrees). Convert slope from Degrees to Radians, enter Radians into the equation, then run it: Radians = (degrees)(
/180) due Raster Calculator’s tangent function problem.

Thanks to Ronda Strauch at UW for providing the correct value for specific upslope contributing area (As). Sorry for any confusion I have caused.

** For questions regarding degrees vs. radians with respect to slope, see this LINK1 and this one LINK2 **

twi_mitasova1

Image courtesy of Helena Mitasova, http://skagit.meas.ncsu.edu. Created in GRASS.

twi_mitasova2

Image courtesy of Helena Mitasova, http://skagit.meas.ncsu.edu. Created in GRASS.

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 (unitless relative values)
a = area of each pixel in m
2 if 30m pixels are used (30m x 30m = 900m2)
FLOWACC = name of flow accumulation raster
SLOPE = name of slope raster

Refs:
Bevan and Kirkby (1979)
Boettinger et al. (2010) Digital Soil Mapping, Springer 473 pgs.
Bohner and Selige (2006)
CSDMS TOPMODEL LINK
ESRI Forum on Degree/Radian units issue LINK
Frazier, B.E., T.M. Rodgers, C.A. Briggs, R.A. Rupp (2009) Remote area soil proxy modeling technique, Soil Survey Horizons, v. 50, p. 62-67
Gessler et al. (1995)
Mitasova’s Lab at NCSU LINK
Moore et al. (1991)
Moore et al. (1993)
Remote Area Soil Proxy (RASP) Model by Rodgers/Roberts/Meirik/Frazier/Rupp:
LINK1 LINK2 LINK3 LINK4 LINK5
Roberts, P.H. (2009) Digital landform mapping and soil-landform relationships in the North Cascades National Park, Washington, MS thesis, Washington State University, 134 pgs. LINK
Rodgers, T.M.(2000) Modeling soils of the Sawtooth and Pasayten Wilderness
Areas with a GIS, MS Thesis, Washington State University
Meirik, E.E. (2008) Mapping vegetation with remote sensing and analysis of soil-vegetation interactions in North Cascades National Park, WA, MS Project Report, Washington State University, 90 pgs.
Speight (1974)
Western et al. (1999)

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