DEM-Hydro Processing scripts

Compound Index calculation

Next to Overland Flow Length, a number of scripts (routines) are given here to calculate a number of maps that are relevant with respect to watershed management, soil erosion and conservation type of studies.

The wetness index sets catchment area in relation to the slope gradient.
This is basically the famous:

w = ln(As/tan(�)) (Beven and Kirkby, 1993)

An idea of the spatial distribution and zones of saturation or variable sources for runoff generation is obtained.
The stream power index is the product of catchment area and slope and could be used to identify suitable locations for soil conservation measures to reduce the effect of concentrated surface runoff.
Finally, the sediment transport index accounts for the effect of topography on erosion. The two-dimensional catchment area is used instead of the one-dimensional slope length factor as in the Universal Soil Loss Equation.

The equations for the indices applied are those given by Burrough and McDonnell (1998).

Dialog box options:

Input DEM:	Select an input raster map which contains height values, i.e. your DEM.
Flow Accumulation Map:	Select a raster map that is the output of a previous Flow accumulation operation.
Output Wetness Index Map:
	Type a name for the output raster map that will contain for every pixel the wetness index.
Output Stream Power Index Map:
	Type a name for the output raster map that will contain for every pixel the stream power index.
Output Sediment Transport Index Map:
	Type a name for the output raster map that will contain for every pixel the sediment transport index.

Short explanation of the calculations by the script:

The DFDX and DFDY filters are used on the Digital Elevation Model to calculate the first derivative in X and Y direction.
Slope maps are calculated, in percentages and degrees.
Possible zero-values are removed from the slope map in degrees.
The Wetness Index is calculated as:
OUTMAP_wetness_index = LN((flowacc_rasmap * PIXSIZE(%1) * PIXSIZE(%1)) / (TAN(DEGRAD(temp_slopemap_deg))))
The Stream Power Index is calculated as:
OUTMAP_stream_power_index = (flowacc_rasmap * PIXSIZE(%1) * PIXSIZE(%1)) / (TAN(DEGRAD(temp_slopemap_deg)))
The Sediment Transport Index is calculated as:
OUTMAP_sediment_transport_index = POW(((flowacc_rasmap * PIXSIZE(%1) * PIXSIZE(%1)) / (22.13)), 0.6) * POW(SIN(DEGRAD(temp_slopemap_deg)) / 0.0896, 1.3)
Finally, temporary raster maps are removed.

Tip:

For more information on the MapCalc functions used in this script, please refer to the following topics: DEGRAD, LN, PIXSIZE, POW, SIN, TAN, or to Map and Table Calculation.