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I managed to get subbasins for my catchment (7 km^2) with their areas but I haven't figured out how to get the LS factor (see Revised Universal Soil Loss Equation) from it.

I have the vector file for my subbasins and I have the DEM (digital elevation model) file. I extracted the LS raster file from the DEM file (via r.watershed) but I dont know what to do afterwards. I figured I can isolate the subbasins within the LS raster file and calculate the average LS factor of those areas. Do you know how to do this?

On a side note, is there a way to split up the subbasins since there are slopes that are steep with slopes that are flat? Thanks in advance.

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2 Answers 2

for L

X=flowAccumulation * cellsize

for flow Accumulation go to Arcmap - Arctoolbox - Spatial Analyst tools - Hidrology

for m

B=(sin Q / 0.0896) / (3 * sin Q ^ 0.8  + 0.56)

L is meter

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

What is "Q"? Can you explain how this answer relates to the more general (and well supported) formula given by @markusN in this thread? –  whuber Nov 12 '12 at 20:22

This question is better asked in the GRASS Users mailing list. However, for the formula, see for example "2.2.1 Universal Soil Loss Equation" in http://skagit.meas.ncsu.edu/~helena/gmslab/reports/cerl99/rep99.html:

The modified LS factor at a point on a hillslope is

LS = (m+1) [A / 22.13]^m [sin b / 0.0896]^n

where A[m] is upslope contributing area per unit width, b[deg] is the slope angle, m and n are parameters. Exponents m and n should be calibrated for a specific prevailing type of flow and soil conditions, if the data are available.

or the GRASS GIS book (Springer).

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