# How to calculate LS Factor of Watershed for RUSLE Calculation?

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.

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).

for L

``````L=(X/22.13)^m
X=flowAccumulation * cellsize
``````

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

for m

``````m=B/B+1
B=(sin Q / 0.0896) / (3 * sin Q ^ 0.8  + 0.56)
``````

L is meter

• 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

I developed the GISus-M to compute the topographic (LSfactor) and cover and management (C-factor) from methods using remote sensing data. The other factors necessary to use the USLE, including soil erodibility, rainfall erosivity, and conservation practices, are also integrated in this tool.

In the GISus-M system, the calculation of the LS-factor on a grid is based on digital elevation models. The GISframework of GISus-M is ideally suited for calculating the LS factor using the LS-TOOL proposed by Zhang et al.(2013). As the LS-TOOL uses ASCII DEM data to calculate the LS-factor, we changed the source code to work with RASTER DEM data to be compatible with input data in GISus-M.