What does the max_slope_length option do in GRASS r.watershed?

Question

What does the max_slope_length option do to the r.watershed program in grass?

According to the manual, it should limit the length of a stream.

Does this mean a max_slope_length equal to the raster resolution means no stream flow?

It appears to have no effect. I'm trying to determine if this is something I'm not understanding, or a bug in the program, but have not had luck with the grass mailing list (no replies at the time of this writing, see thread here)

Here is an MWE with the NC data set that shows no effect from using the option:

r.mapcalc "rain = 1"
r.watershed -s -a --v elevation=elev_srtm_30m accumulation=acc drainage=fdir flow=rain
r.watershed -s -a --v elevation=elev_srtm_30m accumulation=acc.len drainage=fdir.len flow=rain max_slope_length=15
g.gui.mapswipe first=acc second=acc.len

Answer 1

It is meant only for USLE computations. You don't specify output length_slope, so max_slope_length does not influence anything.

Answer 2

If you have a Digital Elevation Model, you could calculate the distance travelled downhill (actual distance along flow paths, not the distance in a straight line) by a drop of water from a point of origin to a point of interest (let's say a raster cell). This is called the slope length and is used in modelling soil erosion.

The r.watershed module can export two maps useful for studying soil erosion: a slope length and a slope steepness map. The max_slope_length parameter you are refering to impacts only the slope length map, not the other outputs.

So basically, if you do not intend to study soil erosion you probably don't need a soil length map, so there's no need to set a value for max_slope_length.