r.watershed and max_slope_length

[mankoff]

I'm not sure if this is a bug, but I think it might be. The max_slope_length seems to have no effect in the r.watershed program. Below is an example that should work on the NC data set. I'd expect there to be different accumulation maps based on setting max_slope_length, but they appear identical.

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

Also mentioned at ​https://www.mail-archive.com/grass-user@lists.osgeo.org/msg31665.html and ​https://gis.stackexchange.com/questions/187312/what-does-the-max-slope-length-option-do-in-grass-r-watershed

[mankoff]

Turns out that this works, but only influences the length_slope output, not the accumulation output, which is why the example above doesn't appear to work.

Source: ​https://gis.stackexchange.com/questions/187312/what-does-the-max-slope-length-option-do-in-grass-r-watershed

In this case, this could be considered a documentation bug?

[mmetz]

Replying to mankoff:

I'm not sure if this is a bug, but I think it might be. The max_slope_length seems to have no effect in the r.watershed program. Below is an example that should work on the NC data set. I'd expect there to be different accumulation maps based on setting max_slope_length, but they appear identical.

The max_slope_length option only affects RUSLE outputs length_slope and slope_steepness. From the manual: "This input is used for the RUSLE calculations".

[mankoff]

This issue was submitted many years ago and appears to be due to my misunderstanding some of the features of r.watershed and RUSLE.