[GRASS-user] Watershed delineation result problem

[Massimiliano Alvioli]

Hi all,

I would like to briefly weigh in in this discussion, especially about
the following:

> 2) Of greater importance is that you can pick either one; it doesn't matter
> in the real world. In 1994 I was the first environmental consultant
> authorized by Oregon's Department of State Lands to use GPS receivers to
> delineate wetland boundaries. They had insisted that only professional land
> surveyors could do this and they set a 2cm accuracy standard. Really?
> Wetland boundaries are transistion zones that can be several meters wide,
> depending on topography, soils, and antecedent precipitation conditions when
> the boundary is flagged. A stream bank is an exception to this broad
> transition area. When I made the case that there is no sharp line of
> demarkation between wetland and upland they accepted my delineations.

I couldn't agree more on this point - at times people really have fancy
interpretations of maps' resolution and what they can actually tell about
the real world. I also would like to add that watershed delineated from
a digital topography may depend on the scale at which one looks at the
landscape. I mean, not the DEM resolution by the very definition of a
watershed - if we agree that this definition depends on the number of
cells draining on the outlet (parameter "threshold" on r.watershed), we
get very different results depending on this parameter.

And now for a little advertising: we discuss these topics and their use
to produce slope unit tools (in GRASS GIS) and maps in these papers:

"Automatic delineation of geomorphological slope units with r.slopeunits v1.0
and their optimization for landslide susceptibility modeling "
https://doi.org/10.5194/gmd-9-3975-2016

"Parameter-free delineation of slope units and terrain subdivision of Italy"
https://doi.org/10.1016/j.geomorph.2020.107124


M