[GRASS-dev] r.watershed2 with MFD

[Dylan Beaudette]

On Fri, Dec 5, 2008 at 1:55 AM, Markus Metz <markus_metz at gmx.de> wrote:
> I took the request for MFD support in r.watershed by Helena and Dylan to
> heart and implemented it, but still need a few more days to clean up the
> code, then I want to submit it as r.watershed2.mfd to grass-addons.
>
> A first result for flow accumulation is here:
> http://markus.metz.giswork.googlepages.com/rw2_mfd_large.jpg
> This output has been calculated from elev_lid1972_1m in the North
> Carolina sample dataset.
>
> Please compare to the original result and flow accumulation of other
> modules as available here:
> http://www.grassbook.org/gallery/chapters/s050404f070_flow.jpg
> a) old r.watershed
> b) r.flow
> c) r.terraflow with MFD
> d) r.sim.water
>
> References:
> original MFD algorithm, also referenced to by r.terraflow
> Quinn, P., Beven, K., Chevallier, P., and Planchon, 0. 1991.  The
> prediction of hillslope flow paths for distributed hydrological
> modelling using digital terrain models, Hydrol. Process., 5, 59-79.
>
> The algorithm of Quinn et al. (1991) was modified by Holmgren (1994) and
> is implemented here:
> Holmgren, P. 1994. Multiple flow direction algorithms for runoff
> modelling in grid based elevation models: an empirical evaluation.
> Hydrol. Process., 8, 327-334.
>
> I added a convergence factor after Holmgren (1994): a lower convergence
> factor results in more diverging flow, output will be similar to
> r.terraflow in MFD, a higher convergence factor will result in more
> converging flow, getting more similar to, but not identical to SFD.
>
> Additional memory requirements are not noticeable ( <2 KB) and
> independent of the region size, additional time required is a few
> seconds for elev_lid1972_1m, increasing by a few more seconds with
> higher convergence factor.
>
> The color coding suggested by Hamish with absolute logarithm works nice
> for most outputs, I have hardcoded it for flow accumulation output.
> The color coding of the example image above has been adjusted a bit to
> show more detail, but abs log of rainbow looks nice too!
>
> Markus Metz
>

Very impressive Markus! I certainly wasn't expecting someone to
implement such a complex feature so quickly. Is there any way to
cleanly merge this with the existing r.watershed code in SVN so that
we can test it?

Great job!

Dylan