[GRASS-dev] r.watershed2 with MFD

[Markus Metz]

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