[GRASS-dev] r.watershed flow option

[Markus Metz]

Helena Mitasova wrote:
> Maybe I am overlooking it but the man page for r.watershed does not say what units
> the option "flow" should be in - I am guessing it is fraction of 1 (or percent given as decimal - e.g. 50% will be 0.5)
>
> so should flow amount  read as fraction or something like that, so that if
> all rainfall turns to runoff = 1
> only 50% rainfall turns to runoff (rest is captured by vegetation or infiltrates) = 0.5

I was thinking about this flow option a bit more, and understand it
even less. Using infiltration rates or some value representing the
amount of rainfall captured by vegetation might not make sense here
because these values have a time component, e.g. x liter per
squaremeter and hour are absorbed by soil/vegetation, and surface
runoff as calculated by r.watershed does not have a time component. If
all but one cell have flow = 1 and this cell has flow = 0.5, and this
cell receives surface runoff from 1000 upstream cells, this cell would
distribute a surface runoff value of 1000.5 downstream, assuming that
none of the water coming from upstream is absorbed, only the rainfall
this particular cell receives is absorbed, in this example 50%. The
real absorption capacity might well allow to absorb some of the runoff
received from upstream.

IOW, considering that surface runoff is cumulative and
time-independent, I don't know what hydrological mechanism/theory
would give me different initial surface flow values for different
cells (e.g. the Montgomery option in r.stream.extract weighs the
accumulated surface flow (SCA to be precise), no the initial one), but
then I am not a hydrologist.

The reason why a given input flow map is read as CELL is most probably
that back then (1991, version 4.0, r.watershed no longer alpha as in
3.2) GRASS supported only CELL raster maps.

Markus M