[GRASS-user] flow accumulation

[Markus Metz]

On Thu, Jul 21, 2011 at 3:30 PM, frans-joost <fjboogert at gmail.com> wrote:
> Dear GRASS users,
>
> I'm trying to implement the Morgan Morgan Finney model for erosion
> estimation.
> One problem I'm having is with the accumulation of surface runoff.
> Is it possible to sum all cells that are uphill, meaning each individual
> cell has an amount of water that flows downslope.
> It should be possible to use a flow direction map to accomplish this.

Flow direction is usually an output, not an input for hydrological
modeling. At least in GRASS, r.terraflow and r.watershed take
elevation (plus other optional parameters) as input and produce
amongst others flow accumulation and flow direction as output.
>
> r.terraflow does not have an option to accumulate using another map
> r.watershed gives me very high and negative values (not sure what happened
> there)

r.watershed flow accumulation output [0]: The absolute value of each
cell in this output map layer is the amount of overland flow that
traverses the cell. This value will be the number of upland cells plus
one if no overland flow map is given. If the overland flow map is
given, the value will be in overland flow units. Negative numbers
indicate that those cells possibly have surface runoff from outside of
the current geographic region. Thus, any cells with negative values
cannot have their surface runoff and sedimentation yields calculated
accurately.

Markus M

[0] http://grass.osgeo.org/grass64/manuals/html64_user/r.watershed.html