[GRASS-user] Interpreting r.watershed accumulation map

[Rich Shepard]

On Mon, 10 Oct 2016, Laurent C. wrote:

> According to the r.watershed manual: "Output accumulation map contains the
> absolute value of each cell in this output map 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.

   Yep, that's what I read, too.

> So basically the darker the cell, the larger the drained area. Darker
> cells therefore show the flow courses according to the topography.

   I thought this to be the case, but the legend doesn't reflect it.

> Note that Itzï does not need an output to run. If you set a simulation
> with only the DEM, an uniform friction coefficient and an uniform
> rainfall, the water will simply accumulate in the depressions, giving you
> an insight of the flow patterns.

   Now, this is very useful information that I did not pick up from reading
the manual page.

Thanks, Laurent,

Rich