[GRASS-user] question on r.watershed

[Boris Orlowsky]

dear all,

i have a question on the output of r.watershed.

my analysis goal is the following: from climate model output (from the 
WATCH dataset, eu-watch.org), i have runoff on a 0.5 degree grid (entire 
globe). i want to compute the accumulated flow for each grid cell, 
taking the inflow from other grid cells into account. my idea is to use

r.watershed elevation=SRTM_1km at PERMANENT flow=runoff threshold=100 
accumulation=discharge basin=basin

from grass gis (i have tried 6.4.1 and 7.0), using a MASK according to 
the SRTM data.

i am a bit confused because the basin maps that i obtain differ strongly 
between a run which includes the flow=runoff and a run without this 
information. in the first case, catchments are lost over most of the 
globe (random colors), without the flow=runoff i get the expected 
basins. maybe i don't understand correctly what r.watershed is supposed 
to do, but i thought that the basins should depend only on the dem. is 
that correct?

the resulting discharge-map using flow=runoff looks reasonable.

any help would be most appreciated!

thanks a lot and best regards, boris.