A new GRASS addon r.accumulate
is available. This module only requires a flow direction map to calculate weighted flow accumulation.
If you have a drainage map from r.watershed
,
r.accumulate direction=drain_directions accumulation=flow_accum
or using its GUI,

In this example, the yellow arrows and color-coded numbers show flow directions and accumulation, respectively.

Optionally, you can perform weighted flow accumulation using the weight option.
r.accumulate direction=drain_directions accumulation=flow_accum weight=flow_weight
If you have a FDR map (FDR.TIF for example) from ArcHydro or ArcGIS,
# import ArcHydro FDR
r.in.gdal input=FDR.TIF output=fdr
# convert FDR to drainage
r.mapcalc expression="drain_directions=int(8-log(fdr,2))"
# calculate accumulation
r.accumulate direction=drain_directions accumulation=flow_accum
Hydrological Tools | Flow Accumulation | D8 and Rho8 Flow Accumulation
and other tools. (Flow direction
=Flow Pointer
).