In reply to David Barratt:
don't often see "hydrological" in these forum posts, I do that sort of stuff for a living.
what are you doing this for? (out of interest)
It’s straightforward (built in function) using Spatial Analyst in ArcGIS,
Can’t help on QGIS tho
Generally it’s not totally reliable, can be woefull where there’s urban stuff or it’s a really flat area.
Mostly it’s just a quick to do it by hand as you have to check everything anyway.