Anecdotal evidence: I have a 3x20 foot (1x6m) reedbed for our greywater
system. Our summers are essentially rainless (western Oregon, US), very
low humidity, with temps in the 80s F (low 30s C) much of the summer.
The reedbed is in partial shade in the midafternoon, which I would
recommend for dry climates if you can. It's also well mulched. I get
plenty of water from it; never measured the outflow versus inflow, but
at 25-75 gallons (okay, you do the conversion to metric this time!) a
day input, there's lots to irrigate with.