Allotments 4 All

I've been asked to look into rainwater harvesting for our new site. I have the data for the annual average rainfall but I need to know how much water is used on an allotment in a year. Of course there are so many variables i.e soil type, location, growing systems, weather patterns and so on.
So all I'm asking is does anyone have any figures for annual water use (metered)
on allotment sites. Litres used and number of plots.
Thanks, Rich...

Birmingham allow 4 cubic metres per plot (I don't know whether that's standard or for our site, which has double size plots).