There's been a good deal of research recently on 'embedded' or 'virtual' water.
My understanding of this is: the quantity of water used to grow a crop, per harvested unit of that crop. Some data on common crops is available. For wheat for example one estimate is that 1,400 litres of water are needed to grow a kg of wheat (never mind the specifics for now). For rice the figure is 3,000 litres of water for a kg of rice (harvested).