http://gisgeography.com/agriculture-maps-global-farming/

Food and Agriculture Organization of the United Nations- AQUASAT- global water information system:
http://www.fao.org/nr/water/aquastat/main/index.stm 

Nat. Geo. Global Water Footprint of Key Crops (+ other resources):
http://environment.nationalgeographic.com/environment/freshwater/global-water-footprint/

Interesting point: "Soil water is the limiting factor for photosynthesis"
http://drought.unl.edu/ranchplan/DroughtBasics/GrassesDrought/SoilMoisturePlantGrowth.aspx 

The Importance of Water in Plants & Crop Agriculture: A Climatic Factor:
http://www.cropsreview.com/importance-of-water.html 
     (Some key words- mesophytes, hydrophytes, xerophytes, resurrection plants)