In water scarce places, what food products use most water? For example how do tomatoes compare with goats, or sugar beet with cows? In a place with lots of sunshine and good soil, but little water, what are the best and worst foods to produce?
Should certain products from water scarce regions even come with a warning label? Thinking about regions where finite groundwater levels are being lowered for increased agriculture.
Water is complex. At the most basic level the data on absolute use is available and as importantly the return on application of water estimated through irrigation trials for a particular transpiration rate. The vast majority of water used by a plant is simply translocated carrying plant nutrients to where it is needed and then expelled into the atmosphere - most water is not removed with the crop output. In some intensive systems the water is condensed and recycled to be used multiple times.
Few would argue against sustainable water use whether depletion of aquifers or draining the Aral sea for cotton. I suspect that the particular product is less of a concern than the water source: far less water is harvested in the production of wheat than potatoes or tomatoes but it still requires something in the order 400l water per kg of grain in a temperate zone.
Another issue is the indirect consequence. Water use may be sustainable in the sense that there is no risk of irreversible depletion but extraction (e.g. from a river) can destroy existing downstream habitats that may not recover even where winter rainfall results in an annual torrent. Environmental damage rather than loss of water is probably the more important issue in Europe and requires a balance to be made between competing demands.
It would be useful to have a water sustainability symbol added to products and should be part of even the most basic farm assurance scheme.