Last evening I went to Macalester College for Fred Pearce's lecture based on his book When the Rivers Run Dry. The main premise is that the downside of the Green Revolution, which prevented even more widespread hunger, is that there are now water shortages in different parts of the world. These shortages may be responsible for conflicts in the future.
Many crops that we enjoy, such as cotton, alfalfa, and coffee, require huge amounts of water for their growth. The amount of "virtual water" that gets traded in the forms of goods is enormous; there is no feasible way to move that amount of actual water around the globe.
As dire as some of this talk was, he highlighted the use of different technologies, such as drip irrigation, that made efficient use of water for crops in some countries.
Reasonable and just water usage is something that we as a human family need to examine carefully.