I'm ambivalent about this question. On the one hand, it's great that someone is trying to back up their arguments with a calculation, and calculations are at least somewhat on-topic. On the other hand, the OP is making several assumptions that invalidate his argument, which is not mathematical. So do we close as off-topic or what?
I voted to close before any of the answers appeared. I think, there is very little, if any, mathematical content in the question, since the discussion about scarcity of water versus oil is largely an ecological and economical one. The main question here is "what do we actually want to calculate" and not "how to calculate it". Since the OP seems to have a very faint or no idea how to answer that former question, he has come to the wrong place.
I think this question can be counted as "applied math." There are (to my knowledge) no other SE sites where this could go (for instance, the proposed economics.SE is probably too advanced), so I think it should stay open. I don't think questions like this show any threat of taking over the website.
It has already attracted two good answers (presumably because mathematicians tend to be sensible folks). Ideally if "Popular Natural Science".SE or "Science".SE (see Area51 if you like that!) were alive, this question really should be migrated there. At present, I don't think migrating it to physics.SE is a good idea.
Anyway, personally I think it is a bit off topic, since there is no question really with the mathematics side of the problem. (Also, why is it tagged calculus?) I would vote to close if my vote weren't binding.