Drinking water is such a tiny proportion of total water use that it is essentially irrelevant.

Water for farming and power stations are the things that will be hit first.

The drinking water is just part of the issue (as you said). Water is used in countless industrial processes, farming, EVERYTHING. if the water goes, so does everything else.

And it’s not just water going away—it’s impingement by salt as well.

> Drinking water is such a tiny proportion of total water use

A lot of water infrastructure needs minimum levels to function. Drinking water may be a small fraction of use. But if the big users deplete a reservoir below its minimum operational level, the fact that the dead water is enough to keep Tehran alive is more trivia than solace.