To reply to your edit: Have you looked around to see what professional water management people have been saying about that very thing? I bet you a dollar that

To reply to your edit: Have you looked around to see what professional water management people have been saying about that very thing? I bet you a dollar that they've talked about that very thing several times. If you read that Slate Star Codex post (if you haven't, you should), you'll see that already we capture and use most of the water that we receive. But, for the heck of it, what water sources do you want to see captured, why, how much water do you expect to get out of that source, and what will be its impact on the rest of the ecosystem?