Actually it says the desalinated water is too expensive even for farming, it’s only used for heavy industries, so it’s certainly not a solution for the domestic supply of 9 million people.

And don’t confuse moving the capital city with actually relocating Tehran. Tehran’s not going anywhere. What they’re proposing is building a new capital city, but it’ll be the rich and the political and religious elite who move there. The millions of poor and powerless living in Tehran will get left behind. Some will be able to migrate south, but many won’t.