They basically need another nation to reroute a river for them, which seems highly unlikely to happen. Desalination doesn't really work well especially if you want to avoid killing off your fishery.
I never really looked deeply into it, but I remember hearing a small shortfall caused a mass causality event a while back.
I didn't read this link, and just including it due to the web domain:
https://www.atlanticcouncil.org/blogs/iransource/iran-water-environment-us-policy/
