The qanat system was effectively destroyed by the Mongols when they invaded Iran centuries ago. That contributed to the desertification of Iran and its overall decline.
Hmm. How did the mongols do that? Qanat’s are not magical water making devices, they are just a long tunnel which leads the water already in the ground to the surface. Even if the Mongols were actively demolishing the tunnels the water is still there underground.
Maybe there is some second order effect I am not aware of, but this statement sounds fishy to me.
Iran was basically a throphy throne for Turkic strongmen to roleplay Persian King of kings for most of its post-Islam history. I assume the collapse of a strong nation state post mongols destroyed any hope of a top level national reconstruction effort. Water was sourced from north and flowed down south. It's like saying there we still have coal when someone has come and ripped up the entire railway network. More aptly, think Christmas lights in series. Now imagine the lights are little oasis towns, and far flung. Water stops flowing one day and it never comes back. That's likely what happened. (Also remember that Iran is ~1/3 the size of US and the dry parts are in the interior.)
Mongols did this throughout the Islamicate world and did it to South Asia also. They destroyed tons of existing infrastructure and by the time civilizations here started recovering, they had to deal with an ascendant Western Europe.
https://en.m.wikipedia.org/wiki/Water_scarcity_in_Iran