I live on an Island in Canada population 140,000 right now it's cool 18C/64F but even we have water restrictions and have been for the last three years.
I can't even imagine a desert climate with millions of people even finding any water for all those people boggles my mind.
This is the irony of water shortages. They're not a function of how much water is available. They're a function of how much money people are willing to spend on it. Water, it turns out, is a renewable resource but it costs money to produce.
In my area on and island of about 200 miles long by 30 miles wide it's mainly farming and also the majority ~75% of the population lives in the capital city. Too much demand for too little volume and modern conveniences of coffee shops, car washes but also golf courses.
I can't even imagine a desert climate with millions of people even finding any water for all those people boggles my mind.