The water-use by agriculture is unsustainable whether there is population or not. Compared to agriculture use of water, all other water uses are negligible. So

The water-use by agriculture is unsustainable whether there is population or not. Compared to agriculture use of water, all other water uses are negligible. So if you eliminate the residential population and empty the cities, agriculture will still use all the water and fail. But if you eliminate the agriculture, there will be plenty of water for the cities and population. A desert has enough water to support a population. It was foolish to introduce industrialized agriculture in a desert.