while the drought was the last straw, i think the mismanagement of their water resources by the regime (for embezzlement of public funds, direct or indirect, into insider pockets etc) is the true root cause. There's "enough" water to last thru the current drought, if it was better utilized in the past.
Supporting this:
https://e360.yale.edu/features/iran-water-drought-dams-qanat...
“The government blames the current crisis on changing climate [but] the dramatic water security issues of Iran are rooted in decades of disintegrated planning and managerial myopia,” says Keveh Madani, a former deputy head of the country’s environment department and now director of the United Nations University’s Institute of Water, Environment and Health.