Decades long droughts are associated with a fair few collapsed civilisations. I heard a fascinating discussion on my country's version of NPR from an author of

Decades long droughts are associated with a fair few collapsed civilisations. I heard a fascinating discussion on my country's version of NPR from an author of a book, who started that the Mayan rulers derived their legitimacy and power from their ability to provide abundant and clean water reserves during the dry season. The water lilly was often a symbol of Mayan kings (and allegedly it can only grow in clean water), although I suspect there's a lot more to it than just that. Then a decades long drought hits the Southern Lowland Mayan cities, and they collapsed, and the author claimed the close association between water and kingship hastened this (although the term collapse is contentious, to paraphrase Gaiman/Pratchett, some historians think that they "didn't so much collapse, as gently saunter downwards")