There is a new paper (pdf), “Irrigation and Autocracy,” by Jeanet Sinding Bentzen, Nicolai Kaarsen, and Asger Moll Wingender. Here is the abstract:
We show that societies with a history of irrigation-based agriculture have been less likely to adopt democracy than societies with a history of rainfed agriculture. Rather than actual irrigation, the empirical analysis is based on how much irrigation potentially can increase yields. Irrigation potential is derived from a range of exogenous geographic factors, and reverse causality is therefore ruled out. Our results hold both at the cross-country level, and at the subnational level in premodern societies surveyed by ethnographers.