A green roof test plot at the University of Georgia retained 88% of precipitation for small storms (<1 inch), 48% for larger storms (>3 inches), and delayed the peak flow by an average of 18 minutes for 31 rain events between Nov 2003 and Nov 2004.
Carter, Timothy L., and Todd C. Rasmussen. (2006). Evaluation of the hydrologic behavior of green roofs. Journal of the American Water Resources Association 42(5), 1261-1274.