Here are Denver Botanic Gardens irrigation rates for their cafe green roof. Some years no supplemental water was used. http://navigate.botanicgardens.org/weboi/oecgi2.exe/INET_ECM_DispTour?TOURCODE=GREEN_ROOF

Urban Drainage used the data from 3 years of study at Denver Botanic Gardens - Mordecai green roof to show 60% to 80% of runoff was slowed and contained by the green roof. Protecting the waterways from polluted street water.