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Controlled studies have indicated that maturing green roofs may have improved water retention properties <ref>Simon De-Ville, Manoj Menon, Xiaodong Jia, George Reed, Virginia Stovin. 2017. The impact of green roof ageing on substrate characteristics and hydrological performance, In Journal of Hydrology, Volume 547, Pages 332-344, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2017.02.006.</ref>. In a study to investigate how green roof runoff reduction performance is affected by climate, Talebi et al. (2019) used water balance and evapotranspiration models to simulate runoff produced by green roofs in six Canadian cities (Vancouver, Calgary, Regina, Toronto, London and Halifax).  Results showed that predicted runoff reduction performance varied from 17% to 50% for low water use plants.  The best performance was predicted to occur in Regina and Calgary (drier climates), while the poorest performance was predicted for Halifax and Vancouver (wetter climates).  Average annual runoff reduction was highest in Toronto and London and lowest in Vancouver.<ref>Talebi, A., Bagg, S., Sleep, B.S., O'Carroll, D.M. 2019. Water retention performance of green roof technology: A comparison of Canadian climates. Ecological Engineering, 126, (2019), 1-15. https://www.sciencedirect.com/science/article/abs/pii/S0925857418303835?via%3Dihub</ref>

Controlled studies have indicated that maturing green roofs may have improved water retention properties <ref>Simon De-Ville, Manoj Menon, Xiaodong Jia, George Reed, Virginia Stovin. 2017. The impact of green roof ageing on substrate characteristics and hydrological performance, In Journal of Hydrology, Volume 547, Pages 332-344, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2017.02.006.</ref>.

The key hydrologic benefit which green roofs have over other forms of LID is the proportion of the water returned to the atmosphere through [[Evapotranspiration| evapotranspiration]].   

The key hydrologic benefit which green roofs have over other forms of LID is the proportion of the water returned to the atmosphere through [[Evapotranspiration| evapotranspiration]].