Green roof performance potential in cold and wet regions

•Green roof performance in wet/cold regions was evaluated with a water balance model.•Stormwater retention varied as a function of local precipitation and temperatures.•Annual retention was more sensitive to plant water use than to roof storage capacity.•Local storage capacities required to avoid drought was proposed.•A high risk of permanent wilting prevents the use of more efficient vegetation.

Extensive green roofs have become a frequently used option for stormwater retention across many different climates including cold and wet regions. Despite the extensive documentation of green roof technology for stormwater management, the knowledge about their function and potential use in wet and cold regions is deficient. Using historic data on daily temperature and precipitation in a green roof water balance model coupled with the Oudin model of evapotranspiration (ET), we evaluated the effects of maximum green roof storage capacities (Smax) and ET on stormwater retention along climatic gradients in Northern Europe. Large differences in potential annual stormwater retention were found between locations, driven by differences in temperature and precipitation amounts. Highest retention in absolute values was found for the wettest locations, while the warmest and driest locations showed highest retention in percentage of annual precipitation (up to 58% compared to 17% for the lower range). All locations showed a considerable retention of stormwater during summer, ranging from 52% to 91%. Storage capacities accepting drought conditions once every 3.3-3.9 year were found to be about 25 mm in the cold and wet locations increasing to 40-50 mm in the warmer and drier locations. Corresponding storage capacities to prevent wilting of non-succulent vegetation was on average a factor of 1.5 larger (not including Sheffield and Malmö). Annual retention increased both with an increase in plant water use (specific crop factors, Kc) and with an increase in Smax, but was found to be more sensitive to changes in Kc than to changes in Smax. Hence, ET was the limiting factor for green roof retention capacity in the cold and wet locations, but relatively large changes in evapotranspiration would be needed to have an impact on retention. The potential to use vegetation with higher water use to better restore the storage capacity between storm-events in these regions was however limited by the risk of permanent wilting of non-succulent vegetation, even on the wettest locations. A considerable increase in roof storage capacity and substrate thickness would be required to reduce this risk; still the increase in stormwater retention would be marginal.