Runoff reduction from extensive green roofs having different substrate depth and plant cover

•The relationship between green roof runoff reduction - substrate moisture - rainfall depth was analyzed.•A mathematical expression describing this relationship was formulated and tested.•Higher green roof runoff reductions were observed for lower initial moistures and smaller rainfall depths.•Estimations of green roof runoff coefficients and the Soil Conservation Service Curve Number parameters were provided.•The adaptive transpiration capacity of Origanum onites has the potential to maximize the hydrological benefits of green roofs.

The current study aims to systematically analyze the relationship between runoff reduction from different types of shallow green roof systems, and the initial substrate moisture conditions, and total rainfall depth. The experimental study comprised of 30 specialized lysimeters equipped with green roof layering. The lysimeters had two different substrate depths (8 cm or 16 cm) and three different plant covers [either succulent plants (Sedum sediforme (Lacq.) Pau), or xerophytic plants (Origanum onites L.), or turfgrasses (Festuca arundinacea Shreb.)]. In addition unplanted lysimeters and lysimeters without a green roof system were utilized as controls. The results of the experimental study were used in order to formulate a mathematical expression, which effectively described the relationship between all the above mentioned parameters (runoff reduction, initial substrate moisture conditions and rainfall depth). The derived relationship was then utilized to provide effective estimations of the rational method runoff coefficients, which constitutes one of the main prescribed methodologies for water resources planning, and management, as a function of initial moisture content and total rainfall depth. Furthermore, the Soil Conservation Service Curve Number model was calibrated using the collected rainfall-runoff data in order to estimate the corresponding curve number (CN) values. Finally, the applicability of a simple linear relationship between total rainfall depth and total runoff depth was investigated. The observed runoff reduction ranged between 2% and 100% for the total runoff depth and between 17% and 100% when the peak runoff rate was considered. Higher reductions were observed in deeper substrates (16 cm) combined with O. onites vegetation cover as well as in cases with lower initial substrate moisture contents and smaller rainfall depths. A strong multiple correlation between rainfall depth, runoff reduction and initial substrate moisture was observed. The developed equation describing this correlation was successfully fitted to the experimental data. In addition, the SCS-CN model was successfully calibrated using the experimental rainfall-runoff data of this study. The obtained CN values were generally high and varied from 88 to 95.5. The lowest CN value was obtained for O. onites vegetation cover combined with deeper substrate depth (16 cm), indicating an increased runoff mitigation potential for such vegetation cover types.