Ecological Engineering 102 (2017) 80–89 Contents lists available at ScienceDirect Ecological Engineering jo ur nal home p ag e: www.elsevier.com/locate/ecoleng Runoff reduction from extensive green roofs having different substrate depth and plant cover Konstantinos X. Soulis a,∗ , Nikolaos Ntoulas b , Panayiotis A. Nektarios b , George Kargas a a Department of Natural Resources Management and Agricultural Engineering, Division of Water Resources Management, Agricultural University of Athens, Greece b Department of Crop Science, Lab. of Floriculture and Landscape Architecture, Agricultural University of Athens, Greece a r t i c l e i n f o Article history: Received 3 October 2016 Received in revised form 15 January 2017 Accepted 29 January 2017 Keywords: Hydrology Modeling Lysimeter Runoff coefficient Soil Conservation Service Curve Number Xerophytes Succulents Turfgrasses a b s t r a c t The current study aims to systematically analyze the relationship between runoff reduction from differ- ent 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 layer- ing. 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 tur- fgrasses (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 for- mulate 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) com- bined 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 success- fully 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. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Green roofs are emerging as one of the most promising manage- ment practices aiming to ameliorate the environmental problems and hydrological risks associated with urbanization (Booth and Jackson, 1997; Carbone et al., 2014, 2015; Hilten et al., 2008). One of the most important services provided by green roof systems is ∗ Corresponding author at: Division of Water Resources Management, Agricultural University of Athens, 75, Iera Odos str., 11855 Athens, Greece. E-mail address: soco@aua.gr (K.X. Soulis). related to their ability to retain a portion of the rainfall and to dis- tribute runoff over a longer period of time. In this way, green roofs are considered as an effective methodology for reducing hydrolog- ical risks in urban regions. Green roof systems typically consist of three major compo- nents: a vegetation layer, a lightweight substrate medium and a water storage/drainage layer placed on top of a waterproof mem- brane (Carbone et al., 2015; Carson et al., 2013; Yang et al., 2015). According to the depth of the growing substrate layer, green roofs are commonly classified as extensive or intensive. Generally, green roofs with substrate depth less than 15 cm are classified as exten- sive and their vegetation consists of shallow rooting, drought http://dx.doi.org/10.1016/j.ecoleng.2017.01.031 0925-8574/© 2017 Elsevier B.V. All rights reserved.