Modelling the hydrological responses of green roofs under different substrate designs and rainfall characteristics using a simple water balance model

Abstract

Abstract In spite of many well-known benefits of green roofs, their widespread adoption as source control measures in urban stormwater management requires the use of adequate modelling tools. The purpose of this study was to develop a simple water balance model for green roof runoff simulation based on conceptual hydrological processes and integrated into green roof designs. The model was calibrated and validated with a pilot experimental dataset that included various green roof configurations. The validated model was applied to simulate the hydrological responses of green roofs under different substrate designs and rainfall characteristics. Results showed that the model Nash-Suttcliffe efficiency (NSE) values under calibration and validation period ranged from 0.933 to 0.982, and the volume errors of relative percentage difference (RPD) varied from −1.45% to 5.35%, which indicated the simulated runoff processes of green roofs were satisfactorily accurate. The sensitivity analysis of parameters (e.g., increased parameters by 50%) showed the most sensitive parameter for simulating green roof runoff was the initial substrate water content, which changed by −74.5% for runoff volume and 58.5% for time to runoff. For a 50% increase in the saturated substrate hydraulic conductivity, it increased runoff volume by 60.2%; as well as the substrate depth increased runoff volume by 56.5% and changed time to runoff by −46.3%. The simulated hydrograph indicated that decreasing the saturated substrate hydraulic conductivity distinctly delayed time to runoff and increased runoff retention. Similarly, the runoff rate declined and runoff generation time was delayed with an increase in the substrate depths. Runoff retention percentages of the green roofs declined as rainfall depths increased. With rainfall intensity decreasing, mean time to runoff of the green roofs significantly increased from 30.0\xa0min to 113.3\xa0min. Runoff detention effects of green roofs were distinctly enhanced with the peak rainfall intensity delayed. This study provided a simple modelling tool for simulating hydrological responses of green roofs under a wide set of substrate characteristics and rainfall conditions in order to guide green roof design.