Hydrological Modeling of Green Roofs Runoff by Nash Cascade Model

Green roofs (GRs) technology has gained increasing interest in recent years since it offers multiple benefits to urban environments, citizens and buildings. Eco-covers can capture some water nutrient pollutants, filter air pollutants and moderate the urban heat island effect. Beyond these benefits, abundant literature stresses the role played by the GRs from the hydrological perspective. They allow to face the increasing stress on the traditional urban drainage systems by reducing the annual stormwater runoff. In light of this, the hydrological behavior prediction of a vegetated cover is essential for urban planners, policy makers and engineers in order to quantify runoff mitigation potential so as to optimize their application.

The aim of the present research is to meet this need by testing the accuracy of Nash cascade model in predicting the stormwater production of GR systems.

The proposed model has been calibrated against hourly data of thirteen rainfall-runoff events observed at two experimental sites, both located within the campus of the University of Salerno, southern Italy. Event scale model calibration, aimed at the identification of the storage coefficient parameter, has been based on the optimization of a number of error statistics.

Despite its simplicity, the investigated model appears adequately to be able to predict the runoff production from the experimental green roofs with a good degree of accuracy, as described by the Nash-Sutcliffe Efficiency index, which ranges between 0.54 and 0.94.