Nine-month evaluation of runoff quality and quantity from an experiential green roof in Missouri, USA

A better understanding of green roof stormwater performance is needed to assess and predict benefits of proposed green roof projects. A nine-month green roof pilot study was conducted in mid-Missouri to evaluate runoff quantity and quality under field conditions for two different media, both tested under planted and unplanted conditions. Water quantity results showed a ∼40% reduction in runoff from the unplanted growing media and a ∼60% reduction in runoff from the planted growing media. A water balance model was developed that incorporated water storage in the media and evapotranspiration (ET) from the media based upon local weather conditions using the Penmen–Monteith ET method. Water quality monitoring showed a first-order decline of excess nutrients in the first few months of green roof operation. Total phosphorus >30 mg-P/L and total nitrogen concentrations >60 mg-N/L were observed in green roof runoff initially, with concentrations decreasing over nine months to ∼5 mg-P/L and ∼10 mg-N/L, respectively. In addition, elevated total organic carbon concentrations were observed, with concentrations of 500 mg/L initially, decreasing after a few weeks to below 50 mg/L. Media type and age were the largest influences on carbon and nutrient loading from the green roof media tested.