Parameterizing a Water-Balance Model for Predicting Stormwater Runoff from Green Roofs

Crop coefficients (kc) were calculated for three different species of common green roof succulents from March to November in 2011, to parameterize the Food and Agricultural Organization of the United Nations (FAO) Penman-Monteith equation for use in a mechanistic green roof water-balance model. Seasonally averaged kc values for each species for 2011 were used to predict plant evapotranspiration (ET) in 2012. The adjusted FAO Penman-Monteith equation predicted the total annual ET within 3-13mm, a substantial improvement over model predictions with kc set to 1, which overpredicted ET by 100mm or more, depending on the species. The adjusted equation was inserted in water-balance models, which predicted runoff within 2-13% of measured totals for 2012. This discrepancy may be explained by variability in maximum water-holding capacity, which is difficult for two-dimensional models to predict. Nevertheless, these results provide increased confidence in the use of models to predict stormwater runoff from green roofs, and evaluate performance. Monitoring multiple green roof installations with cost-effective sensor networks will increase the ability to identify the key components to enhance green roof function, reduce stormwater runoff, and inform future design. (C) 2016 American Society of Civil Engineers.