A modified FAO evapotranspiration model for refined water budget analysis for Green Roof systems

Many metropolitan cities around the world are promoting the implementation of Green Roofs (GR) as effective stormwater management systems to mitigate adverse effects of urbanization. Hourly water balance calculations are necessary for accurate prediction of the antecedent soil moisture conditions of a GR, a key parameter for rainfall-runoff modeling. However, existing evapotranspiration (ET) models have substantial errors for hourly ET predictions over a range of moisture conditions to accurately assess the hydrological performance of the GRs during storm events. Therefore, a modified Penman-Monteith equation was developed to provide improved prediction of hourly ET specifically for GR applications. ET rates predicted by the modified and established Penmen-Monteith equations were compared with ET data collected from a GR lysimeter over the summer and fall of 2016. The proposed modified Penman-Montieth equation eliminates the water availability factor (θiθfc) included in the equation's advection term and separates the effect of plant coefficients on radiation/energy and advection terms. The modified equation was found to improve ET estimates by 8-9% (RMSE: 0.535-0.493, MAD: 0.375-0.343), under non-water limited conditions. For water-limited conditions, errors were significantly reduced. RMSE improved by 37% (0.703-0.440) and MAD was improved by 31% (0.429-0.294).