Vegetated Roof Water-Balance Model: Experimental and Model Results

A five parameter, daily vegetated roof water balance model (VR-WBM) was developed, calibrated, and validated by using experimental vegetated roof data from the Seacoast, New Hampshire region. The lysimeter experiment on a sedum canopy characterized water storage with a 0.051 mm resolution. Overall, the results show that the average stormwater runoff reduction was 32% for the study period, and an average reduction per storm was 57%. Average daily evapotranspiration (ET) rates were $1.24\mathrm{mm}/\mathrm{day}$ during the warmest month and $0.52\mathrm{mm}/\mathrm{day}$ during the coolest month. For well-watered conditions, the ET losses were well-modeled by using a grass reference evapotranspiration (ET) value with a crop coefficient of 0.53 for the study’s sedum canopy in which the onset of stomatal closure occurs when the soil moisture is $0.11{\mathrm{m}}^3/{\mathrm{m}}^3$. When soil moisture content values are lower than $0.11{\mathrm{m}}^3/{\mathrm{m}}^3$, evapotranspiration rates decrease linearly with declining soil wetness. The VR-WBM does an excellent job predicting runoff ($R^2=0.98$) and storage ($R^2=0.94$). Although ET had a lower $R^2$ value, ($R^2=0.59$), the average ET values were within 3% of the observed values, and they do not appear to affect storage and runoff predictions. Additionally, the model demonstrated an ability to accurately quantify antecedent soil moisture and its effect on runoff generation.