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

Abstract

Crop coefficients (k c ) were calculated for three different species of common green roof 5 succulents from March to November in 2011, to parameterize the FAO Penman-Monteith 6 equation for use in a mechanistic green roof water-balance model. Seasonally averaged k c values 7 for each species were then used to predict plant evapotranspiration (E T ) in 2012. The adjusted 8 FAO Penman-Monteith equation predicted total annual E T within 3-13 mm, a substantial 9 improvement over model predictions with k c set to1, which over-predicted E T by 100mm or 10 more, depending on species. The adjusted equation was inserted in water balance models which 11 predicted runoff within 2-13% of measured totals for 2012. This discrepancy may be explained 12 by variability in maximum water holding capacity which is difficult for two dimensional models 13 to predict. Nevertheless, these results provide increased confidence in the use of models to 14 predict stormwater runoff from green roofs and evaluate performance. Monitoring multiple green 15 roof installations with cost-effective sensor networks will increase our ability to identify the key 16 components to enhance green roof function, reduce stormwater runoff, and inform future design. 17

Observation and Estimation of Evapotranspiration from an Irrigated Green Roof in a Rain-Scarce Environment

“…The RMSEs of simulated sedum ET a are close to Marasco et al [35] (1.19 to 1.32 mm d −1 ), who used a modified P-T equation, an A-A equation, and a storage model to simulate ET a for sedum green roofs in a rain-rich environment. The NSEs and PBIASs of ET a simulations for the studied sedums generally fall within the range of Berretta et al [43] (NSE: −0.15 to 0.98; PBIAS: −74.02 to 15.13) based on a study also using the FAO56 scheme and the Hargreaves equation for sedums in a rain-rich environment; and NSEs of FAO56 and T-M methods are higher than the reported value (0.31) of Starry et al [34]. Overall, the results of the tested three methods fall within the normal range of the aforementioned previous studies.…”

“…Although the model performance statistics are not particularly good, they fall within the range of most previous studies. The corresponding R 2 for the sedum cover is slightly lower than the best estimate (R 2 = 0.51) of Starry et al [34] who used the FAO56 scheme to estimate sedum ETa in a rainrich climate, but falls within their reported range (0.07 < R 2 < 0.51). The R 2 of sedum ETa simulation is also lower than the reported R 2 value of DiGiovanni et al [33] using the T-M method for sedums in a rain-rich climate, but the linear fitting equation of that study fixed the intercept at zero which could Although the model performance statistics are not particularly good, they fall within the range of most previous studies.…”

“…The R 2 of sedum ETa simulation is also lower than the reported R 2 value of DiGiovanni et al [33] using the T-M method for sedums in a rain-rich climate, but the linear fitting equation of that study fixed the intercept at zero which could Although the model performance statistics are not particularly good, they fall within the range of most previous studies. The corresponding R 2 for the sedum cover is slightly lower than the best estimate (R 2 = 0.51) of Starry et al [34] who used the FAO56 scheme to estimate sedum ET a in a rain-rich climate, but falls within their reported range (0.07 < R 2 < 0.51). The R 2 of sedum ET a simulation is also lower than the reported R 2 value of DiGiovanni et al [33] using the T-M method for sedums in a rain-rich climate, but the linear fitting equation of that study fixed the intercept at zero which could lead to a higher R 2 value.…”

“…The P-M equation serves as the keystone of other simplified equations like the Priestley-Taylor (P-T) equation [64] and the Advection-Aridity (A-A) equation [65], or revised forms, including the Food and Agriculture Organization (FAO) of the United Nations' irrigation and drainage method (FAO56) [48] and the American Society of Civil Engineers standardized reference ET scheme [66]. As the P-M equation and its descendants are commonly used for green roof ET estimation in the humid environment [32][33][34][35]42,43], the P-M equation and its two revisions are tested for the two vegetated covers. Non-vegetated cover, without the transpiration part (no crop coefficient and water stress coefficient), was excluded from the ET estimation.…”

“…The FAO56 ET scheme has been widely used as the baseline to estimate green roof ET [32,34,43]. It can be calculated by the following workflow.…”