A novel stomatal resistance equation for modeling heat and mass transfer of extensive vegetative roofs

Abstract

The cooling potential of vegetated roofs depends on the stomatal resistance (rs), leaf area index, and other factors. Sailor (2008) and Tabares-Velasco and Srebric (2012) are among the most cited heat and mass transfer vegetative roof models. They include different equations to estimate rs, which show large differences, and significantly overestimate rs under certain conditions. Therefore, their models offer particular opportunities for improvement to reflect the actual vegetation cooling potential. This is necessary to predict the impact of vegetative roofs more accurately on the building energy performance and the urban heat island effect. The rs of both vegetative roof heat and mass transfer models have been developed based on studies with species that are not commonly planted on vegetative roofs. It was found that both models overestimate the rs under certain conditions that could turn into the underestimation of the transpiration rates and the cooling potential. Consequently, this paper aims to develop a novel equation for rs based on field measurements in a semiarid climate. The species tested were Glandularia berterii (C3), Selliera radicans (C3), Phyla reptans (C3), Aptenia cordifolia (CAM), Sedum palmeri (CAM), and Sedum spurium (CAM). A New Linear Model (NLM) was formulated to estimate rs. The regressors of NLM are minimum stomatal resistance (rs,min), incident solar radiation (Rsh), leaf area index (LAI), substrate volumetric water content (VWC), and vapor pressure difference (VPD). The results show that rs,min and Rsh reflect 64 % of rs; the species with higher cooling potential were Selliera radicans, Sedum palmeri, and Sedum spurium due to higher LAI and lower/moderate rs,min; and, the NLM showed a better agreement with the measured rs, correcting points where Sailor (2008) and Tabares-Velasco and Srebric (2012) equations significantly overestimated rs. Finally, it was demonstrated that the developed NLM better represents the stomatal behavior and distinguishes the responses between C3 and CAM species.