Stomatal transpiration and droplet evaporation on leaf surfaces by a microscale modelling approach

Knowledge on convective water vapour exchange at leaf-air interfaces is required to assess transpiration of leaves via stomata and evaporation of droplets, which can both be considered as microscopic moisture sources, heterogeneously distributed across the leaf. An innovative modelling approach was proposed to investigate such convective mass transport from leaf surfaces, using computational fluid dynamics (shear-stress transport k-omega turbulence model with low-Reynolds number modelling). The main novelty lies in the fact that a large range of spatial scales (10(-5)-10(-1) m) was included and that the individual microscopic sources were modelled discretely. The convective exchange from the leaf model was strongly dependent on three parameters: surface coverage, air speed and source size. The relation between the convective flow rate and both the coverage ratio (CR) and the microscopic Sherwood number, i.e., the ratio of the source size to the viscous sublayer thickness, was quantified. It was shown that well-established convective transfer coefficients, obtained from plates or leaf models for a CR of 100%, can result in a significant overprediction of the convective exchange, compared to more realistic, lower CR. Furthermore, small variations in stomatal density (CR), e.g., due to biological variability, were shown to have a large impact on the convective exchange and droplets were found to evaporate more rapidly at low CR. The decrease in mass transfer rate due to stomatal closure was quantified as well. The proposed numerical modelling approach can be applied to increase our understanding of leaf transpiration and droplet evaporation, but also of the leaf boundarylayer microclimate and the transport processes therein. A critical discussion of the modelling assumptions allowed to identify focus points for future model refinement as well as future research. (C) 2013 Elsevier Ltd. All rights reserved.