Studies on the Bulk Transfer Coefficients over a Vegetated Surface with a Multilayer Energy Budget Model

A multilayer energy budget model for vegetation canopy is developed to describe the fluxes of sensible and latent heat exchanged between the vegetated surface and the atmosphere. The model gives satisfactory results when the calculated radiative surface temperature TR and fluxes of sensible and latent heat are compared with experimental data. This model illustrates that values of the bulk transfer coefficients CH and CE are sensitive to meteorological conditions (incident radiation, wind speed, air humidity) when their values are evaluated using TR as the mean surface temperature. Therefore, the use of TR is inadequate to determine the bulk transfer coefficients. As an alternative, the effective surface temperatures for the energy exchange are defined and the values of CH and CE are determined with these temperatures. These values of CH and CE are only dependent on the vegetation parameters: vegetation type and density, the leaf transfer coefficients (or stomatal resistance), and the moisture availability of the ground. Incorporating the values of CH and CE into the surface energy budget equation, the surface energy exchange and the effective surface temperature can be estimated when the available energy (the incident radiation minus the soil heat storage) is given. Also, an example of the method to determine the surface moisture availability β = CE/CH is shown with use of the measured radiative surface temperature.