A two-leaf model for canopy conductance, photosynthesis and partitioning of available energy I: Model description and comparison with a multi-layered model

An one-layered, two-leaf canopy model which calculates the fluxes of sensible heat, latent heat and CO(2)( )separately for sunlit and shaded leaves is presented. The two-leaf model includes: (1) a simple but robust radiation model, (2) an improved leaf model accounting for the interaction of conductance and photosynthesis and the response of stomata to water vapour pressure deficit and available soil water and (3) a new parameterisation of radiative conductance which simplifies solution of the leaf energy balance equation. Comparisons with a multi layered model show that predicted fluxes of CO2, latent and sensible heat fluxes usually agree within 5% over a range leaf area index typical of a wheat crop grown in a temperate climate. The two-leaf model is computationally 10 times more efficient than the multi-layered model and is suitable for the incorporation into regional and global climate models. For a hypothetical canopy with a leaf area index of 5 under very dry (vapour pressure deficit of air of 2 kPa) and sunny conditions, the net canopy photosynthesis and latent heat fluxes calculated by the two-leaf model agree with those by the multi-layered model within 10% for the whole range of soil water conditions (from very dry to wet) and the sensible heat fluxes of the canopy calculated by the two-leaf model agree with those by the multi-layered model within 25 W m(-2) (or usually within 15%). For a canopy with leaf area index less than 2, the differences in the modelled fluxes of canopy COP, latent or sensible heat are less than 5% between the multi-layered model and two-leaf model. Our results show that the two-leaf model can predict net photosynthesis, latent and sensible heat fluxes of a canopy quite accurately under a wide range of soil water availability and meteorological conditions, as compared with the multi-layered model. (C) 1998 Elsevier Science B.V. Ah rights reserved.