A 2-LEAF MODEL FOR CANOPY CONDUCTANCE, PHOTOSYNTHESIS AND PARTITIONING OF AVAILABLE ENERGY-I - MODEL DESCRIPTION AND COMPARISON WITH A MULTILAYERED MODEL

An one-layered, two-leaf canopy model which calculates the fluxes of s ensible heat, latent heat and CO(2)( )separately for sunlit and shaded leaves is presented. The two-leaf model includes: (1) a simple but ro bust radiation model, (2) an improved leaf model accounting for the in teraction of conductance and photosynthesis and the response of stomat a to water vapour pressure deficit and available soil water and (3) a new parameterisation of radiative conductance which simplifies solutio n of the leaf energy balance equation. Comparisons with a multi layere d model show that predicted fluxes of CO2, latent and sensible heat fl uxes 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 comput ationally 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 n et 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 th e 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 usua lly within 15%). For a canopy with leaf area index less than 2, the di fferences in the modelled fluxes of canopy COP, latent or sensible hea t are less than 5% between the multi-layered model and two-leaf model. Our results show that the two-leaf model can predict net photosynthes is, 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.