Below-canopy fluxes in a simplified one-dimensional theoretical description of the vegetation-atmosphere interaction

Previous work (Shuttleworth, 1976) demonstrated that 'multi-layer' simulation models of the vegetation-atmosphere interaction can be written in analytically continuous form: subsequent work (Shuttleworth, 1978) exploited the analytic nature of the model to simplify the description to a combination equation, similar in form to the Penman-Monteith equation, but requiring a redefinition of 'surface' resistance in order to provide a physically continuous description in all conditions of surface wetness. The present paper extends this work and re-examines the simplification procedure, to demonstrate that it is also possible to include below-canopy flux in a physically realistic, but simple, way, by the introduction of an additional term in the combination equation. It is then demonstrated that the analysis given in this and previous papers can be used to describe the possibility of vertically separated subcanopies in situations where this is relevant. The use of these two extensions is illustrated by reference to experimental data.