A simplified one-dimensional theoretical description of the vegetation-atmosphere interaction

Shuttleworth (1976b) demonstrated that it is possible to write a 'multi-layer' model of the vegetation-atmosphere interaction in analytically continuous form and create a one-dimensional description, of general applicability, with evaporation described by a combination equation similar to the Penman-Monteith equation. The present paper develops and simplifies that analysis in an attempt to provide a more practical description. It is shown that the generalized combination equation can be rewritten in a form which is identical to the Penman-Monteith equation in the single-source limit providing that the canopy resistance is redefined in two alternative ways, according to whether there is large-scale variation in surface wetness. Both of these definitions reduce to the Monteith (1965) form in dry conditions, and indicate zero surface resistance in wet conditions. The version applicable to situations involving large-scale variation in surface wetness, which is considered the general description in rainfall conditions, is consistent with published data in partially wet conditions (Shuttleworth, 1976a). This version allows a separation of the 'interception loss' and 'transpiration loss' components and, although approximate, has potential value in that it allows an experimental test of speculative models of interception loss, and provides the means whereby such models can be merged with Penman-Monteith models of transpiration loss, to provide a model of evapotranspiration. A preliminary test of a simple, single-source model of interception loss shows satisfactory agreement with experimental data.