It is a matter of personal satisfaction that the assignment of an estimated numerical value to the intrinsic resistance has created the opportunity to test whether the Monteith hypothesis 1975), that surface resistance is a realistic representation of the parallel summation of the infernal resistances, can be extended to a partially wet situation (Shuttleworth, 1976a). In his note, Professor Monteith correctly identifies the reason why the extension of his hypothesis fails, viz., that in the general situation, the distribution of surface water makes such a combination of resistances unrepresentative. (It should be remembered, however, that particular situations could exist where this representation may be realistic: a canopy in the process of 'wetting up' by fog droplet impaction might be an example .)
Thorn (1972, 1976) has developed and has better defined the Penman-Monteith model by making explicit the parallel combination not only of the internal resistances of the vegetative elements making up the canopy, but also their boundary-layer resistance. In this way the conflict with the 'partially wet' data presented in Shuttleworth (1976a) becomes more obvious. The realization that this combination of resistances did not provide a general description is part of the justification for Shuttleworth's assertion (1976b) that internal and boundary-layerresistances cannot be separated in general, but are merely part of a total 'surface resistance' for the element. In his note, Professor Monteith attempts to modify the Penman-Monteith model to include the partially wet situation by implicitly acknowledging this assertion for vapour flux. He does not, however, acknowledge the necessary corollary that sensible-heat is also subject to a similar 'surface resistance', equal to the boundurylayer resistance. In consequence, the description provided (in Section 3 of the note) implicitly assumes that the wet and dry needles, making up the partially wet canopy, have the same surface temperature.
In principle, the general description provided by Shuttleworth (1976b) includes any partially wet situation (since the analysis is independent of the choice of wi in Equation A(l. 1)). It includes the assumption that the canopy is made up of a combination of totally wet and totally dry elements, when wi is zero or one only, and the alternative limiting assumption (which may only apply after fog-droplet impaction) that the distribution of surface water resembles the distribution of stomata. It could be argued that the single-source description of a partially wet canopy with Boundary-Layer Meteorology 12 (1977) 385-386.