Modeling energy fluxes from sparse canopies and understorys

Land surfaces are an assemblage of component surface types, for instance overstory vegetation species, understory vegetation species, and bare soil. Often two or more surface types absorb a significant fraction of the available energy to the land surface as a whole. In these cases the interaction of fluxes from the component surfaces may be important to the total land surface energy balance. We compare three models of land surface energy balance: a Penman-Monteith model; a model with two component surfaces that don't interact (patch model); and a model with interacting component surfaces (Shuttleworth-Wallace model). Data from six published studies are used to investigate which models best represent a particular land surface taking account of water supply to the component surfaces and overstory canopy architecture. Flux interaction between component surfaces was only found to be important when there was a large difference between the surface resistances (i.e., water availability to the surfaces). Also, all three models were found to estimate the same land surface energy fluxes (to within 50 W m(-2)) when both surface resistances were >300 s m(-1). The ratio of (aerodynamic resistance between the canopy air space and the reference height) to (mean component surface boundary layer resistance) was useful for indicating the level of interaction between component surfaces.