Modelling the energy balance of a natural jarrah (Eucalyptus marginata) forest

This paper presents the development of the energy balance components of a catchment scale model (COUPLE) that couples soil moisture storage, surface energy balance and the atmospheric boundary layer. The model determines the energy balance at a canopy surface and the soil surface by coupling bulk stomatal resistance and soil surface vapour pressure to the soil moisture content. Heat storage in the trunks and branches is modelled using the “force-restore” method widely used for soil heat flux. We test the parameterisations in the model against field data, and compare values obtained with those from the literature. Literature parameterisations for soil surface vapour pressure and stomatal conductance are assessed. While some existing parameterisations were found to be adequate, others required modification for this environment. In particular, the relationship between canopy conductance and vapour pressure deficit was found to be exponential, rather than the commonly used linear one. Evaporation at the soil surface was well modelled by combining the simple surface vapour pressure representation suggested by Lee and Pielke [J. Appl. Meteorol. 31 (1992) 480], and a catchment scale unsaturated zone soil moisture storage. Energy flux data were collected during two field exercises in a natural eucalypt forest with a canopy height of 25 m during spring and summer, giving data for periods of minimum and maximum water stress. We show that while the inclusion of more locally appropriate parameterisations only slightly improved the overall energy and water balance of the model, the partitioning of moisture flux between soil evaporation and transpiration was substantially improved. The data indicated that heat storage in the deep canopy was a substantial proportion of the energy budget at the beginning and end of the day, with heat stored in the trunks the major component of it. The “force-restore” method proved adequate to simulate this, and is relatively simple to implement.