Energy balance of a natural jarrah (Eucalyptus marginata) forest in Western Australia: Measurements during the spring and summer

The jarrah (Eucalyptus marginata) forest in southwest Western Australia is capable of maintaining vigorous growth through severe dry summers, during which there may be little or no rain for close on 6 months. Hydrologically, the catchments are characterised by very low runoff rates, being 1-10% of rainfall. The work described here aimed to (1) obtain measurements of the various components of jarrah forest evapotranspiration during two contrasting seasons; (2) obtain measurements enabling determination of the components of the energy balance of the forest; and (3) test the hypothesis that these forests have established in these areas in part because they can access the deep soil water to maintain high evaporation rates in the summer months and avoid significant water stress. The results of these measurements are to be used in validation and parameterisation of a model to be described in a separate paper. Measurements of the energy and water balance of this system were taken during 10 days in spring and late summer. Total forest evaporation, tree water use, sensible heat flux above the 25 m canopy and at 2 m above the forest floor, forest floor evaporation, soil heat flux and temperature profiles, canopy heat storage and radiation above and below the canopy were measured. Despite a reduction of leaf area by one-third of the spring value and a reduction of 30% in soil moisture, the forest maintained high leaf conductances during summer, with evapotranspiration rates at around or above 78% of equilibrium evapotranspiration in both seasons. On average, total water use by the trees was slightly, but not significantly, greater in summer than spring (2.8 and 2.5 mm per day, respectively), and in summer soil evaporation was about half its spring rate (0.27 mm per day compared with 0.48 mm per day). Short-wave radiation penetration through the canopy was found to be the same in spring as it was in summer (being 35% of overstorey radiation), despite a 30% reduction in leaf area index (1.32-0.88) from spring to summer. This is probably due to the significance of tree trunks and branches intercepting radiation, relative to the relatively sparse canopy, and the greater component of diffuse radiation during spring. Canopy heat storage, was found to be a significant component in the total hourly energy budget. During the first three daylight hours it was 45% of net radiation, R_n, in spring and 69% in summer. During the last three daylight hours the proportions were -17 and -19% in spring and summer, respectively. The data presented illustrate important considerations for modelling this forest system and others like it. The importance of a deep soil profile which retains moisture from winter rainfall of this Mediterranean climate, and provides the forest with an adequate supply through the long dry summer is clearly significant. Canopy conductance must be linked to the availability of moisture from the deep soil profile throughout the dry summer. The forest also appears to have adapted to its climate in such a way as to maximise water use despite seasonal fluctuations in leaf area, although we only have data from a single seasonal cycle.