Evapotranspiration from eucalypt open-forest savanna of northern Australia

1. Using three independent methods (eddy covariance, heat pulse and open-to p chambers), diurnal and seasonal measurements of evapotranspiration were m ade in a wet-dry Eucalypt savanna of the Northern Territory, Australia. 2. Total annual dry-canopy water loss was estimated to be 870 mm and unders torey evapotranspiration contributed 557 mm to this flux. Understorey evapo transpiration occurred predominantly during the wet season as bare soil eva poration and transpiration of Sorghum spp., a C-4 grass. 3. Annual transpiration from trees was 313 mm, significantly less than the grassy understorey. Despite a very high degree of seasonality in distributi on of rainfall and large changes to soil and atmospheric water content, wat er use by the trees did not differ between wet and dry seasons. This sugges ts that mature trees exploit a large soil volume and this may include extra ction from the capillary fringe of the shallow water table (2-10 m below th e ground surface). 4. The open canopy created an aerodynamically rough surface well coupled to the atmosphere with the coupling coefficient, Omega, ranging from 0.40 to 0.11 over a wet-dry cycle. 5. Leaf area index (LAI) of the overstorey was typically 1.0 in the wet sea son and 0.65 in the dry season. The decline in tree LAI occurred when evapo rative demand showed a similar proportional increase. Consequently overstor ey water use remained relatively unchanged throughout the year. 6. Given the very high rainfall intensities of the monsoonal climate and lo w LAI of the site, canopy interception was set at 5% of rainfall. Including this amount gives an annual evapotranspiration of 958 mm for this savanna.