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Keywords:

  • Eddy covariance;
  • energy balance;
  • heat pulse;
  • tree transpiration;
  • understorey evaporation

Abstract

1. Using three independent methods (eddy covariance, heat pulse and open-top chambers), diurnal and seasonal measurements of evapotranspiration were made 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 understorey evapotranspiration contributed 557 mm to this flux. Understorey evapotranspiration occurred predominantly during the wet season as bare soil evaporation and transpiration of Sorghum spp., a C4 grass.

3. Annual transpiration from trees was 313 mm, significantly less than the grassy understorey. Despite a very high degree of seasonality in distribution of rainfall and large changes to soil and atmospheric water content, water use by the trees did not differ between wet and dry seasons. This suggests that mature trees exploit a large soil volume and this may include extraction from the capillary fringe of the shallow water table (2–10 m below the ground surface).

4. The open canopy created an aerodynamically rough surface well coupled to the atmosphere with the coupling coefficient, Ω, 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 season and 0·65 in the dry season. The decline in tree LAI occurred when evaporative demand showed a similar proportional increase. Consequently overstorey water use remained relatively unchanged throughout the year.

6. Given the very high rainfall intensities of the monsoonal climate and low 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.