Daily and annual water use by four eucalypt species irrigated with wastewater at Robinvale.  J. D. Morris and B. A. Wehner.  September 1987.  31pp. (unpubl.)

SUMMARY

Water use by river red gum, flooded gum, southern blue gum and mahogany gum irrigated with sewage effluent was assessed in a three year-old plantation in north-western Victoria. Measurements were carried out at 4 to 9 week intervals over a period of one year, using a porometric technique to record transpiration rates of single leaves in 24 canopy strata (inner and outer canopy on 4 sides of the tree, at 3 levels in the crown). Multiple linear regression analysis was used to derive empirical models for predicting transpiration rates from short-term meteorological observations and for predicting daily canopy water use from daily maximum temperature and net radiation. Validation on an independent data set suggested that the short-term models underestimated water use during periods of peak transpiration. Similar values of canopy water use were obtained whether derived directly from measured single leaf transpiration or through calculated leaf and canopy conductances.

Transpiration per unit leaf area was greatest in red gum, but this was compensated to some extent by greater leaf area and different crown structure in blue gum and flooded gum; canopy water use by these species was about 90% of that by red gum. Mahogany gum transpired less than half this amount. Monthly water use by red gum varied from around 100 mm in June to over 500 mm during summer. This corresponds to a crop factor of 1.4 to 1.9 times pan evaporation over most of the year. Integration of daily water use values predicted using meteorological records over a four year period indicated that irrigated eucalypt plantations in north¬western Victoria may regularly use up to 4000 mm of water per year.

The high water use values recorded are attributed to high leaf area index developed in irrigated tree plantations, unrestricted availability of water during summer, the arid climate of the study area, and possible morphological effects of irrigation with sewage effluent. The models derived may be applied for estimating water use by young irrigated eucalypt plantations from daily meteorological data in areas with climates similar to that of north-western Victoria.