TheEffect of Irrigation on Blue Gum (Eucalyptusglobulus) Water Uptake:

Implicationsfor Land Disposal of Sewerage Effluent

ReportNo WSAA 102

September 1995




·        Due to lack of quantitative information on the water uptake ofTasmanian Blue Gum (Eucalyptus globulus) plantations, planning studies on theland disposal of Albany wastewater have had to estimate the evapotranspirationcomponent of site water balance models. With the Albany land treatment sitebeing in an early phase of construction, an alternative study site was chose(Wandalup Farm near Mandurah) which had an established Blue Gum plantationadjacent to a nutrient-enriched wastewater supply (piggery effluent).


·        The primary objectives of the present study were a) to measure theseasonal variability of Blue Gum water uptake under irrigated conditions andcompare with rainfed trees, b) relate water uptake data with climatic variablesin order to extrapolate findings from the study site to the Albany landdisposal site, and c) determine the minimum annual rate of tree water useexpected under irrigated conditions.


·        The sapflow rates of up to nine trees were measured for 10-23 months.Three treatments each with three monitored trees were established, a)effluent-irrigated, b) water-irrigated, and c) rainfed. Groundwater levels,meteorological parameters and soil, water and foliar nutrient concentrationswere also monitored.


·        Lower than expected water uptake rates were observed due to the wetsite conditions at Wandalup Farm. Shallow groundwater and winter waterloggingrestricted root development to the top 60 cm of soil. Therefore, the tree wateruptake rates under these conditions should be considered as equivalent to thelower end of the spectrum in transpiration performance expected for Albany.


·        Water uptake in the irrigated treatments increased 30-40% within oneweek of irrigation starting. During the same period water uptake in the rainfedtreatment increased 0-5%.


·        On a cumulative basis, the irrigated treatments had a 44% (or 400 mm)greater total water uptake by the end of the experiment (23 months).


·        The Leaf Area Index of the irrigated treatments increased graduallyover the course of the experiment, whereas the rainfed treatment was eitherstable or decreased.


·        A particularly dry summer during the experiment resulted in the deathof one tree in the rainfed treatment. There was also evidence of a reduction intree water uptake from mid-summer onwards due to soil moisture becoming limitedin the rainfed treatment. The irrigated treatments maintained elevated wateruptake rates throughout summer. This implies that the supply of additionalwater to the trees during the ‘high-energy’ months of the year significantlyincreases the magnitude and duration of elevated water use.


·        During the cooler/wetter months of the year, average water uptake is40-60% lower than summer averages. it is during this time of the year that soilmoisture is no longer limiting the rate of water uptake. However, evaporativeenergy is limiting and irrigation during this time of the year would lead toexcessive waterlogging. The differences in water uptake between the irrigatedand rainfed treatments during late winter were reduced.


·        Water uptake/pan evaporation ratios (WU:PE) were calculated to allowextrapolation to the Albany land disposal site given the average monthly panevaporation data from the Albany airport. Annual PE at Albany is 207 mm lowerthan at Wandalup resulting in 5.7-8.2% lower estimated WU values for Albany.For effluent-irrigated trees, the annual WU at Albany, estimated from Wandalup,is 610 mm. For rainfed trees it is 377 mm. This is representative of treesgrowing in areas of poor drainage which would include the areas of shallow sandover clay/laterite at the Albany site. Estimates of WU extrapolated from astudy at a well drained site (Marshall and Chester, 1991) suggest an annual WUof 842 mm for irrigated trees. Both of these values are significantly lowerthan the estimated 1249 mm in the Planning Study for the Albany Land TreatmentSite.


·        The apparent overestimation of water uptake in the Planning Study is aresult of having a constant WU:PE ratio of 0.93 throughout the year in the water balance model for the site. Thequantitative measurements made in the present study demonstrate that the WU:PEratio varies throughout the year ranging from 0.31 in March to 0.1 in June.Reassessment of the Albany (and future) land disposal site WU rates using theresults of the present study, would result in a more accurate determination ofirrigation timing, flow rates and area of plantation required to transpireestimated water loads.


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