The Effect of Irrigation on Blue Gum (Eucalyptus globulus) Water Uptake
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 32
Due to lack of quantitative information on the water uptake of Tasmanian Blue Gum (Eucalyptus globulus) plantations, planning studies on the land disposal of Albany wastewater have had to estimate the evapotranspiration component of site water balance models. With the Albany land treatment site being in an early phase of construction, an alternative study site was chose (Wandalup Farm near Mandurah) which had an established Blue Gum plantation adjacent to a nutrient-enriched wastewater supply (piggery effluent).
· The primary objectives of the present study were a) to measure the seasonal variability of Blue Gum water uptake under irrigated conditions and compare with rainfed trees, b) relate water uptake data with climatic variables in order to extrapolate findings from the study site to the Albany land disposal site, and c) determine the minimum annual rate of tree water use expected under irrigated conditions.
· The sap flow 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 concentrations were also monitored.
· Lower than expected water uptake rates were observed due to the wet site conditions at Wandalup Farm. Shallow groundwater and winter waterlogging restricted root development to the top 60 cm of soil. Therefore, the tree water uptake rates under these conditions should be considered as equivalent to the lower end of the spectrum in transpiration performance expected for Albany.
· Water uptake in the irrigated treatments increased 30-40% within one week of irrigation starting. During the same period water uptake in the rainfed treatment 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 gradually over the course of the experiment, whereas the rainfed treatment was either stable or decreased.
· A particularly dry summer during the experiment resulted in the death of one tree in the rainfed treatment. There was also evidence of a reduction in tree water uptake from mid-summer onwards due to soil moisture becoming limited in the rainfed treatment. The irrigated treatments maintained elevated water uptake rates throughout summer. This implies that the supply of additional water to the trees during the ‘high-energy’ months of the year significantly increases 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 soil moisture is no longer limiting the rate of water uptake. However, evaporative energy is limiting and irrigation during this time of the year would lead to excessive waterlogging. The differences in water uptake between the irrigated and rainfed treatments during late winter were reduced.
· Water uptake/pan evaporation ratios (WU:PE) were calculated to allow extrapolation to the Albany land disposal site given the average monthly pan evaporation data from the Albany airport. Annual PE at Albany is 207 mm lower than 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 trees growing in areas of poor drainage which would include the areas of shallow sand over clay/laterite at the Albany site. Estimates of WU extrapolated from a study at a well drained site (Marshall and Chester, 1991) suggest an annual WU of 842 mm for irrigated trees. Both of these values are significantly lower than the estimated 1249 mm in the Planning Study for the Albany Land Treatment Site.
· The apparent overestimation of water uptake in the Planning Study is a result of having a constant WU:PE ratio of 0.93 throughout the year in the water balance model for the site. The quantitative measurements made in the present study demonstrate that the WU:PE ratio 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 the results of the present study, would result in a more accurate determination of irrigation timing, flow rates and area of plantation required to transpire estimated water loads.