Science, Technology and Engineering

Environmental Geoscience

Honours Projects

Using water and salt budgets to quantify the effect of blue-gum plantations on water sources in the Eastern Dundas Tableland, western Victoria

Matt Ryan

Supervisor: Dr John Webb

In recent years the introduction of high-density blue gum plantations has occurred at a large scale across the Eastern Dundas Tableland. Very few studies have been undertaken into the water use of young plantations so this has provided a unique opportunity to determine the impact over time of the plantations on recharge and salinity as the trees reach maturity.

A paired catchment study was developed using a three year-old blue-gum (Eucalyptus globulus) plantation and a control catchment, used primarily for sheep and cattle grazing. The two sites are adjacent to each other and both are small catchments with local groundwater flow systems and creeks that usually flow throughout the wetter months. Stream and piezometer loggers, together with a small weather station, were installed to monitor changes to groundwater levels, stream flow, salinity, and climatic events.

Hydrograph data showed that although over 60 per cent of the plantation catchment is planted at 800 trees per hectare, recharge to the local groundwater flow system is still higher than in the farm catchment. Artesian pressures were also recorded in the wetter months, indicating that the plantation is having very little to no effect on recharge to the local system.

Water balance models produced suggest that lateral flow plays a significant role in the water dynamics of both catchments. Transpiration rates of the plantation were shown to be approximately 1.25 times greater than pasture.

Salt budgets constructed show that both catchments are currently net salt exporters. Salt input to both catchments totaled 35 kg/ha in the past year, with the farm catchment exporting 230 kg/ha and the plantation exporting 310 kg/ha. Salt storage in the saturated and unsaturated zone was also higher in the plantation than in the farm catchment. This suggests the trees are causing salt to accumulate in the soil profiles; however there is insufficient data for the catchment prior to the plantations’ establishment to unequivocally prove this.

Errors encountered with data loggers interpreting EC and water levels incorrectly, resulted in the study period being reduced from 14 months to 4 months. Water and salt budgets are usually calculated using many months or years of data. Due to the number of estimates and assumptions in the data, the separate models should be used as broad comparisons only.

This study has provided preliminary data on the water and salt balances of a young plantation. It is expected that the catchments will be monitored until the trees are harvested, and that the data presented in this thesis will play an integral role in determining the effect of large scale commercial plantations on local hydrological systems.