Changes in vegetation cover have the potential to impact groundwater and surface water resources through changes to evapotranspiration. Information on this relationship is required to develop policies for controlling land use change to mitigate possible reductions in water resource availability due to water balance change.
We monitored precipitation, flow, and groundwater over 6 years in seven small upland catchments of south-west Victoria dominated by pasture or blue gum plantations. Rainfall was highly variable (394-978 mm/yr) and annual streamflow made up < 1% to 21% of the rainfall.
Plantation flow approaches or exceeds pasture flow during wetter years in two geologic settings investigated. Low flow in one of the plantations is believed to be due to effects of underlying limestone karst.
There is a strong correlation between winter flow in both plantation and pasture catchments for similar geologic setting. Rain events in other seasons typically produce a greater response in plantations than would be predicted from the winter relationships between land uses. This suggests that summer flows are dominated by overland flow. Thus, the plantations do not have a major streamflow effect in summer.
Groundwater levels declined in the plantations but remained stable in the pasture catchments, although still below the 1990s Millennium Drought levels. Unplanted areas along plantation stream drainages increased recharge, offsetting recharge decline under the trees.
Evapotranspiration ranged between 90-97% of rain in pasture catchments and 100-110% of rain in plantation catchments, indicating trees are transpiring groundwater. Groundwater depths in the plantations are deeper than levels previously considered accessible by tree roots.
The minimal effect on streamflow within the catchments suggests that tree plantations in this setting do not have as significant an effect on water resources as previously suggested. However, the reduced recharge is an important consideration in the recharge areas of significant aquifers.