Water balance

The long-term water balance of a closed groundwater system such as the Gloucester subregion can be simplified greatly such that annual rainfall is equal to evapotranspiration plus streamflow. As the streams are all apparently net gaining under most natural conditions (see companion product 1.1 for the Gloucester subregion (McVicar et al., 2014)), then streamflow is surface runoff plus groundwater discharge, or baseflow. In a closed geological system such as the Gloucester subregion, and assuming steady state, rainfall recharge is equal to groundwater discharge. At the regional scale the amount of groundwater discharge that is apportioned to deep-rooted vegetation is not significant, as the current vegetation cover is overwhelmingly cleared for grazing (see companion product 1.1 for the Gloucester subregion (McVicar et al., 2014, p. 25, Figure 11; p. 32, Figure 16)), with tall vegetation, associated with deep roots, restricted to small remnant plots at the edges of the PAE. Also considered not significant is groundwater pumping, with only about 0.2 GL/year in known usage (see companion product 1.1 for the Gloucester subregion (McVicar et al., 2014, p. 61)) representing about 0.1% of the subregion’s annual mean rainfall.

There are no groundwater inputs or outflows from the geological basin, and there is no hydrogeological connectivity between the geological Gloucester Basin and the surrounding areas.

Recharge has been estimated to occur across the entire Gloucester subregion; the values range spatially from zero to 23% of rainfall. The highest values occur in the alluvium and the flanks of the geological basin where deep strata outcrop (see companion product 1.1 for the Gloucester subregion (McVicar et al., 2014, p. 63, Table 5)).

Hydrological analyses indicate that the long-term (1900 to 2012) mean rainfall across the Gloucester subregion is 1100 mm/year, and the mean streamflow in the Gloucester and Avon rivers (2003 to 2013) is 326 mm/year, or 177 GL/year. The area of the northern part of the Gloucester subregion included in the surface catchments of these two rivers is 181 km2, or 33.4% of the contributing area. Using the simplified two-component annual water balance, rainfall of 199 GL is partitioned into 59 GL of streamflow and 140 GL of evapotranspiration. Using computer models, Parsons Brinckerhoff (2013b) estimated that 1.7 GL/year was transferred upward from the SRL. Using this as a first order estimate of groundwater discharge to the stream, or baseflow, the streamflow is therefore 57.3 GL runoff (97.1%) and 1.7 GL baseflow (2.9%). If the baseflow is equal to mean catchment recharge, then this equates to 0.8% of rainfall, or 9.4 mm/year. As the mapped alluvium associated with the Gloucester and Avon rivers is only 8% of the northern surface catchment area within the subregion, the effective rate over a smaller area would be much larger.

Using baseflow separation based on digital filtering, estimates of baseflow from stream records are 27% for Avon River at Waukivory and 57% for Gloucester River at Gloucester (see companion product 1.1 for the Gloucester subregion (McVicar et al., 2014, p. 74, Section Using a simplified system based on minimum monthly flows, Parsons Brinckerhoff (2014) estimated baseflow of 6% for Avon River at Waukivory and 29% for Gloucester River at Gloucester. Using the ratio of stream and alluvium water salinity as a baseflow proportion, Frery et al. (2018, Section 2.1.6) estimated baseflow is 3% for Gloucester River at Gloucester, and about 13% for Avon River but with less than 40 data points.

For the southern part of the Gloucester subregion including the Mammy Johnsons and Karuah rivers, a similar simplified water balance can be constructed. The catchment area within the subregion is 166 km2 and the water balance calculations continues to use only long-term mean rainfall value and assume uniform runoff production. The 1968 to 2013 mean flow of the Karuah River at Booral is 270 GL/year, which is the gauge at the southernmost tip of the Gloucester Basin. Within the subregion is 17% of the Karuah river basin area, yielding 183 GL of rainfall partitioned into 46 GL of streamflow and 137 GL of evapotranspiration. Using the range of stream salinity at this gauge of 100 to 350 µS/m (see companion product 1.1 for the Gloucester subregion (McVicar et al., 2014, Section 1.1.5)) and assuming the same alluvial salinity as for the northern part of 3000 µS/m, the ratio of salinities yields 3.3 to 11.7% as a first order baseflow estimate. The volume this represents is 1.5 to 5.4 GL, leaving runoff as 44.5 to 40.6 GL, respectively. Baseflow separation by digital filtering estimates baseflow at 34% for Mammy Johnsons River and 40% for Karuah River at Booral (see McVicar et al., 2014, Section

Last updated:
23 October 2018