1.1.5.3.1 Monthly and annual flow characteristics
Sixty streamflow gauges within the subregion have been selected for the analysis of monthly and annual streamflow characteristics. All the selected gauges have more than ten years of data since 1981.
Figure 42 shows monthly streamflow boxplots for the 60 streamflow gauges in the Hunter subregion. The monthly flow is distributed unevenly. High streamflow, that is wet season, occurs in February, March and June. Low streamflow, that is dry season, occurs from September to December.
A streamflow gauge (gauge number 210083) on the Hunter River was selected as an example to analyse inter‑annual streamflow variability. Note that annual flow data in Figure 43 are missing for two years (1984 and 1985) because of missing daily flow data; for other years there are no missing daily streamflow data. For the past 40 years, the streamflow for the Hunter River has shown strong inter-annual variability. Wet years or dry years appeared every 5 to 10 years, indicating strong climate variability. Higher annual streamflow often occurred in the 1970s and later 1980s, while relatively lower annual flows occurred in the early 1980s, early 1990s, and early 2000s. In the 2010 to 2012 La Niña years, annual streamflow evidently recovered (Figure 43).
Figure 43 Annual flow time series for a streamflow gauge in the Hunter river basin
Gaps represent missing annual flow data.
1.1.5.3.2 Baseflow index analysis
Streamflow includes two components, quickflow and baseflow. Quickflow is the part of streamflow that originates from precipitation and soil water directly flowing into the stream, while baseflow is the part of streamflow that originates from groundwater seeping into the stream. Daily total streamflow is separated into baseflow and quickflow using a one-parameter filtering separation equation (Lyne and Hollick, 1979) that is expressed as:
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(1) |
where Qt is the total daily flow, Qb is the baseflow, i is the time step (day) number and α is a coefficient, usually taken to have a value of 0.925 (Aksoy et al., 2009; Gonzales et al., 2009).
Baseflow index (BFI) is here defined as the ratio of mean annual Qb to mean annual Qt. Figure 44 summarises the BFI for major rivers in the Hunter River basin and the Macquarie Tuggerah basin, respectively. BFI varies from 0.40 to 0.66 for the eight rivers (Glennies Creek, Goulburn River, Hunter River, Moonan Brook, Pages Creek, Paterson River, Williams River, and Wollombi Brook) located within the Hunter river basin, and varies from 0.44 to 0.49 for the two rivers (Ourimbah Creek and Wyong River) located within the Macquarie Tuggerah basin.
The BFI for the Hunter River varies from 0.49 at Singleton to 0.66 at Glenbawn, with the mean value of 0.57 obtained from 13 gauging sites (Figure 44). This means that 57% of the total streamflow for the Hunter River is contributed by baseflow, and 43% is contributed by quickflow.
The first eight rivers are within in the Hunter river basin; the last two rivers are within in the Macquarie Tuggerah basin. Each green dot represents a value for a gauging station.
