1.1.5.3.1 Monthly and annual flow characteristics
Four available streamflow gauges in the Gloucester River Basin are Avon River at Waukivory (208028), Gloucester River at Gloucester (gauge 208020), Barrington River at Relfs Road (gauge 208031), and Gloucester River at Doon Ayre (gauge 208003). Two available gauges in the Karuah River Basin are Mammy Johnsons River at Crossing (gauge 209002) and Karuah at Booral (gauge 209003). The locations of these gauging stations are shown in Figure 27. Out of the six gauges, gauge 208031 has the least data length (less than three years), while others have data lengths ranging from around ten years for gauges 208020 and 208028, to more than 45 years for gauges 208003, 209002 and 209003.
Figure 28 shows monthly flow distribution for the four gauges in the Gloucester River Basin. The monthly flow for each gauge is distributed unevenly. The maximum flows occur in February for gauges 208028, 208020 and 208031, and in March for the downstream gauge 208003. The minimum flow, however, occurs in different months. The driest month is January for the Avon River (gauge 208028), and September for the Gloucester River (gauges 208020 and 208003). The low flow occurs in April, May, and September for the gauge 208031 in the Barrington River.
Figure 29 shows monthly flow distribution for the two gauges in the Karuah River Basin. The maximum flow occurs in March, followed by a second peak in June. The minimum flow occurs in September, the same month as for the Gloucester River Basin.
Figure 28 Monthly flow distribution for four streamflow gauges in the Gloucester River Basin
Figure 29 Monthly flow distribution for two streamflow gauges in the Karuah River Basin
Figure 30 shows annual flow variation for the four streamflow gauges in the Gloucester River Basin. Note that annual flow data in Figure 30 (and Figure 31) are missing for some years because of missing daily flow data. However, there are no years with zero flow, so all gaps in the figures represent years with missing data. Furthermore, there is no missing data in any of the remaining annual totals. Among the four gauges, only the lower reach gauge 208003 has more than 50 years of observed data, while the rest have fewer than ten years of data (Table 7). For the last 20 years, the annual flow for the Gloucester River is much less than that for the pre-1980 period. The mean annual flow at gauge 208003 is about 550 GL/year, more than 70% of which is contributed by the Barrington River.
Figure 30 Annual flow time series for the four streamflow gauges in the Gloucester River Basin
Gaps represent missing annual flow data.
Table 7 Streamflow data length and climatology for the six streamflow gauging stations in the Gloucester subregion
Figure 31 shows annual flow variation for the two gauges in the Karuah River Basin. Both have long-term annual flow data, and show large interannual variability. For the Mammy Johnsons River, higher annual flows often occurred in the 1970s and 1980s, while relatively lower annual flows occurred from 1990 onwards, with only three such years (2001, 2008 and 2011) being above the long-term average. For the Karuah River, the annual flow was significantly greater than average in five years since 1990 to 1999, 2001, 2008, 2009, and 2011.
Figure 31 Annual flow time series for the two streamflow gauges in the Karuah River Basin
Gaps represent missing annual flow data.
1.1.5.3.2 Baseflow index characteristics
Streamflow includes two components, quick flow and baseflow. Quick flow 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 quick flow using a one-parameter filtering separation equation (Lyne and Hollick, 1979) that is expressed as:
|
|
(2) |
where Qt is the total daily flow, Qb is the baseflow, i is the time step (day) number and 
is a coefficient, usually taken to have a value of 0.925 (Aksoy et al., 2009; Gonzales et al., 2009).
Baseflow index (BFI) is defined as the ratio of annual Qb to annual Qt. The BFI for the Avon River at Waukivory is only 0.27, while those for the other three gauges in the Gloucester River Basin are similar with BFI values of 0.57 to 0.58 (Figure 32). This means that 43% of the total flow for the Gloucester River at Gloucester is contributed by quick flow and 57% is contributed by baseflow, while for the Avon River 73% of total flow is contributed by quick flow and 27% is contributed by baseflow. For the two gauges in the Karuah River, the BFI value is 0.34 for the Mammy Johnsons River at Crossing, and is 0.40 for the Karuah River at Booral (Figure 33), which is lower than that for the Gloucester River.
Gaps represent missing annual flow data.
Figure 33 Baseflow index time series for the two gauges in the Karuah River
Gaps represent missing annual flow data.
