Melbourne
Water overview
Significant water events
The volume of water received from the Thomson Reservoir during the 2013–14 year (132,197 ML) to the Melbourne region was over five times the volume delivered during the previous year (see Surface water inflows).
Melbourne Water completed 13 environmental flow events in the Yarra, Tarago, and Werribee rivers, providing more than 19,000 ML of water to improve river health and meet ecological objectives (Melbourne Water 2014b).
Following the completion of the Eastern Treatment Plant Tertiary Upgrade project in 2012–13, Melbourne Water was able to improve quality of recycled water produced at the plant from Class C to Class A. Melbourne Water delivered Class A recycled water from the Eastern Treatment Plant direct to Water Infrastructure Group. The Water Infrastructure Group via Eastern Irrigation Scheme provides recycled water to its customers for horticulture, open-space irrigation, and industrial processes (Melbourne Water 2014b).
Soil moisture
Figure W1 shows that soil moisture was generally average to below average in the Melbourne region during the 2013–14 year except for a small area in the southeast (compared against the 1970–2014 period). The soil moisture is based on a simple conceptual representation of soil water storage and transfer processes averaged over a 5 km x 5 km grid cell derived from the Australian Water Resources Assessment system landscape model (AWRA-L) version 3.0. The model produces a soil moisture grid for the region at a daily time step, which is then averaged over the entire year to produce an annual soil moisture grid. The annual grid represents the mean daily volume of soil moisture across the region during the year.
Figure W1 Annual soil moisture deciles for the Melbourne region during the 2013–14 year
Soil moisture distribution shown in Figure W1 aligns with average to below average rainfall conditions experienced across the region during the 2013–14 year (see Rainfall in Climate overview section). Average to below average rainfall and generally dry conditions in the soil translated into below average streamflows in the rivers.
Streamflow
There are four primary river catchments within the Melbourne region:
- Bunyip
- Yarra
- Maribyrnong
- Werribee.
More details on these river catchments are provided in the Water resources section of the 'Contextual information'.
Consistent with the average to below average rainfall conditions and dry conditions in the soil, total annual flows for all four rivers were below long-term averages at selected gauging stations. Figure P5 in the Water resources section shows the location of these gauging stations and the catchment boundary used to prepare hydrographs for the four main rivers in the Melbourne region. Figures W2–W5 compare the total monthly flow for the Bunyip, Yarra, Maribyrnong, and Werribee rivers respectively with long-term average and percentile flows.
Figure W2 Total monthly flow for the Bunyip River compared to long-term average and percentiles for the river
Total annual flow for the Bunyip River for the 2013–14 year was 7% less than the long-term average for the period 1962–63 to 2013–14. This observation is consistent with the average to below average rainfall and soil moisture conditions. Generally below average streamflow reflects below average rainfall, but there were some months of above average rainfall. In August 2013, this resulted in above average streamflow, but above average rainfall in April and June 2014 did not trigger high streamflow presumably because soil water stores were relatively low after the dry summer and autumn period.
Figure W3 Total monthly flow for the Yarra River compared to long-term average and percentiles for the river
Total annual flow for the Yarra River for the 2013–14 year was 14% less than the long-term average for the period 1976–77 to 2013–14. Diverting a part of catchment runoff to storages and relatively low rainfall in the western part of the catchment were likely reasons for a lower percentage of streamflow at the gauging station compared to the Bunyip River. A strong relation between monthly streamflow and monthly rainfall was not observed for the Yarra River at this gauging station for the 2013–14 year. Below average streamflow was observed in all months except November to December 2013 and May 2014 irrespective of rainfall. Yarra catchment was relatively dry compare to the rest of the region (see Figure C1 under Rainfall in Climate overview section) during the year. Therefore, monthly rainfall estimated for the whole region would be an overestimate for the Yarra catchment.
Figure W4 Total monthly flow for the Maribyrnong River compared to long-term average and percentiles for the river
Total annual flow for the Maribyrnong River for the 2013–14 year was 58% less than the long-term average for the period 1985–86 to 2013–14. Diverting a major portion of catchment runoff to Rosslynne Reservoir and relatively low rainfall in the eastern part of the catchment (see Figure C1 under Rainfall in Climate overview section) likely contributed to low streamflow conditions observed at the gauging station. Generally below average streamflow reflects below average rainfall, but above average rainfall in August 2013 and April and June 2014 did not trigger high streamflow most likely due to upstream regulation for the Rosslynne Reservoir and relatively low soil water stores after the dry summer and autumn period.
Figure W5 Total monthly flow for the Werribee River compared to long-term average and percentiles for the river
Total annual flow for the Werribee River for the 2013–14 year was 5% of the long-term average for the period 1957–58 to 2013–14. Extensive diversions for Pykes Creek Reservoir, Bacchus Marsh Weir, Melton Reservoir, and Werribee Weir upstream of the gauge were likely the main reasons for observed low streamflows. As a result, there is hardly any relation between monthly streamflow and monthly rainfall at this gauging station for the 2013–14 year.
Major water initiatives
During the 2013–14 year, Western Water initiated and completed the $6.5 million Gisborne Recycled Water Scheme, and $2.0 million Romsey–Lancefield water supply interconnection (Western Water 2014).
Gisborne Recycled Water Scheme supplies Class B recycled water to agricultural businesses in the Gisborne South. Eleven customers have connected to the scheme, receiving a combined total of 137 ML annually. The Romsey–Lancefield pipeline improves water supply security for both towns of Romsey and Lancefield.