Allocations – Bacchus Marsh
The 2017-18 season allocation for customers in the Bacchus Marsh Irrigation District is currently:
- High Reliability 35%
- Low Reliability 0%
Note that from 2014-15, allocation may be affected by carryover rules.
Allocation is reviewed every fortnight. Changes in allocations are announced via this website, SMS and sometimes by media release.
Allocation prediction for Werribee and Bacchus Marsh
Allocation prediction for Werribee and Bacchus Marsh
The graphs are based on the starting allocation for this season, and then show increases as we receive more water into the reservoirs (inflows). This is based on historical data over 30 years.
The four graphs show how the current season might progress based on four scenarios:
- a wet year
- an average year
- a dry year, and
- a drought year.
The actual current seasonal allocation is shown by the dotted green line and this will be updated as the season progresses. You can see in each of the graphs how the current season is tracking against each scenario. This can help to predict what the allocation may look like under each scenario.
We are currently tracking closely with a wet season scenario.
Every year, we set allocation levels for the Western Irrigation District, which tells customers what % of their river water entitlement they can take. This is separate to any recycled water entitlements.
We announce allocations every fortnight.
A season lasts 12 months, from 1 July to 30 June.
Before the recent drought, allocations would usually go above 100% of high reliability water shares and 100% of low reliability water shares by the end of the season. During the last years of the drought, they dropped to as low as 5% for the whole season.
These big differences have led us to improve the model that we use to decide on allocations. We need something that is flexible, and that ensures that we don’t over-allocate.
Under our Bulk Entitlement, we have to provide data to support our allocation decisions.
Rules for good seasons
These are the rules we follow to decide allocations when the allocation exceeds 50% high reliability water shares (HRWS). This allows us to keep aside water for the next season before we reach 100% HRWS allocation in the current year. Allocate all river water for the current year up to 50%
Allocation increases will be in rises of 5%. If there’s more than 0% but less than 5%, it will stay in storage until the next announcement. After we reach 50%, we take out dead storage at Pykes Creek (1,922 ML), then share any further water equally between the current year and the next year. For example, if the current available resources are at 60%, the allocation for this year would be 55%, and 5% would be put aside for next year.
Between 50% and 75%, if available water is less than 5% for each season, the allocations will swap between this season and next season, starting with the current season. Sharing continues until the allocation is 75% for the current season and 25% for the next season. Once we reach 75%, we stop allocating for the next season. All new water goes into the current year until it reaches 100%. When the current season has 100% allocation, 25% allocation (less evaporation) is guaranteed for the following year.
Once we reach 100% HRWS, further water is shared between low reliability water shares (LRWS) for the current season and HRWS for the next season
Rules for poor seasons
If the ‘full season” allocation would be less than 25%, then the allocation shall be made based on the available resources in the following sequence:
1. Fix the allocation period to 30 November, and increase allocation to 5% in 1% increments.
2. Fix the allocation at 5%, and extend the allocation period, firstly to 28 February, secondly to 30 April, and then to 30 June.
3. Fix the allocation period to 30 June, and increase allocation in 2% increments until a 25% allocation is reached.
Thereafter, our normal allocation procedure would apply
Data that we use to work out allocations amounts
We use a range of data to work out the amount of water we can allocate. Some of this is actual data and some of it is assumed data.
|Reservoir||Maximum capacity||SRW’s share||Other shareholders||Dead storage|
|Pykes Creek||22,119 megalitres (ML)||99.74%||Western Water 0.26%||1922 ML (in drought, SRW can use floating pumps to reduce dead storage to 468 ML)|
|Melton||14,364 ML||100%||141 ML|
|Merrimu||32,516 ML||20%||Western Water 70% DELWP 10%||570 ML|
High and Low Reliability Water Shares
The figures for these are in the Victorian Water Register and our Irrigation Planning Module (IPM).
Total usage to date
This figure comes from the IPM system. It includes data from meter readings, usage estimates since last meter readings, and orders lodged in the system.
We assume that we are able to harvest all flows into both Pykes Creek, Melton, other than the legal passing flows.
Flows into Merrimu are shared between SRW, Western Water and the State Government.
We use a model to work out evaporation, based on average evaporation, area of storage and a 12 month period.
Delivery and river transfer losses
Delivery and river transfer losses can vary a lot, depending on weather, leakage and demand.
Based on past data, we set losses at:
- 35% if allocation is below 50% (for both delivery and river transfer losses)
- 30% for delivery losses and 20% for river transfer losses if allocations are above 50%
Any unused water at the end of a season will be part of the opening allocation at the start of the new season.
Guaranteed inflows are estimated by skilled staff.