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Allocation – Werribee

The 2017-18 season allocation for customers in the Werribee and Bacchus Marsh Irrigation Districts is currently:

  • High Reliability 45%
  • Low Reliability 0%
  • Carryover (equivalent to HRWS) 70%

The allocation does not include any carried over water from last season.

Allocation is reviewed every fortnight. Changes in allocations are announced via this website, SMS and sometimes by media release.

Allocation Methods

Allocations

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 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.

Actual data

Volumes:

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.

Assumed data

Inflows

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.

Evaporation

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%

Unused entitlement

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

Guaranteed inflows are estimated by skilled staff.

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