Frequently Asked Questions About Water Storages
General
- Why does the storage figure sometimes not go up after we have a big storm?
- What is the difference between a catchment, reservoir and dam?
- What is the difference between an on-stream and off-stream storage reservoir?
- Why are some reservoirs close to full and others close to empty?
- What happens to excess runoff if a dam is full?
- What impact does snowmelt have on dam levels?
Storage reporting
- What are the changes to storage reporting?
- Why haven’t you previously measured reservoir capacity in megalitres (ML)?
- What is a ‘non-catchment’ source?
- Why are you including water in Eildon Reservoir in Melbourne’s figures without adjusting the storage capacity?
General
1. Why does the storage figure sometimes not go up after we have a big storm?
This is probably the question we’re asked most often. Here’s a full explanation: Media Release - 'Sponge Effect' Limits Impact Of Rain
2. What is the difference between a catchment, reservoir and dam?
Catchment
In broad terms, a catchment is an area that catches rainfall and directs it to a creek, river, reservoir or (in built-up areas) a gutter. Our water supply catchments feed the reservoirs that provide our drinking water.
Most of our drinking water comes from 157,000 hectares of closed water catchments in the Yarra Ranges. These native forests filter rainwater as it flows across land into creeks, rivers and our reservoir storages.
Reservoir
Water from the catchments is normally stored in one of Melbourne’s 10 reservoirs. Water is stored for long periods of time to help improve its quality. Over several months, sediment in the water settles to the bottom of the reservoir and sunlight helps to disinfect the water.
In Melbourne there are five on-stream reservoirs that collect water directly from catchments – Thomson, Upper Yarra, Maroondah, O’Shannassy and Tarago - and five off-stream reservoirs that store transferred from on-stream reservoirs – Cardinia, Sugarloaf, Slivan, Greenvale and Yan Yean.
Dam
A dam is massive barrier built across a catchment to block the flow of water. The dam wall creates a reservoir and allows water to continue flowing downstream of the dam.
Dams create a permanent supply of water for the community to use. Water is released into a network of pipelines that supply homes and businesses with water.
3. What is the difference between an on-stream and off-stream storage reservoir?
An on-stream reservoir is fed by a catchment. An off-stream reservoir is filled with water transferred from on-stream reservoirs.
4. Why are some reservoirs close to full and others close to empty?
Melbourne’s 10 reservoirs vary in size from the smaller O’Shannassy (3 billion litres) to the massive Thomson (1,068 billion litres). While O’Shannassy might be reported as 100% full, that equates to just 0.3% of what the Thomson can hold. Conversely, a 20% full Thomson equates to 70 full O’Shannassy reservoirs.
The total volume of all reservoirs is the most important figure – the volume of any one reservoir is not representative of how much water Melbourne has. The ten reservoirs are interconnected, so we can move water around depending on where the rain falls.
5. What happens to excess runoff if a dam is full?
Generally, the only dams that tend to spill are our two smallest - O’Shannassy and Maroondah.
When either of these dams spill, water flows over a concrete ‘spillway’ and into the Yarra River. This water is then pumped from the Yarra into the Sugarloaf Reservoir, which has a much greater capacity.
6. What impact does snowmelt have on dam levels?
Only a small proportion of the four major harvesting catchments is above the snow line (about 100km2 or 10%).
When snow melts it produces roughly 10-30% of its volume as water, depending on the snowpack density.
The majority of snow melt happens towards the end of the snow season, typically starting in early spring. The proportion of this melted snow that finds its way into the reservoirs depends on the weather and catchment conditions at the time it melts.
Storage reporting
7. What are the changes to storage reporting?
As of 1 July 2010, we have adjusted our water storage reporting to reflect:
- A new national water accounting initiative
- The addition of non-catchment sources such as water being held in Lake Eildon for Melbourne.
We are harmonising our reporting to match a national water reporting initiative. It’s a small change that makes no difference to the total amount of water in storage – which we have always reported down to the nearest megalitre.
There will be very minor changes to the capacity of every reservoir except Thomson and the total system capacity. It makes little difference overall (some rounded up, some rounded down) and is around 0.1%.
8. Why haven’t you previously measured reservoir capacity in megalitres (ML)?
Rounding the capacity of our dams off to the gigalitre was a user-friendly way of representing their size. But to avoid confusion we are harmonising them with a national approach to reporting capacity.
It’s important to note that the actual amount of water in storage has always been reported in megalitres.
9. What is a non-catchment water source?
A non-catchment water source is any water that has not been harvested in Melbourne’s traditional water supply catchments.
10. Why are you including some of the water in Lake Eildon in Melbourne’s figures without adjusting the storage capacity?
We report the volumes stored within our water supply storages that have defined capacities. Lake Eildon is part of the Goulburn-Murray system and not strictly part of the Melbourne water supply system. As a result, allocated volumes which will be diverted to Melbourne over time are treated separately to reservoir capacities.
Put another way…
Storage capacity is relevant when the dam you’re reporting on has the ability to fill to that point with water that is yours. In other words, if it is dynamic. By contrast, water for Melbourne in Lake Eildon is a fixed amount each year. Once allocated and stored in Eildon, the amount will not increase even if there is rain. In other words, it is static.