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Water losses

All rivers use or consume a portion of the water they hold through the natural processes of evaporation, water seeping into the ground and when water is used by growing plants. This water is often considered 'lost' but should really be thought of as the natural amount of water a river needs simply to keep flowing along its length.

In the Murray–Darling Basin, losses on many rivers, including the Murray, are often quite high due to the region's typically hot and dry climate, particularly in summer.

This means some water is naturally 'lost' from rivers in the Basin system.

When water is managed and delivered, these losses need to be taken into account. It is the MDBA's role to do this for the River Murray because it operates this part of the Basin. Other rivers are managed by Basin state governments or their nominated agencies. These bodies also consider losses when delivering water.

When the MDBA moves water through the River Murray system to areas where it is needed, it must take into account the fact that some of the water will be 'lost' through these processes. Water is also lost through evaporation when it is in storage, waiting to be delivered.

The MDBA works with other organisations that manage water to minimise losses, as much as possible.

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What the term 'water losses' means

'Water losses' is a term used when the MDBA and the states or water corporations are accounting for water.

The MDBA has processes in place to calculate how much water is expected to be lost in operating the river—this amount is known as conveyance. It also calculates the amount of water that has been lost through natural processes when water is moved or stored, so it can tell the states how much water is currently available to them.

What causes water losses

Water losses happen when water:

  • evaporates
  • is used by plants
  • seeps into the ground.

Water in the Basin generally has a high rate of evaporation given the variable climate in Australia. This is because the land in the Basin is very flat, meaning rivers flow slowly. This means the water moves slowly and takes a long time to get from place to place, giving it time to evaporate. Water evaporates at a faster rate in the summer climate  due to hot conditions.

Some storage locations are more 'efficient' than others at storing water. Dartmouth Dam is an example of a very efficient storage. It's the largest dam in the southern Basin and when full, holds more than 40% of the River Murray system's stored water. The evaporation rate from this dam is extremely low.

In areas where the rivers are lined with forest, particularly river red gums, a lot of water can be lost when the trees draw water from soil (or directly from the river). The water taken from the soil is then replaced by water from the river.

The amount of water lost varies depending on weather conditions.

  • Hot weather increases the amount of water that evaporates.
  • Wind also increases the amount of water that evaporates.
  • Dry soil (caused by hot weather and low rainfall) absorbs more water than usual
  • Low rainfall and increased demand for water can mean more water is needed to move water through the system. This results in larger losses.

Conveyance water

Conveyance water (or 'conveyance losses') is the water needed to carry out the day-to-day operations of the river, including the extra water used to move water from one place to another.

As water moves through the rivers, some of it evaporates and some of it soaks into the ground. To make sure that entitlement holders get all the water they were allocated, extra water needs to be released to make up for the amount that will be lost along the way. The MDBA calculates how much water will be lost due to climate and weather factors and then works out how much extra water to release. This extra water is called 'conveyance water'. This water is accounted for as part of the operation of the River Murray system.

Water losses and entitlements

When the MDBA calculates how much water is available in the River Murray system for each state according to the Murray–Darling Basin Agreement, it accounts for the water that is likely to be lost through conveyance, evaporation and seepage. This water is considered conveyance water. This means that the amount of water calculated for each state (the state's 'share') is the amount of water actually available. The shares do not include the conveyance water.

Conveyance water is set aside at the start of the water year, and the amount of water used to get water where it needs to go is recalculated every month.

Once each state knows the amount of water available, it decides how to allocate the water to entitlement holders according to the internal rules established by that state. The MDBA's use of conveyance water does not come out of entitlement holders' allocations. In fact, conveyance water is necessary to make sure that all the water allocated gets to where it needs to go.

In the news: Water losses in the Barmah Choke

The Barmah Choke is a narrow stretch of the River Murray that runs through the Barmah–Millewa forest. At times the forest can be inundated either naturally or intentionally by the MDBA's river operators where a water order has been received.

In a 2019 episode of 60 Minutes, the MDBA was accused of mismanaging the water to cause detrimental inundation in the Barmah-Millewa forest. However, this report was inaccurate and the MDBA issued a complaint to 60 Minutes highlighting factual errors.

Misunderstandings around environmental watering resulted in people believing that the area was flooded to deliver water downstream.

At times, the MDBA deliberately increases releases to make water flow onto parts of the Barmah-Miilewa Forest. This is done to meet a water order designed to replicate or enhance natural flows into this forest. These flows also occur naturally from time to time following rain events. The flows are delivered within the levels permitted for operating the Choke and are well below actual flood levels for the Murray. In most cases, the water that enters the forest returns to the river and flows further downstream. This is known as return flows.

It is important to note that all losses related to providing water for the environment are absorbed by environmental water holders. Environmental water holders also work to effectively manage flows through the Choke and where possible, aim to water outside of peak irrigation season.

What the MDBA is doing to manage water losses

The MDBA runs the River Murray as efficiently as possible to minimise losses. This is something that is considered in the day-to-day operation of the River Murray. Making decisions on when to move water is based both on how to best reduce the amount of loss and the potential for water to spill.

For example, in the water year from July 2018 to June 2019 (one of the driest on record) it was decided to move water to reduce losses.

With less water flowing from rivers in New South Wales and Victoria, there wasn't enough water flowing into Lake Victoria to meet summer demand for water from South Australia and the lower Murray. To meet that demand, the MDBA transferred water from the Hume and Dartmouth dams to Lake Victoria.

The water was moved through the Barmah-Millewa Forest to bypass the Barmah Choke.

The move was timed so there would be minimal loss from:

  • evaporation
  • any potential spills from Lake Victoria if there was rain in spring leading to more water flowing into the system.

This was also to reduce the risks of 'system shortfall' over the summer, when there is not enough water in the river to meet water orders.

In general, if water is at risk of being lost through evaporation or spills, or if there is a risk of shortfalls, the MDBA may need to move water into another area.

Gauges at various points along the river measure the amount of water leaving and entering the river system so that accurate calculations can be made.

More on water losses

Updated: 30 Sep 2020