WITH Rocklands Reservoir experiencing above-average inflows the last few months and being at its highest level since 1997, some locals have expressed dismay about an imminent planned release of water into the Glenelg River.

The Spectator spoke to one resident who was concerned that the reservoir was rarely this full and therefore the water should be kept for the future in case of drought.

The resident expressed their concern that it would be a waste to release water at anything less than 100 per cent capacity.

Rocklands Reservoir is the largest reservoir managed within Grampians Wimmera Mallee Water’s (GWMWater) water supply system - at full supply volume (FSV), it holds 348,300 megalitres of water; the current volume is sitting at approximately 203,260ML, which is just under 70 per cent capacity.

The high levels of rainfall in the region over the past few months has meant that the inflow during September was approximately 121 per cent of average, October saw inflow at 465 per cent of average, and November inflow was 860 per cent of average.

The current reservoir volume has come as a relief to Rocklands’ residents as the region is vulnerable to drought due to its semi-arid climate.

Adequate water levels in the Rocklands Reservoir can be a major anxiety for residents as low rainfall and droughts have serious impacts on the economy, environment, and the social fabric of the south-west’s communities.

Despite being able to hold another 145,000ML, the current operating guidelines restrict maximum operating volume (MOV) to 296,000ML, about 85 per cent.

The MOV of a water storage facility may be set at a lower level than the FSV for reasons such as: to allow for operational flexibility, ensure the safety and stability of the facility, to protect biodiversity, and to conserve water resources.

As of December 21, 2022, the water volume of the reservoir was at 203,260ML (see attached graph to compare the volume of the reservoir over the months for 2002, 2012, 2021, and 2022).

While the Rocklands Reservoir is fundamental to the livelihoods and survival of Rocklands’ community, it is equally fundamental to the environment and those who live downstream.

It supplies water to the Grampians water system which serves five urban areas of Balmoral, Cavendish, Hamilton, Tarrington, and Dunkeld.

The reservoir also supplies water to a number of rural customers that are essential to local commerce located along the main supply pipelines and is typically used for livestock watering, plant, dairy yard washing down, and domestic supply.

While Hamilton’s water supply is mainly obtained from the western slopes of the Victoria Range in the Grampians National Park, the supplies are topped up with surface water from Rocklands Reservoir.

The water is delivered via a 52-kilometre pipeline commissioned by Wannon Water and completed in 2010.

The pipeline greatly improved the security of Hamilton’s supply and meant water restrictions could be averted even with a repeat of dry conditions like those between the years 2006-2009.

The release of water from the Rocklands Reservoir into the Glenelg River is subject to bulk entitlements and in most cases, full entitlements will not be available in dry years.

Bulk entitlements are part of the operating rules that have been developed to maximise the yield from the combined water supply sources while mitigating water quality risks associated with the higher salinity supply from Rocklands Reservoir.

These bulk entitlements fix the total annual volume able to be harvested from the Glenelg River and they specify requirements such as where the water can be taken, daily extraction limits, and minimum passing flows.

Aside from domestic and economical use, the entitlements are important in protecting an amount of water to go down the Glenelg to restore some of the natural flow of water down the river which has been halved since the construction of the reservoir wall.

Grampians Wimmera Mallee Water (GWMWater) handle the bulk entitlements and environmental entitlements granted for the Wimmera-Mallee System and earlier this month updated the seasonal allocations for December 2022.

GWMWater said water releases were imperative to improve the health of the river and protect biodiversity.

A key objective of the water storage system is that Rocklands water levels are not drawn down below its MOV, and basic needs will always be prioritised above other possible uses for water, including transfers to downstream reservoirs.

But above MOV, water releases help to manage risks associated with water contamination, structural failures, drought, flooding, and other environmental impacts.

Water contamination was also a risk if the water became contaminated, as it would disrupt the entire water system of the south-west that relies on it for environmental health, and domestic and economic use.

Salinity (salt) levels are monitored closely - when the salt levels are within suitable levels, water is preferentially retained in Rocklands Reservoir.

However, if the salt levels get too high, the reservoir water must be mixed with better quality water from the southern Grampians streams, like that in Hayes Reservoir.

GWMWater said releasing water downstream to other reservoirs was also an important part of water management because in a situation of high rainfall - like this season - it was preferable that to fill reserves such as Hayes, Cruckoor, and Hartwichs Reservoirs as much as possible to achieve an overall higher amount of water stored.

GWMWater water resources manager, Kym Wilson said, “Grampians catchments are collectively holding more than 439 Gigalitres, sitting at 78.43 per cent capacity.”

In some cases, reservoirs can actually increase the risk of flooding if not managed properly; for example, the Rocklands Reservoir dam wall is concrete and could be vulnerable to damage - this could lead to structural failure and an unprecedented inability to regulate water releases.

Also, if Rocklands was filled to capacity and experienced heavy rainfall, excess water released downstream could cause flooding.

Ms Wilson said evaporation was also a concern - Rocklands is losing over 200ML per day to evaporation - a figure which will only increase as the weather warms up.

“When water is more plentiful, water transfers aim to optimise where water is stored across the headworks for a range of outcomes including water efficiency, water security, and recreation,” she said.

“The Glenelg River is entitled to passing flows released under bulk entitlement rules as well as a share of environmental water allocations.”

Ms Wilson said water releases and transfers to other reservoirs are part of normal operations; more information can be found by visiting bit.ly/3BWDGAi