By all reports, 2020 was a wet year.
This has obviously been a great relief following the 2019–20 bushfire season, and as a result, data released by the Department of Environment, Land, Water and Planning (DELWP) and Victoria’s 19 water corporations shows that water restrictions are not likely to be required for any regional city, town or licence holder.
But we should not grow complacent.
With warmer temperatures occurring over summer and the long-term future, water resource managers must remain vigilant to the security of water supplies for those who rely on it—which is everybody; industry, agriculture, the environment, communities and business.
To observe and project: an evidence base for water planning
This is where the Victorian Water and Climate Initiative (VicWaCI) comes in.
The initiative, a four-year research partnership between DELWP, CSIRO, the Bureau of Meteorology and The University of Melbourne, has released a synthesis of its findings.
Titled Victoria’s Water in a Changing Climate, the report allows for a better understanding of how Victoria’s climate and hydrology are changing through a mix of historic observations and future projections, and, perhaps most importantly, how this will impact the state’s water resources in the short and long term.
“VicWaCI has been a key part of providing the evidence base that underpins many of the water resource planning decisions made across the Victorian water sector," says Geoff Steendam, Senior Manager of the Hydrology and Climate Science team of DELWP’s Water and Catchments Group.
“This report is a response to the need to improve our understanding around climate and water in order to better meet community expectations around water and is one of the actions in the Victorian Government’s water plan, Water for Victoria."
The science around Victoria’s changing rainfall
Victoria’s Water in a Changing Climate is an important resource for the water sector.
Understanding all facets of our changing climate and hydrological cycles including where and when it is raining will make for better decision making and will help water resource planners to prepare for the challenges that we already know lie ahead regarding water security.
There is a large range of uncertainty around rainfall projections. Rainfall declines over the past two decades have generally (and unfortunately) been much greater than the change simulated by most climate models.
“For instance, research shows us that Victoria is receiving less rainfall in the cool season of April to October, which is the time of year when Victoria generally receives most of its rainfall, and when storages are typically replenished," explains the Bureau’s Principal Research Scientist Dr Pandora Hope.
Hope adds that, “conversely, we also know that over recent decades the warm season of November to March has gotten wetter in the northern part of the state".
Shifts like this mean that there will be less water making its way into our catchments and waterways. Even in the state’s north, the warm season weather accelerates the process of evapotranspiration, which is the transfer of water vapour to the air directly from the soil or through plants.
Somewhat perversely, VicWaCI research also found that extreme, short duration rainfall events (those exceeding both 12mm and 18mm) are becoming more frequent and more intense in many places. This adds another layer to Victorian water planning as these can have a substantial impact on, drainage, urban runoff and infrastructure investment.
Reckoning with being a sunburnt country
Victoria, of course, is not unfamiliar with water restrictions, with extensive efforts made by the community to save water during the Millennium Drought (typically recognised as 1997–2009) most notably.
The initiative’s findings show that the Millennium Drought had a dual effect on water resources. Firstly, researchers at University of Melbourne and Monash University found that one-third of studied catchments have not recovered to their pre-Millennium Drought runoff status (predominantly those in the state’s west). Secondly, that hydrologic models do not replicate well the lower runoff observed during the drought. This means that it may become harder to predict runoff during a long drought.
“Catchments can respond to and recover from drought in distinctly different ways," says The University of Melbourne Senior Lecturer Dr Murray Peel.
"The relief of the catchment, so how steep or flat the catchment is, as well as the pre-drought climate, are good indicators of a catchment’s resilience or vulnerability to drought. Drier, flatter catchments are more vulnerable to shifts.”
Understanding the uncertainty of our water cycle
Among the most significant findings that we can be certain about is that the previously reliable patterns behind Victoria’s rainfall have indeed changed.
There is a large range of projections for rainfall, as highlighted by the research initiative’s work with the CSIRO. But while the science in the VicWaCI report discusses many large-scale climate drivers, all of this is done within the context of an expected warmer and drier future.
“It's important to note that Victoria’s climate variability will remain high,” recognises Dr Francis Chiew, a globally recognised researcher at CSIRO.
“We'll still have wet years or wet multi-years, but the dry periods that we encounter will become more frequent. It is a big challenge not limited to just Victoria or Australia.”
Nevertheless, Chiew goes on to recognise that, “the understanding of Victoria's water future continues to improve due to research initiatives like the Victorian Water and Climate Initiative.”
All of this suggests that water resource managers must plan for a range of possible futures, something that is being aided by another DELWP-produced document, the Guidelines for Assessing the Impact of Climate Change on Water Availability in Victoria, released late in 2020.
The range of potential water futures for Victoria will also depend on whether, for instance, global governments’ responses to the Paris Agreement will be enough to stabilise or limit further changes in the global climate and dominant hydrological processes.
All of the factors detailed here as well as in the Victoria’s Water in a Changing Climate report highlight the importance of using climate change projections, like those developed through the research program, when planning for future water availability.
So, what can we glean from current storage levels and a forecast for above-average winter rainfall across most of Victoriain 2021? Geoff Steendam suggests that it is important we don’t see the short-term water story as being indicative of the long-term future.
“Although we will continue to see wet and dry periods into the future, over the longer term we can expect to see an overall trend towards a decline in streamflow across our catchments. With the help of all of our stakeholders across the Victorian water sector, we are working to apply the findings to water resource decisions.”