South West water resources

Streamflow, dam inflows and regional groundwater levels have been declining across the South West since the 1960s. The effects of decreasing rainfall on ground and surface water resources varies with location. Some of the worst affected areas are those with clay soil covered in forest such as the Blackwood Plateau. Some of the least affected areas are locations with sandy soils, high water tables and no perennial vegetation like parts of the Swan and Scott Coastal Plains (CSIRO, 2009).

South West drinking water

The South West’s drinking water supply is delivered by two water service providers – Bunbury Water Corporation (Aqwest) and Busselton Water (Corporation) , which abstract approximately 6.5 GL/year and 5.2 GL/year of groundwater, respectively. In 2016/17, Aqwest and Busselton Water delivered drinking water to 17 273 and 13 174 customers (properties), respectively.

Over the next fifty years, demand for drinking water supply across the South West may more than double from 12 GL/year to nearly 30 GL/year. The Water Corporation, in conjunction with Aqwest and Busselton Water, are focusing on reducing water use, increasing water recycling and developing new water sources to meet this challenge – Water for Growth: Urban.

Water-dependent ecosystems

Waterways in the South West are the most vulnerable to climate change in Western Australia, due to the long-term decrease in rainfall and streamflow and added population pressures, which drive increased demand for land development and water.

Declines in river flows, groundwater levels, increased drought, extreme flooding events, and salt-water intrusion can all result in changes to freshwater ecosystems. Existing threats to freshwater ecosystems like altered hydrology, habitat loss, algal blooms, fish kills and invasive species will be intensified by climate change, increasing the risk of species extinctions and shifts in the benefits that healthy waterways provide for people.

We use information from climate models to identify the risks of different climate futures to water resources, and this informs our water planning, flood advice and management of water-dependent ecosystems. We aim to enable ecologically sustainable development of the state’s water resources by recognising and protecting water-dependent ecosystems and the benefits and values they provide.

Sea-level rise and flooding

Since 1993, the rates of sea-level rise to the north, west and south-east of Australia were higher than the global average, and rates of sea-level rise on the central-east and southern coasts of the continent were closer to the global average.

Sea levels recorded at Fremantle indicate a long-term average rise of 1.5 mm each year from 1897 to 2004. More recent shorter-term data from the Hillarys monitoring station indicates a greater increase of about 9 mm each year between 1991 and 2011. Modelling of future climate scenarios predicts a 100 mm increase from 1990 to 2030 for the median case.

As sea levels rise, low-lying coastal freshwater floodplain and wetland ecosystems are at risk as the incidence of inundation events increases and vegetation has less time to recover after more and more regular flooding by seawater.

The Department of Planning Lands and Heritage uses climate model information about sea level change to inform Western Australia’s state coastal planning policy.

It is still not clear how climate change is affecting the frequency and severity of flooding Western Australia. Climate change is expected to affect the intense rainfall events that cause flooding, but the changes are likely to vary depending on the rainfall duration, rainfall volume and the season.

This variability, combined with the indirect influence of changing annual rainfall on runoff generation and average storage volumes in reservoirs and lakes, means that even with higher rainfall intensities, there is no certainty that flooding will increase.

Adapting to less rainfall in the South West

We take past trends and future projections of climate into account through water allocation plans, meeting demand, water and land use planning, water licensing, and provide advice on flooding and ground and surface water ecosystems.

We have developed a standard, state-wide approach to integrate projections of future rainfall, evaporation and temperature into modelling of water resources. We use this modelling to inform water availability through water allocation and water supply plans.

We can’t reliably predict rainfall for a given season or year with climate modelling, but we do consider the impact of dry months, dry years and series of dry years in managing water resources. In systems with large dams or connected infrastructure, the department uses monitoring and adaptive management to adjust management from year-to-year in response to rainfall variability. In systems where we have less control and ability to vary water use within and between years, we set annual limits and rules for water abstraction.

Water supply and allocation

We tailor water management around the type of water resource and the infrastructure used to access it. This ensures the best use of available water as the climate changes. How and where you access water is important – either from mains water supply, from a dam or pump on a river, or from a groundwater bore. It will shape how reliably you can access water and how your water use impacts on other users as well as the environment.

We are managing water shortages around the South West through water allocation plans by using allocation limits, licensing rules, recycling and efficiency measures. The case studies below are examples of how the we have used future climate projections to make water supply and allocation planning decisions for different systems across the South West.

Refining allocation limits in the Cockburn groundwater area
Linking water releases from South West dams to inflow – Managing releases from the North Dandalup Dam and Managing releases for the Serpentine River
Rebalancing the Gnangara Groundwater System
Optimising and securing water resources in the South West
Rural case studies on water efficiency and innovation – Shire of Wyalkatchem and Shire of Kellerberrin