How is the groundwater system performing?
Rainfall, streamflow and recharge to groundwater have been declining across south-west Western Australia since the 1970s. Most of our rain falls in winter and this is when groundwater is recharged, so when there is less winter rain there is less recharge to groundwater.
Rainfall has reduced faster than changes to groundwater use and this has shifted the groundwater system out of balance. High groundwater use and lower rainfall have resulted in declining groundwater levels and drying of wetlands.
In 2019–20 water levels at 16 out of 30 representative wetland and vegetation sites were lower than the criteria levels set by the Minister for the Environment in Ministerial statement no. 819 to protect the groundwater system and dependent ecosystems.
A system out of balance also puts water quality at risk in some locations. Enough groundwater needs to flow out to the river and coast to prevent saltwater intrusion into aquifers. Saline water has already moved inland in the Superficial aquifer in some places. Water quality can also deteriorate inland if pumping draws the watertable down to expose acid sulfate soils. Low groundwater levels have already caused acidification and associated ecological impacts at some of the wetlands we monitor.
If we act together to rebalance the system, we can adjust to changes in rainfall, increase water security and reduce adverse impacts on water users and the environment.
Gnangara Mound water levels
The Gnangara Mound is the common name for the Superficial aquifer located north of Perth. The watertable in the aquifer forms a groundwater mound –rainwater recharges groundwater at a higher rate at the top of the mound, and then water flows horizontally through the aquifer, ultimately discharging into the rivers and the ocean. In areas where they are connected, groundwater can move vertically between the Superficial aquifer and the deeper Leederville and Yarragadee aquifers.
Groundwater levels across the Gnangara Mound have generally been in decline for the last 40 years because of decreasing rainfall, continued use of groundwater for a range of important uses, and pine plantations limiting recharge. This means more than 1000 gigalitres (1000 billion litres) of groundwater storage has been lost from the Superficial aquifer since 1980.
In some areas, the rate of groundwater level and storage decline has slowed and stabilised in recent years (see graph here and below). This is a positive response to our previous actions to reduce and redistribute groundwater abstraction for public water supply, as well as a result of more consistent rainfall recently, following very low rainfall in the late 2000s (2006 and 2010 were record dry years). Land use changes such as pine management and urbanisation have also led to higher recharge in some areas.
The department generates the storage level graph below using a method which uses measured groundwater levels from between 329 to 517 monitoring bores. The range in the number of bores reflects the growth in installation of monitoring bores by the department between 1979 and 2020.
Average water levels rose in 2016–17 after an unusual summer rainfall event where 90 mm fell over three days in February 2017 (see graph below). But since then, the dry years of 2019 and 2020 have led to groundwater levels and storage returning to a declining trend. These changes highlight climate variability and the responsiveness of the system to wet and dry years. As a result of climate change, low rainfall years are likely to occur more often. The latest climate models project with high confidence that the future climate for south-west WA will be warmer and drier.
The department generates the average groundwater level graph below based on monthly water level measurements at 43 monitoring bores at a range of locations across the Gnangara Mound. For each bore we compare these monthly measurements to the highest recorded groundwater level. We then average the results to show relative changes in groundwater levels over time.
