Gnangara groundwater system
The Gnangara groundwater system is Perth's largest source of good quality, fresh water. It provides a crucial part of Perth’s public water supply as well as water for public open spaces, an extensive area of local agriculture and our gardens. It also supports nationally significant groundwater-dependent ecosystems, such as wetlands and Banksia woodlands. These groundwater-dependent ecosystems support biodiversity, help create our ‘sense of place’ and make our city an attractive place to live.
Stretching about 2200 square kilometres along the coastal plain north of the Swan River to Gingin and east to the Darling Scarp, the Gnangara groundwater system comprises four main aquifers:
- The shallow, unconfined Superficial aquifer (the Gnangara Mound).
- The shallow, semi-confined Mirrabooka aquifer.
- The deep, mostly confined Leederville aquifer.
- The deepest, mostly confined Yarragadee aquifer
The Gnangara groundwater system
What is the Gnangara Mound?
The Gnangara Mound is the common name for the Superficial aquifer in a large mound of sandy soil located north of Perth. The watertable in the aquifer forms a groundwater mound, where rainwater recharges groundwater at a higher rate than water flows horizontally through the aquifer.
Water use from the Gnangara groundwater system
Gnangara groundwater provides over 40 per cent of Perth’s drinking water each year, as well as local water for agriculture, public open space and domestic gardens. Gnangara groundwater also supports ecosystems such as lakes and wetlands that provide important environmental functions and contribute to the social and recreational value of our communities.
Groundwater for public water supply comes from all of the aquifers in the system, with less taken from the Superficial aquifer to reduce impacts on groundwater-dependent ecosystems. More water is taken from areas of the Leederville and Yarragadee aquifers that are not connected to the Superficial. Private licensed use mostly comes from the Superficial aquifer.
Water use from the Gnangara groundwater system is managed through the Gnangara groundwater areas allocation plan, which defines how much water is available for use and guides water licensing from the system within the set allocation limits.
See how we are tracking against the plan objectives in the Gnangara groundwater areas allocation plan: Evaluation statement 2011–2014.
Gnangara Mound groundwater levels
Groundwater levels across the Gnangara Mound have generally been in decline for the last 40 years. Groundwater levels have declined because of declining rainfall, continued use of groundwater for a range of important uses, and pine plantations on the mound. Since 2011, the rate of groundwater level decline has slowed and possibly stabilised in some areas (see graph below). This is a positive response to the department’s actions to reduce and redistribute groundwater abstraction and reflects more consistent rainfall since the very low rainfall in 2010. We had another very dry year in 2015 and end-of-summer average levels were the lowest on record. After well-distributed rain during the 2016 winter and unseasonal heavy rainfall in early 2017, average groundwater levels are now better than recent years for this time of year.
The seasonal rises in average groundwater level relate to rainfall recharge to groundwater over winter and the falls relate to groundwater abstraction and evapotranspiration over summer. Before 2006 groundwater levels generally declined each year. Since then, levels have stabilised except for step-downs after very dry years (2006 and 2010). This shows that actions to reduce abstraction and move it away from environmentally sensitive areas has made a difference but that more interventions are needed to establish a more sustainable balance in a drying climate.
The average groundwater level graph below is based on monthly measurements at 43 monitoring bores across the Gnangara Mound (Superficial aquifer), which are averaged to show relative changes in groundwater levels compared to the average highest recorded groundwater level.
Bores for the graph were selected from our existing monitoring network on the Gnangara Mound, with a minimum of ten years of data. To achieve the single monthly average figure, monthly water levels are offset against the highest historically recorded groundwater level for each bore. These offset water levels are then averaged across all 43 bores to produce a single number for the month.
During winter months, we also have a monthly winter rainfall summary to track rainfall against Gnangara groundwater levels recorded at the end of the previous winter.
Average groundwater levels of the Gnangara Mound (Superficial aquifer).