By Derek Eamus
Groundwater is extensively used right across the Australian continent, which is why we should take very seriously a new study which says supplies are on the decline, writes Derek Eamus.
The loss of groundwater stores poses serious threats to humans that need it to drink, crops that are irrigated with it, and natural ecosystems that rely on it for their survival.
That’s why a new NASA study is cause for concern, particularly in a dry country like Australia.
Two satellites, launched in 2002, are able to make detailed measurements of the Earth’s gravity field in the Gravity Recovery and Climate Experiment (GRACE).
Short-term fluctuations in the Earth’s gravity field can occur in response to large-scale changes in sea-ice and sea-level rise. However, changes in the gravity field in terrestrial locations far away from oceans and ice have been detected. These fluctuations occur in response to changes in the total mass of water stored within lakes, soil and groundwater.
Using independent data sets, it is possible to remove the effects of lakes and soil water store from the GRACE signal, thereby providing, for the first time ever, large-scale assessments of the changes in total groundwater store within massive aquifers at monthly, seasonal, annual and inter-annual time-scales.
About 40 per cent of the world’s population live in arid and semi-arid regions where groundwater is vital for human consumption but also for the maintenance of ecosystem health. It is not widely understood that vegetation and many streams and rivers are supported by the availability of groundwater, either as discharge into streams and rivers or through groundwater uptake by plant roots directly.
Australia is of course a very dry country so it is no surprise to find that groundwater is extensively used right across the continent. Perth relies heavily on the Gnangara Mound aquifer for its water supply, but the water table has been dropping for the past 40 years or more because of reduced rainfall, increased extraction, and probably because of decreased recharge arising from vegetation water-use.
The Great Artesian Basin (GAB), underlying about 1.7 million square kilometres of Australia, contains about 65,000 km3 of water, but the water is up to 2 million years old so it is easy to extract this resource far faster than it is being replenished.
As the pressure in the GAB has declined and the water table drops, mound springs (where groundwater is pushed to the ground surface under pressure) have begun to dry up in South Australia and Queensland. Associated paperbark swamps and wetlands are also being lost and it gets more and more expensive to extract the groundwater for irrigation and other commercial applications.
On average, rates of groundwater extraction across Australia has increased by about 100 per cent between the early 1980s and the early 2000s, reflecting both the increased population size and commercial usage of groundwater stores.
The GRACE satellites have also been used to monitor the impact of the Australian millennial drought on the Murray-Darling Basin (MDB). The MDB accounts for about 30 per cent of the gross value of Australia’s agricultural production. From 2001 to 2006, the total rainfall deficit (the difference between the actual volume of rainfall received and what would be expected in rainfall was the average) for the basin was estimated to be about 520 km3. Using GRACE data Marc Leblanc and co-workersshowed that there was an almost total loss of surface waters (lakes, rivers) within two years of the start of the drought but depletion of groundwater stores continued for six years or more after the start of the drought. Between 2001 and 2007, there was about 104 km3 of groundwater lost from the MDB. The average annual loss of surface water and groundwater was 20 km3 which is 150 per cent of the total water usage in a normal year. Despite rainfall rebounding in 2007 and 2008, the GRACE data showed a continued decline in groundwater storage.
The international study released by NASA showing declines in groundwater resources globally should alert us to the pressing need to manage groundwater resources sustainably. Australia is not immune to the challenges posed from declining groundwater resources.
Derek Eamus is Professor of Environmental Sciences at the University of Technology