The world is in the midst of a largely unheralded boom in groundwater use. The rate of extraction from aquifers more than doubled in the 40 years to 2000. It has continued to soar since then.
Professor Craig Simmons, who directs Australia's National Centre for Groundwater Research and Training, says as much as 40 per cent of humanity's total water supply now comes from underground. It is not only used for drinking, but is also needed to grow food and support many industries.
Yet as he and other international water scientists meet in Sydney this week, they are warning of a growing threat to groundwater supplies from over-extraction and from pollution by farm fertilisers, pesticides and mining residues. They say that major aquifers in some countries will start to run low by 2030 unless immediate steps are taken to better manage the resource.
Fresh water is critical for sustaining life on earth. But much of it is stored underground, out of sight and often out of mind. It is nature's buried treasure, a storehouse for future use in case surface water in mountain glaciers, lakes, rivers and streams runs low or is diminished by global warming and climate change. Underground water is less prone to pollution than surface water. It is also the only source of water in many dry areas where water quickly evaporates in the heat.
By some estimates, water stored in the ground has become a key source for drinking for almost half the world's population. Much of it has filtered down far below the surface into reservoirs known as aquifers that have taken hundreds or even thousands of years to accumulate.
Until recently, it has been difficult to measure the extent, levels and recharge rates of below-ground reserves. But as the means for doing so using lasers, special cameras and water-dating techniques have enabled more accurate underground water mapping, scientists have become increasingly alarmed at evidence major aquifers in some countries that are important drivers of global economic growth are being depleted much faster than they are being naturally replenished.
A recent satellite study revealed falling groundwater tables in China, India, North Africa, the Middle East and United States, where expanding agriculture and cities have raised water demand.
Water shortage, particularly in northern China where Beijing is located, is emerging as one of the main constraints to Chinese growth. Underground water nourishes 40 per cent of China's food and supplies 70per cent of its drinking water. Yet water levels in aquifers in some regions are sinking by a meter or more a year. Over 650 Chinese cities have polluted and inadequate water supplies, both on the surface and, increasingly, underground as well.
In India, the world's second most-populous nation after China, the Central Ground Water Board has reported that in the 10 years to 2011, there has been a more than 4m decline in aquifers that supply six major cities, including the political capital, New Delhi, and the commercial capital, Mumbai.
Over-extraction is not confined to developing countries. Advanced economies - among them Australia and the United States, both leading food exporters - face a growing problem of aquifer depletion. Underground water now provides one-quarter of the US supply and over half of all Americans rely on it for drinking. As many as 800,000 new wells to tap aquifers are being drilled each year in the US.
The underground water crisis is generated by competition for increasingly scarce water supplies between megacities, the energy sector, mining, manufacturing and farming. It has been hastened by availability of cheap pumps and relatively cheap energy from fossil fuels, making water easy to extract from aquifers at ever deeper levels.
In Australia, one of the driest continents, CSIRO scientists say that underground water supplies are in fair to good condition. However, a recent study indicated that underground water was heavily over-allocated in seven of the 20 irrigation areas in the Murray-Darling river basin in southeast Australia, the country's main agricultural production zone.
Drought adds to the strain on aquifers. Professor Simmons says that when rainfall declines, most of the available moisture is taken by evaporation and surface vegetation, and has little chance to filter underground. ''Thus, a small decline in rainfall can lead to a very large reduction in aquifer recharge,'' he says. ''Yet the first thing people do when a drought comes is to start extracting more groundwater.'' Professor Simmons says that Australia must accept that droughts will be part of its future. ''At the same time, we have an increasing population as well as growing demands on water from agriculture, industry and mining. We need to start thinking now about [how] we will meet Australia's water requirements in future droughts - and with, potentially, double our present population.'' In water-stressed countries, efforts are underway to increase rainwater harvesting and storage, and conserve surface water use. But underground water governance has lagged. It needs to catch up - urgently.