Water shortages could occur if we don't manage our resources properly.
Credit: AFP
SYDNEY, 25 August 2006: Over two thirds of the Earth's surface might be covered in water, but threats of increasing water scarcity around the world are becoming increasingly common.
The amount of water in all the rivers of the world is estimated at 2000km3, yet the annual water withdrawal rate is calculated at 3800km3 per year.
According to a new study, the problem does not only lie with how much freshwater exists, but also how much of the renewable freshwater resources are cycled.
"I think the world should pay more attention to the water scarcity, which is not necessarily driven by physical water shortages but by social issues," said Taikan Oki, co-author of the study published in today's edition of the U.S. journal Science, and associate professor at the University of Tokyo, Japan.
"Even now, there are billions of people who have not enough access to safe water resources. Population growth and economic growth will put further burden on it in terms of both quantity and quality," said Oki.
Australia is one of the driest continents in the world but our water usage is increasing in line with population growth. Every day, urban Australians use an average of 300 litres of water per person, more than Europeans who use about 200 litres, and significantly more than those in sub-Saharan Africa who survive on 10-20 litres per day.
A combination of factors such as human demand, climate change and drought, along with poor water infrastructure and resource mismanagement, is making this a global crisis.
But according to Oki and co-author Shinjiro Kanae, the main focus of preserving global renewable freshwater resources should be the monitoring of freshwater cycles.
Water circulates and naturally forms closed hydrologic cycles. The cycles start when water evaporates and changes from liquid to a gas, and then recondenses as a liquid over time. Water absorbed during photosynthesis becomes part of carbohydrates stored in plants, but returns to water by decomposition.
"In a sense, all the water on the earth is believed to be circulating. However, there are fast tracks and slow tracks," said Oki.
Water flow is considered the most important measure of water resources, but the speed of water circulation is crucial. Speed can be measured by the amount of time water molecules stay in a given reservoir, known as the mean residence time, and is calculated by dividing the volume of the reservoir by the average flow into and out of it.
Rivers unaffected by human impacts have a relatively fast mean residential time, but groundwater aquifers can be quite slow at recharging, with mean residential times of hundreds and thousands of years.
"We can assess the speed by the mean residential time of water for each reservoir, for example, it is less than ten days for water in the atmosphere (water vapour) and two and half weeks for water in rivers," said Oki. "On the contrary, water in a deep aquifer is believed to have a residential time of more than a few hundred years."
Increasing human demands, especially from developed countries, could threaten the supply of renewable freshwater resources. But there is an upper limit to the amount of renewable freshwater resources available for human use and at present, the current withdrawals on a global scale are well below that limit.
"Developed countries are equipped with better facilities to store, regulate, intake, and utilise the freshwater resources," said Oki. "The loss is also much less than developing countries, and as a result, developed countries have capability to use more water effectively than other parts of the world even if climatic conditions are similar."
So what about the threat of climate change? The anticipated global warming will accelerate the water cycle. Precipitation will increase and therefore runoff and river discharge will be increased, thus available freshwater resources will be increased on a macro scale.
"However, the more intense and intermittent precipitation characteristics anticipated in the future under global warming may not allow us to be optimistic because such characteristics will require more infrastructure and better management of freshwater resources," said Oki.
The authors claim that water management is crucial in order to provide renewable freshwater resources for not only human demands in the future, but also for ecosystems and the environment.
