is salt water ocean and two percent is frozen. Less than one percent of the earth's surface water can be used for the water needs of humans, animals and plants.
The International Atomic Energy Agency (IAEA) is showing countries how to tap the world's many underground springs.
"The management of water resources is the most important thing for water scarcity, because you have to know where it is and how much is there in order to minimise or avoid water shortages," said Pradeep Aggarwal, head of the IAEA's isotope hydrology department.
Isotope hydrology is not a new science and has been around as long as nuclear science. But the IAEA, which has helped some 45 countries set up isotope hydrology labs, says the simple and relatively cheap technique is still not used widely enough.
The idea is simple. Water contains natural isotopes, which are atoms of the same element but with different atomic weights. So-called heavy water contains more heavier isotopes.
By tracking these isotopes -- which give the water source its "fingerprint" -- water engineers can quickly obtain information that would normally require half a century of measuring rain, river and water body levels.
"Because these are naturally existing tracers, we don't have to inject anything into the ground," said Aggarwal. "And they contain the history of water movement and that history is crucial in hydrology."
EVERY LAST DROP
The technique of analysing underground water sources with isotope hydrology is becoming more and more popular in Africa, which is heavily reliant on ground water.
Callist Tindimugaya, head of hydrology at Uganda's Ministry of Water, Lands and Environment, told reporters at a recent IAEA gathering on isotope hydrology that the technique has become crucial for his country's 23 million people, 80 percent of whom rely on underground springs.
The entire Ugandan city of Kisoro relies on a single spring for its water needs, though local officials and water engineers had no information about the origin and movement of this water.
"The worry was the sustainability," Tindimugaya said. "They were worried this water would get polluted and that at the end of the day they would not have clean, safe water."
Local officials mistakenly thought the water originated from Uganda's mountain lakes. The Austrian government even sent water specialists to help Uganda analyse the water source to find out whether it was sustainable and would remain unpolluted.
When conventional techniques failed, the Austrian consultants turned to the Vienna-based IAEA for help. By using isotope hydrology the mystery was solved -- the spring came from rainwater that moved slowly through a network of volcanic caves.
"Rainwater enters volcanic vents, sort of caves, and the caves are covered by papyrus (leaves), so the water is not exposed to atmospheric evaporation," Tindimugaya said.
"And we found that it takes a long time for this water to come through the spring. This water stays 50 years underground before it is discharged."
This was crucial information. First of all, the Ugandans are now able to take action to protect the source from pollution.
"But if pollution is ever detected (at discharge), it means a lot of water along the way is already polluted," he said.
Seifu Kebede, a scientist from Ethiopia's University of Addis Ababa, said that the potential water crisis is often misunderstood as being the result of a lack of clean water.
"The problem is not only the lack of clean water," said Kebede. "There is water, but the problem is (managing), understanding how it moves and how the hydrologic system functions, rather than the lack of water."
Thanks to isotope hydrology, Ethiopia is learning to trace the movement and size of hidden water sources to ensure that they are useable and sustainable for the country's rural population, 90 percent of whom rely on ground water.
"But part of the challenge is convincing the water engineers to implement the isotope hydrology data because the science is new to the country and the scientists are new to it," said Kebede. "They are sometimes suspicious."