17th December 2025
Only 2.5% of the Earth’s water is fresh water – the rest is salt water found in the seas and oceans.
Fresh water exists in two forms, blue water and green water.
Blue water is the water that is in rivers, lakes , underground aquifers and as frozen water in glaciers and ice sheets. Amazingly about 60% of blue water is locked away as ice.
Green water is the water stored in the soil and drawn up by plants.
Every year some 500,000km3 of water evaporates from the surface of the oceans, with 450,000km3 returning as precipitation directly back into the ocean(usually within a period of about 9 days) and 50,000km3 falling on land.
“The highest rate of evaporation from the oceans occurs in winter for both the Northern and Southern Hemispheres. The location of greatest evaporation is found on the east coasts of continents. This is due primarily to winter storms that move off the east coasts of continents, which tend to have strong winds. These winds help carry water vapor away from its source, thereby allowing more evaporation to take place.
“Another factor is the warm ocean currents that move pole-ward along the east coasts of continents. The cold winter-time air masses that move over the water allow for large differences in air and sea temperatures, so evaporation is also large. When these differences in air and sea temperatures are then combined with strong winds, it makes evaporation in these regions very efficient.” (1)
Water molecules from the ocean would struggle to reach the inner parts of the continents. Here precipitation is dependent on moisture that has been released by plants through evaporation, forming clouds etc. Plants provide about 70,000km3 of such precipitation. Vegetation cover is key to ensuring that precipitation spreads right across continents.
The other key source of water for inland areas is from melting snow and ice: high mountain regions capture precipitation and then release it throughout the year as meltwater-fed rivers. Water molecules that are exist within these frozen states may take a thousand years to complete the journey from precipitation to their return to the ocean! This is also highlights as we loose glaciers because of global warming, they will be hard to replace.
The Green Tau 