We all know that the amount of precipitation falling from the sky varies a lot, depending on where we live: some places get loads of water and other places get hardly anything. Ever wonder how much water falls over the entire earth over the course of a year? According to a recent article in Scientific American (August 2008), the amount is 110,000 cubic kilometers - that's nearly 10 times the volume of lake Superior!
That huge amount of water would be enough to easily fulfill the requirements for everyone around the planet, if the water arrived where and when we needed it. Unfortunately, this is not the case. Not only is the rainfall supply highly variable in time and space, most of the water that does fall cannot be captured.
Here's a rough breakdown of where all that precipitation goes:
- 61.1% absorbed by soil and plants and then released back into the air, unavailable for withdrawal.
- 36% ends up in the oceans, also unavailable for withdrawal.
- 1.3% evaporates from open water, also unavailable for withdrawal.
- Only 1.5% is directly used by people. Of this amount, 1.4% is used for farm irrigation and 0.1% is used by municipalities and industry.