Showing how water moves through the environment, can help illustrate the importance of protecting our water resources. The first thing to know is that the amount of water on the planet has been roughly constant since the Earth was formed more than four and a half billion years ago. The water you drink today could be composed of the same water molecules that Canada’s first Prime Minister, Sir John A. MacDonald, drank in the mid-19th century, or even the same molecules drank by a prehistoric animal many millions of years ago.

The Hydrologic Cycle. Illustration by Tom Schultz

The same water molecules have been moving about the Earth for all those years. This endless circulation of water between the atmosphere and the Earth and back to the atmosphere is called the Hydrologic Cycle. There are three basic steps to the Hydrologic Cycle: evaporation and transpiration; condensation; and precipitation - as seen above.

The diagram also shows that when water lands on the earth's surface it can seep into the soil through a process called infiltration, and deeper underground to the water table by percolation. If water lands on impenetrable surface e.g. a tarmac road, it will flow over the surface until it reaches soil or a river, stream or lake. This water is called runoff.

Next: Why is it important?

Adapted from Pollution Probe's Source Water Protection and Drinking Water Primers.