The endless circulation of water from the atmosphere to the Earth and back to the atmosphere is called the hydrologic cycle.
The basic stages of the hydrologic cycle are: evaporation, transpiration, condensation and precipitation.
EVAPORATION: Water molecules bind together to form a body of water, such as a lake, river or even a puddle. When the sun heats the molecules on the surface of a water body they become energized and break away from each other. The molecules evaporate and rise as invisible vapour into the atmosphere.
TRANSPIRATION: Plants give off water vapour into the atmosphere in much the same manner as humans perspire. Water that has been absorbed by the plants moves to the surface of the plants’ leaves and evaporates into the air.
CONDENSATION: As the water vapour rises into the atmosphere, it cools and condenses, often attaching itself to tiny particles of dust in the air. When water vapour condenses, it turns back into a liquid. The water particles collect together and form clouds.
PRECIPITATION: When the clouds become heavy with water particles, the particles fall back to the Earth as precipitation in the form of snow, rail, freezing rain or hail.
PERCOLATION: Some of the precipitation that falls to the Earth seeps into porous soils and cracks in rocks. This water moves downward through the Earth’s surface until it settles in an aquifer (a collected of underground water).
SURFACE RUNOFF: Some of the water that lands on the ground flows over the surface of the land and runs off into nearby streams, rivers and lakes. The greater the slope of land, and the less porous the soil, the more runoff there will be.
Source: The Source Water Primer, 2004, Ministry of Environment and Pollution Probe.