• The hydrologic cycle involves evaporation, condensation, run-off, infiltration, percolation, and transpiration
• The bulk of the earth's water remains in the oceans for thousands of years before participating in the hydrologic cycle
• Most precipitation on land returns to the atmosphere by evaporation and transpiration
• Groundwater represents two-thirds of all water on land

he distribution of water on the land is dependent upon the complex interaction between atmosphere and oceans that we call climate. The circular path of the hydrologic cycle links evaporation, condensation, run-off, infiltration, percolation, and transpiration. These processes cause water to change state (vapor, liquid, solid) as it moves between different elements of the earth system.

Simple animation of the hydrologic cycle. A description of the steps in the cycle is available at the University of Illinois Department of Atmospheric Sciences website.

The oceans are the ultimate source for all water on or below the land surface. The average residence time - the length of time water remains in a given location - for oceanic water is 3,000-4,000 years. The bulk of evaporation (85%) occurs over oceans and is greatest in areas of warm climates at low latitudes. Water vapor cools and relative humidity of the air increases as it rises in the atmosphere. Condensation (water vapor converted to liquid) forms tiny moisture droplets that may coalesce to form clouds form when the the air becomes saturated with water vapor (100% humidity). Atmospheric circulation patterns may redistribute the saturated air prior to precipitation.

Water balance on earth. The majority of earth's water is in the oceans.

Precipitation is concentrated over areas of rising air (e.g. along equator or above mountains) and is least in areas of descending air (e.g. along the tropics). The volume of moisture in the atmosphere is equivalent to ~25 mm of precipitation.  It is estimated that moisture in the atmosphere is recharged 40 times a year (residence time ~9 days) as the average annual precipitation for the world is approximately 1000 mm. Nearly a third of all water falling as precipitation completes the circuit to the oceans by surface run-off in streams (average residence time 14 days) . Most of the rest returns to the atmosphere by evaporation or through the transpiration of plants.

Land areas receive more moisture by precipitation than they supply by evaporation.  The difference is made up by evaporation from oceans. This excess water is returned to oceans by  surface run-off (streams).

A slim fraction of water falling as precipitation infiltrates below the surface through bedrock or soils to form groundwater.  Some of the soil moisture is lost to evaporation or taken up by vegetation and the remainder recharges the groundwater system. Groundwater flow is termed percolation and occurs at rates from meters per day to millimeters per year. Consequently the residence time for groundwater may be measured in intervals of weeks or thousands of years. Even the slowest flow rates will eventually return the groundwater to the ocean, completing the hydrologic cycle.

Approximately 3% of the world's water exists on land, mainly the form of groundwater or ice. Most (~90%) of groundwater contains concentrated salts and is too saline for human use.