The Water Cycle

Main Idea

Water is continually recycled through the Earth system via evaporation, condensation, precipitation, and collection, powered mainly by solar energy.

Key Concepts

• The water (hydrologic) cycle moves water between oceans, atmosphere, land, and living things.
• Solar energy drives evaporation from oceans, lakes, and rivers.
• Water vapor condenses into clouds and falls back to Earth as precipitation.
• Plants release water vapor through transpiration, adding to atmospheric moisture.

Definitions

• Evaporation: liquid water turning into water vapor, mainly from oceans.
• Condensation: water vapor cooling and turning back into liquid droplets, forming clouds.
• Precipitation: water falling from clouds as rain, snow, sleet, or hail.
• Transpiration: the release of water vapor from plant leaves.
• Runoff: water flowing over land into rivers, lakes, and oceans.

Processes

• The sun heats surface water, causing evaporation into the atmosphere.
• Rising water vapor cools at higher altitudes and condenses around particles to form clouds.
• When droplets grow heavy enough, precipitation falls, replenishing rivers, lakes, and groundwater.
• Some water infiltrates the soil to become groundwater, while some flows as surface runoff back to the ocean.

Real-World Examples

• Rainforests recycle enormous amounts of water through transpiration, influencing regional rainfall.
• Snowmelt from mountains feeds rivers used for drinking water and agriculture downstream.
• Groundwater aquifers, like the Ogallala Aquifer in the US, store water that took thousands of years to accumulate.

Common Mistakes

• Thinking the water cycle only involves rain, ignoring evaporation and groundwater.
• Believing water is "used up"; it is recycled, not destroyed.
• Confusing condensation (vapor to liquid) with evaporation (liquid to vapor).