Water moves continuously through ecosystems, linking the atmosphere, living organisms, soil, rivers, lakes, and groundwater. This movement is called the water cycle, and it helps control climate, shape habitats, and supply freshwater for plants and animals. In ecosystems, water is not just stored in oceans and clouds, it is also held in leaves, roots, soil pores, wetlands, and aquifers.
Understanding the water cycle helps explain floods, droughts, plant growth, and the availability of clean water.
Key Facts
- Evaporation changes liquid water into water vapor when energy, usually from sunlight, is absorbed.
- Transpiration is water vapor released from plant leaves through stomata.
- Evapotranspiration = evaporation + transpiration.
- Precipitation occurs when condensed water in clouds falls as rain, snow, sleet, or hail.
- Runoff is water that flows over land into streams, rivers, lakes, and oceans.
- Infiltration rate greater than rainfall rate favors groundwater recharge, while rainfall rate greater than infiltration rate favors runoff.
Vocabulary
- Evaporation
- Evaporation is the process in which liquid water becomes water vapor and enters the atmosphere.
- Transpiration
- Transpiration is the release of water vapor from plant leaves, mostly through tiny openings called stomata.
- Precipitation
- Precipitation is water that falls from the atmosphere to Earth as rain, snow, sleet, or hail.
- Runoff
- Runoff is water that travels across the land surface into streams, rivers, lakes, or the ocean.
- Aquifer
- An aquifer is an underground layer of rock, sand, or gravel that stores and transmits groundwater.
Common Mistakes to Avoid
- Thinking plants only use water and do not return it to the atmosphere is wrong because transpiration can move large amounts of water from soil to air.
- Confusing evaporation and transpiration is wrong because evaporation happens from surfaces like lakes and soil, while transpiration happens through plant leaves.
- Assuming all rainfall becomes runoff is wrong because some water infiltrates soil, is taken up by roots, or recharges groundwater.
- Ignoring groundwater in the cycle is wrong because aquifers can store water for long periods and slowly feed rivers, lakes, and wetlands.
Practice Questions
- 1 A forest receives 40 mm of rain in one day. If 12 mm becomes runoff and 18 mm infiltrates into the soil, how many millimeters remain for evaporation, transpiration, or temporary surface storage?
- 2 A small lake loses 6,000 liters of water by evaporation in a day. Nearby plants release 4,500 liters by transpiration. What is the total evapotranspiration for that day?
- 3 A paved parking lot and a forested hillside receive the same heavy rain. Explain which area is likely to produce more runoff and why plants and soil structure change the path of water.