The hydrologic cycle describes the journey of water as water molecules make their way from Earth's surface to the atmosphere and back again. This system, powered by the sun, is a continuous exchange of water between the oceans, the atmosphere, and the land. The same water you will drink tomorrow was probably also consumed by a dinosaur millions of years ago.
The Hydrologic Cycle: A brief introduction.
Evaporation: Water on Earth's surface returns to the atmosphere through evaporation. Surface water, such as lakes and oceans, is a major source of water for evaporation. Water may also evaporate from rivers, soil, glaciers, the skin of sweaty animals and other surfaces.
Transpiration: Plants return water to the atmosphere through transpiration. Plants draw water from the soil into their roots. The water moves through the stem, branches and leaves of the plant. Water is released through the leaves during photosynthesis and is evaporated from the surface of the leaf. Earth scientists often group evaporation and transpiration together using the term evapo-transpiration.
Condensation: Clouds form in the atmosphere through condensation. Water that returns to the atmosphere through evapotranspiration will condense as it cools at high elevations. Condensed water vapor can be seen in the form of clouds and fog.
Precipitation: When molecules of water vapor completely saturate the air, clouds are formed. When this same body of air quickly cools, the vapor condenses, forming water drops which merge and grow until they are too big to be suspended in the atmosphere by air currents. Depending on the temperature of the air through which these water droplets are falling, they will return to Earth as rain, snow, sleet or hail.
Runoff: Runoff is the process of rain being funneled across the surface of Earth into streams, rivers and eventually lakes and oceans. Runoff can happen both on Earth's surface and underground. Not all water that hits Earth's surface will immediately become runoff. Some water soaks into the soil. This process is called infiltration. (Some teachers may prefer to present infiltration as a separate step in the water cycle.) Water can still move downhill under the soil. Water that soaks deep into the ground, penetrating underground rock layers, is called groundwater. Groundwater also flows downhill, although very slowly. It can take hundreds of thousands of years for groundwater to reach a lake or ocean!
Together these five processes make up the Hydrologic Cycle.
- Pour 3-4 tablespoons of sand and some water into each bag.
- Make the sand look like mountains and a valley.
- On the outside of the bag draw the new landscape with a black permanent marker.
- Tape the bags to the window in full sun.
- Come back a little later and notice the condensation.
They have just created a small world where the Hydrologic Cycle has occurred.
How does your small world differ from the real world? What does the Hydrology Cycle mean to living things on the Earth?
Included National Parks and other sites:
Utah Science Core:
4th Grade Standard 1 Objective 1,2
5th Grade Standard 1 Objective 2