The Water Cycle

1. The Water Cycle AKA The Hydrologic Cycle

3. Water • 3 states • Solid • Liquid • Gas • The 3 states of water are determined mostly by temperature. • Even though water is constantly changing states, the total amount of water on Earth remains constant. • Over 70% of the Earth’s surface is covered by oceans • Water is constantly being cycled between the atmosphere (air), hydrosphere (water), and lithosphere (land).

4. Evaporation • The sun (temperature) is the energy force that powers the water cycle • It heats oceans, lakes, rivers and causes water to change from the liquid state to the gaseous state • The oceans contribute to about 80-90% of the water vapor in the atmosphere. • During evaporation, the impurities (for example, Salt) are left behind. • This is important because about 97% of the water on Earth is salt water (oceans) and only 3% is freshwater (rivers, streams, lakes, ponds, and in the ground).

5. Condensation • When atmospheric temperature decreases, the water vapor (gas) changes back into a liquid. • Condensation is the opposite of evaporation. • Small water droplets are formed in the atmosphere. • Collections of water droplets form clouds in the sky or fog at ground level. • You can see condensation on drinks in the summertime or leaves in the morning.

6. Precipitation • Tiny water droplets bounce around in a cloud and as they hit each other, they stick together and become larger. • The clouds get heavy and eventually water falls back to the Earth. • Precipitation can occur in the form of rain, freezing rain, sleet, snow, or hail. • Most precipitation falls back into the oceans or onto land. If precipitation falls in the form of snow it can accumulate in the form of ice caps or glaciers. • Most of the condensed water in clouds does not actually fall as precipitation.

7. Surface Runoff • About 1/3 of the water that returns to the Earth as precipitation runs off the surface of the land, down hill, into streams, rivers, lakes, and oceans. • The other 2/3 of precipitation is evaporated, transpired, or is infiltrated into ground water. • Surface Runoff is a very important part of the water cycle because it returns water once again to the bodies of water, where evaporation occurs. • For example, when snow melts

8. Infiltration • Not all surface runoff water flows back into streams, rivers, lakes, and oceans. Some of it soaks into the ground. • Infiltration is the downward movement of water from the land surface into soil or underlying rock layers. • This water can replenish aquifers, which store large amounts of freshwater that can be removed from the ground using a water well. • Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge. • Some groundwater finds openings in the land surface and comes out as freshwater springs.

9. Transpiration • Water is returned to the atmosphere by plants. • Water is absorbed by plants (usually through the roots) from water that is in the soil. • The water travels up through the plant and then is evaporated back into the atmosphere from the plant surface (usually the leaves).

10. Sublimation • Sublimation is the conversion between the solid and gaseous form of water, with no intermediate liquid stage. • This occurs when there is low atmospheric pressure. • An example of this is when snow and ice change into water vapor in the air without first melting into water. Phase Diagram for Water