The Water Cycle
AKA The Hydrologic Cycle
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).
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).
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.
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.
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
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.
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).
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
