Water Cycle Reading
I. States of Matter& Intro to Hydrologic Cycle
There are 3 major states of matter- solid, liquid and gas.
Over 70% of Earth’s surface is covered by
water. Only 3% of the Earth’s water is freshwaterthe remaining water is found in the world’s oceans
as salt-water. Of the 3% of freshwater, 79% is
frozen in icecaps and glaciers, 20% is stored as
groundwater and 1% is found in bodies of water
(lakes, rivers, ponds etc.).
The hydrologic (water) cycle describes the
continuous movement of water on, above and below the
surface of the Earth. Water never leaves the Earth and
cannot be destroyed- it is cycled through the atmosphere,
ocean and land in different states (gas, liquid, solid). The
water cycle is driven by energy from the sun. The sun
heats water on earth, causing it to evaporate and move
throughout the water cycle. The water cycle consists of
evaporation, precipitation, transpiration, sublimation
condensation, runoff, and infiltration.
II. Water: Liquid to Gas
A. Evaporation describes when
water changes from a liquid to a gas
called water vapor. This occurs when the
sun beats down on sources of water
(lakes, ponds, oceans) on Earth to heat the
atoms up to change from a liquid to a gas,
where it enters the atmosphere as water
vapor. 90% of the moisture found in the
atmosphere is supplied by evaporation
from bodies of water. Factors that affect
evaporation include temperature, wind
and humidity. Increases in temperature
and wind also increase the rate of
evaporation. Humidity is the amount of water vapor present in the air. It is easier for water to
evaporate into dryer air than into more water saturated air.
B. Transpiration is the process of
evaporation from plants (kind of like plants
sweating). Evapotranspiration describes
when plants take water in through their roots
and eventually release the water as gaseous
water vapor through small openings called
stomata in their leaves. Evapotranspiration
accounts for about 10% of the water in the
atmosphere.
C. Respiration is a process used by animals to
breakdown food into energy and as a result, they release water
vapor from their lungs into the atmosphere. In humans, the water
vapor is visible when breathing when it is cold outside and when
we breathe on mirrored surfaces like glasses.
III. Water: Solid to Gas
Ordinarily, melting occurs when solid snow/ice changes into liquid water. Sublimation is
the process where water changes from a solid (snow/ice) directly into water vapor (gas) without
changing into liquid first. This occurs naturally at higher altitudes such as on Mt. Everest where
there is ample sunlight, high winds and low humidity.
IV. Water: Gas to Liquid
Condensation is the process where water change s from a gas to a liquid. Condensation occurs
in the atmosphere when warm air containing water vapor rises, cools and loses its capacity to
hold water vapor. As a result, excess water vapor condenses mixes with particles to form clouds.
Condensation is the opposite of evaporation. Condensation is also responsible for ground-level
fog and the water droplets on your cold drink on a hot d ay.
V. Water: Liquid to Liquid/Solid
Precipitation is the process of water changing from a liquid to a
liquid or from a liquid to a solid. Precipitation comes from clouds
in the atmosphere and returns water to the Earth’s surface in the
forms of rain, snow, sleet or hail. Freezing occurs when liquid
water changes into solid ice/snow/sleet.
Surface runoff is the process where water flows over the Earth’s surface from excess storm
water or melt water from melting snow/ice and eventually flows into nearby streams, river or
other larger bodies of water. Runoff serves to recharge the ground water when running slowly
enough that the ground can absorb it. Surface runoff is affected by the ground cover- how many
plants, trees and shrubs are present to slow down runoff, allowing the surface water to soak into
the ground. In areas with good
ground cover, there is little
runoff. In developed areas with
pavement, driveways and roads,
the runoff is near 100%. There
can also be runoff underground
from the groundwater- this is
called subsurface runoff and it
eventually leads to the ocean.
VI. Groundwater/Infiltration
Groundwater is the water found underground in the cracks and spaces in soil, sand and rock.
Groundwater is recharged from surface runoff infiltrating through the soil into the water table.
Infiltration is the process by which water on the surface enters into the soil. The amount of
water depends on soil size- the larger the soil particles, the more space water has to easily pass
through the soil. There are many terms associated with groundwater.
Soil Size
The zone of aeration is the area where the open spaces between soil particles are filled with
both air and water. We cannot pump water from this zone.
The zone of saturation is the area where the open spaces between soil particles are filled with
water only. The water in this zone is called groundwater and is available for pumping.
An aquifer is the area made up of all the soil, rock and water in the zone of saturation where
groundwater is stored. Water can easily move through aquifers because they have large spaces
that make the permeable (allowing objects to pass through it). Water in aquifers can be brought
to the surface through a spring or eventually discharged into lakes or streams.
The water table is the dividing line
between the zone of aeration and the
zone of saturation. The water table can
be a few feet below the surface or
hundreds of feet below ground. It can
rise or fall depending on heavy
rain/snowfall or excess pumping of
groundwater for human use.
Groundwater supplies drinking water
for 51% of the total U.S. population and
64%o of groundwater is used for
irrigation to grow crops.