The Hydrologic Cycle
Water is the source of all life on earth. The distribution of water, however, is quite varied; many
locations have plenty of it while others have very little. Water exists on earth as a solid (ice),
liquid or gas (water vapor). Oceans, rivers, clouds, and rain, all of which contain water, are in
a constant state of change (surface water evaporates, cloud water precipitates, rainfall
infiltrates the ground, etc.). However, the total amount of the earth's water does not change.
The circulation and conservation of earth's water is called the "hydrologic cycle".
The Earth's Water Budget
Water covers 70% of the earth's surface, but it is difficult to comprehend the total amount of
water when we only see a small portion of it. The following diagram displays the volumes of
water contained on land, in oceans, and in the atmosphere. Arrows indicate the annual
exchange of water between these storages.
Diagram adapted from: Peixoto and Kettani (1973)
The oceans contain 97.5% of the earth's water, land
2.4%, and the atmosphere holds less than .001%, which
may seem surprising because water plays such an
important role in weather. The annual precipitation for
the earth is more than 30 times the atmosphere's total
capacity to hold water. This fact indicates the rapid
recycling of water that must occur between the
earth's surface and the atmosphere.
To visualize the amount of water contained in these
storages, imagine that the entire amount of the earth's
annual precipitation fell upon the state Texas. If this
was to occur, every square inch of that state would be
under 1,841 feet, or 0.3 miles of water!
Water Transfer Through the Hydrologic Cycle
Water is transferred from the surface to the atmosphere through evaporation, the process by
which water changes from a liquid to a gas. Approximately 80% of all evaporation is from the
oceans, with the other 20% coming from inland water and vegetation.
Water in the atmosphere is transported around the globe, mostly from over the oceans to over
land. Some of the earth's moisture transport is visible as clouds, which themselves consist of ice
crystals and/or tiny water droplets. Clouds are propelled from one place to another by either
the jet stream, surface-based circulations like land - sea breezes, or other mechanisms. Most
water is transported in the form of water vapor, which is actually the third most abundant gas
in the atmosphere.
As winds transport evaporated water around the globe, it influences the humidity of the air
throughout the world. For example, a typical hot and humid summer day in the Midwestern
United States is caused by winds blowing tropical oceanic air northward from the Gulf of
Mexico.
Condensation is the change of water from its gaseous form (water vapor) into liquid water.
Condensation generally occurs in the atmosphere when warm air rises, cools and loses its
capacity to hold water vapor. As a result, excess water vapor condenses to form cloud
droplets.
Precipitation is the primary mechanism for transporting water from the atmosphere to the
surface of the earth. There are several forms of precipitation, the most common of which for
the United States is rain. Other forms of precipitation include; hail, snow, sleet, and freezing
rain. A well-developed cyclone could be responsible for the generation of any or all of these
forms of precipitation.
Runoff is the movement of water on the land to the oceans, chiefly in the form of rivers, lakes,
and streams. Runoff consists of precipitation that neither evaporates, transpires nor penetrates
the surface to become groundwater. Even the smallest streams are connected to larger rivers
that carry billions of gallons of water into oceans worldwide. Excess runoff can lead to
flooding, which occurs when there is too much precipitation.
Groundwater is all the water that has penetrated the earth's surface and is found in one of
two soil layers. The one nearest the surface is the "unsaturated zone", where gaps between soil
particles are filled with both air and water. Below this layer is the "zone of saturation", where
the gaps are filled with water. The water
table is the boundary between these two
Unsaturated
layers. As the amount of groundwater
Zone
water increases or decreases, the water
table rises or falls accordingly. When the
entire area below the ground is
saturated, flooding occurs because all
subsequent precipitation is forced to
remain on the surface.
The amount of water that can be held in the soil is called "porosity". How easily water flows
through the soil is its "permeability". The faster and more easily water can flow through the soil,
the higher its permeability. Soils hold different amounts of water and absorb water at different
rates depending on the types of materials (ex. sand, silt or clay) in the soils. Surface
permeability is extremely important for hydrologists to monitor because as a surface becomes
less permeable, an increasing amount of water remains on the surface, creating a greater
potential for flooding. Flooding is very common in southern Arizona during a monsoon
because the surface permeability is overwhelmed by the amount of rainfall, causing most
rainwater to become runoff.
Water that infiltrates the soil flows
downward until it encounters
impermeable rock (bedrock), and then
travels laterally. The layers where water is
stored underground and moves laterally
are called "aquifers". Groundwater
Groundwater flow
returns to the surface through these
aquifers, which empty into lakes, rivers,
and the oceans. Under special
Impermeable layer
circumstances, groundwater can even
flow upward in artesian wells. The flow of groundwater is much slower than runoff, with speeds
usually measured in centimeters per day, or even centimeters per year.
This text reading was adapted from an online guide to the hydrologic cycle from the University
of Illinois found at: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/hyd/home.rxml.
Questions: Hydrologic Cycle Reading
Score: _______/20
_____1. Evaporation
_____2. Condensation
_____3. Precipitation
_____4. Runoff
_____5. Groundwater
_____6. Water table
Name:_____________________
Date:___________ Period:____
A. Transfer of water from the atmosphere to Earth’s surface.
B. Transport of water over Earth’s surface into rivers and then to oceans.
C. Boundary between the saturated and unsaturated zones.
D. Transfer of water from Earth’s surface to the atmosphere.
E. Water that has infiltrated the subsurface and flows underground.
F. Droplets of water that form clouds as the air temperature cools.
____7. Moisture in the atmosphere is transported from one region to another by
A. Precipitation
B. Winds
C. Evaporation
D. Runoff
____8. The water table in Tucson is typically at depths of more than 50 ft. The surface
soil (down to a depth of ~50 ft.) is part of the
A. Saturated zone
B. Water table
C. Groundwater flow
D. Unsaturated zone
____9. Soil that allows water to easily flow through the empty spaces between
individual grains has
A. High porosity
B. Low porosity
C. High permeability
D. Low permeability
10. USING YOUR OWN WORDS, explain why flooding is common during a monsoon in
Tucson.
11 – 16. Place the letter for the correct term on the diagram below:
A.
B.
C.
D.
E.
F.
Evaporation
Condensation
Precipitation
Run-off
Groundwater
Infiltration