Transcript Water 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 frequent 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.
• The oceans contain 97.5% of the earth's water, land
2.4%, and the atmosphere holds less than .001%.
• There is rapid recycling of water between the earth's surface
and the atmosphere.
Water Cycle Components
• Evaporation of water
• Condensation of water
• Transportation of water
• Water is transferred
from the surface to the
atmosphere through
evaporation - water
changes from a liquid
to a gas.
• Approximately 80% of
all evaporation is from
the oceans, with the
remaining 20% coming
from inland water and
vegetation.
• Winds transport the
evaporated water around
the globe, influencing the
humidity of the air
throughout the world.
Evaporation
On average, more water is evaporated from the ocean
than is returned by precipitation. More water is
precipitated over the land than evaporates. The
difference is returned to the ocean by rivers and
streams.
Most evaporated water exists as a gas outside of clouds and
evaporation is more intense in the presence of warmer
temperatures. This is shown in the image above, where the
strongest evaporation was occurring over the oceans and near the
equator (indicated by shades of red and yellow).
• 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
looses its capacity to
hold water vapor. As a
result, excess water vapor
condenses to form cloud
droplets.
• The upward motions that
generate clouds can be
produced by convection
in unstable air, lifting of
air by fronts and lifting
over elevated topography
such as mountains.
Condensation
Condensation by Convection
• Convection refers
primarily to atmospheric
motions in the vertical
direction. As the .earth is
heated by the sun, different
surfaces absorb different
amounts of energy and
convection may occur
where the surface heats up
very rapidly. As the surface
warms, it heats the
overlying air, which
gradually becomes less
dense than the surrounding
air and begins to rise.
The bubble of relatively
warm air that rises
upward from the surface
is called a "thermal".
• The water vapor within rising thermals condenses to
form a cloud.
Condensation in a Cold Front
• In the case of a cold
front, a colder,
denser air mass lifts
the warm, moist air
ahead of it. As the
air rises, it cools and
its moisture
condenses to
produce clouds and
precipitation. Due to
the steep slope of a
cold front, leads to
the development of
showers and
occasionally severe
thunderstorms.
Condensation in a Warm Front
• In a warm front, the warm, less
dense air rises up and over the
colder air ahead of the front.
Again, the air cools as it rises
and its moisture condenses to
produce clouds and
precipitation. Warm fronts have
a gentler slope and generally
move more slowly than cold
fronts, so the rising motion
along warm fronts is much more
gradual. Precipitation that
develops in advance of a surface
warm front is typically steady
and more widespread than
precipitation associated with a
cold front.
Condensation due to Topography
• Air is also lifted by the
earth itself. When air
encounters a mountain
range, for example, air
is forced to rise up and
over the mountains
and if enough lifting
occurs, water vapor
condenses to produce
clouds.
• In the hydrologic
Methods
cycle, transport is the
movement of water
through the
atmosphere,
specifically from over
the oceans to over
land. Some of the
earth's moisture
transport is visible as
clouds. Clouds are
propelled from one
place to another by
either the jet stream,
surface-based
circulations like land
and sea breezes.
of Transporting
• Most water is transported in
the form of water vapor,
which is actually the third
most abundant gas in the
atmosphere. Water vapor may
be invisible to us, but not to
satellites, which are capable
of collecting data about the
moisture content of the
atmosphere. From this data,
visualizations like this water
vapor image are generated,
providing a visual picture of
moisture transport in the
atmosphere.
Bright areas indicate higher
amounts of moisture, often
associated with clouds.
Dark areas indicate less
moisture (relatively drier
air). Moist air does not
always contain clouds.
• 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 are
rain, hail, snow, sleet,
and freezing rain.
Amounts of
precipitation will
vary by location.
Transport by Precipitation
• Groundwater is all the
Transport
water that has penetrated
the earth's surface and is
found in one of two soil
layers. Nearest the surface is
the "zone of aeration", where
gaps between soil 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 layers. The
water table rises or falls
according to the amount of
ground water. When the the
ground is saturated, flooding
occurs because all subsequent
precipitation is forced to
remain on the surface.
by Groundwater
.
• The rate at which water flows through the soil is its
"permeability". Different surfaces hold different amounts of
water and absorb water at different rates. Surface
permeability is extremely important in determining the
potential for flooding.
• Water that infiltrates the soil flows downward until it
encounters impermeable rock (shown in gray), and then
travels laterally. Groundwater returns to the surface
through lakes, rivers, and the oceans. The flow of
groundwater is much slower than runoff.
• Transpiration is the
Transport
evaporation of water into
the atmosphere from the
leaves and stems of
plants. Plants absorb soilwater through their roots.
Plants pump the water up
from the soil to deliver
nutrients to their leaves.
This pumping is driven by
the evaporation of water
through small pores called
"stomata", which are found
on the undersides of
leaves. Transpiration
accounts for
approximately 10% of all
evaporating water.
by Transpiration
• Runoff is the movement of
landwater 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.
Transport by Runoff
A Summary of the Hydrologic Cycle
The cycle begins with the evaporation of water from the surface. Moist
air is lifted, it cools and water vapor condenses to form clouds.
Moisture is transported and returns to the surface as precipitation. On
the ground water may; 1)evaporate back into the atmosphere 2) the
water may become groundwater or runoff. Groundwater and runoff
gather into the oceans, rivers, streams. Water is absorbed by plants
which then release it back into the atmosphere through transpiration.
Water evaporates from the surface to begin the cycle again.