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Water Cycle (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 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
Evaporation
•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.
On the 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 near the equator, where the strongest evaporation is occurring
over the oceans.
Condensation
•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 by Convection
•Convection refers primarily to atmospheric motions in the vertical direction. As the sun
heats the earth, 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 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. 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.
Methods of Transporting
•In the hydrologic 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. Either the jet stream or surface-based circulations like land and sea
breezes propel clouds from one place to another.
•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.
Transport by Precipitation
•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 Groundwater
•Groundwater is all the 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 ground is saturated, flooding occurs because all subsequent precipitation is
forced to remain on the surface.
•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.
Transport by Transpiration
•Transpiration is the 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.
Transport by Runoff
•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.
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, and 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.