DESCRIPTION
OF THE HYDROLOGIC CYCLE
The
scientific discipline in the field of physical geography that deals with the
water cycle is called hydrology. It is concerned with the origin, distribution,
and properties of water on the globe. Consequently, the water cycle is also
called the hydrologic cycle in many scientific textbooks and educational
materials. Most people have heard of the science of meteorology and many also
know about the science of oceanography because of the exposure that each
discipline has had on television. People watch TV weather personalities nearly
every day. Celebrities such as Jacques Cousteau have helped to make
oceanography a commonly recognized science. In a broad context, the sciences of
meteorology and oceanography describe parts of a series of global physical processes
involving water that are also major components of the science of hydrology.
Geologists describe another part of the physical processes by addressing
groundwater movement within the planet's subterranean features. Hydrologists
are interested in obtaining measurable information and knowledge about the
water cycle. Also important is the measurement of the amount of water involved
in the transitional stages that occur as the water moves from one process
within the cycle to other processes. Hydrology, therefore, is a broad science
that utilizes information from a wide range of other sciences and integrates
them to quantify the movement of water. The fundamental tools of hydrology are
based in supporting scientific techniques that originated in mathematics, physics,
engineering, chemistry, geology, and biology. Consequently, hydrology uses
developed concepts from the sciences of meteorology, climatology, oceanography,
geography, geology, glaciology, limnology (lakes), ecology, biology, agronomy,
forestry, and other sciences that specialize in other aspects of the physical,
chemical or biological environment. Hydrology, therefore, is one of the
interdisciplinary sciences that is the basis for water resources development
and water resources management.
The global water
cycle can be described with nine major physical processes which form a
continuum of water movement. Complex pathways include the passage of water from
the gaseous envelope around the planet called the atmosphere, through the
bodies of water on the surface of earth such as the oceans, glaciers and lakes,
and at the same time (or more slowly) passing through the soil and rock layers
underground. Later, the water is returned to the atmosphere. A fundamental
characteristic of the hydrologic cycle is that it has no beginning an it has no
end. It can be studied by starting at any of the following processes:
evaporation, condensation, precipitation, interception, infiltration,
percolation, transpiration, runoff, and storage.
The information presented below is a greatly
simplified description of the major contributing physical processes. They
include:
EVAPORATION
Evaporation occurs when the physical state of water is changed from a liquid state to a gaseous state. A considerable amount of heat, about 600 calories of energy for each gram of water, is exchanged during the change of state. Typically, solar radiation and other factors such as air temperature, vapor pressure, wind, and atmospheric pressure affect the amount of natural evaporation that takes place in any geographic area. Evaporation can occur on raindrops, and on free water surfaces such as seas and lakes. It can even occur from water settled on vegetation, soil, rocks and snow. There is also evaporation caused by human activities. Heated buildings experience evaporation of water settled on its surfaces. Evaporated moisture is lifted into the atmosphere from the ocean, land surfaces, and water bodies as water vapor. Some vapor always exists in the atmosphere.
Evaporation occurs when the physical state of water is changed from a liquid state to a gaseous state. A considerable amount of heat, about 600 calories of energy for each gram of water, is exchanged during the change of state. Typically, solar radiation and other factors such as air temperature, vapor pressure, wind, and atmospheric pressure affect the amount of natural evaporation that takes place in any geographic area. Evaporation can occur on raindrops, and on free water surfaces such as seas and lakes. It can even occur from water settled on vegetation, soil, rocks and snow. There is also evaporation caused by human activities. Heated buildings experience evaporation of water settled on its surfaces. Evaporated moisture is lifted into the atmosphere from the ocean, land surfaces, and water bodies as water vapor. Some vapor always exists in the atmosphere.
CONDENSATION
Condensation is the process by which water vapor changes it's physical state from a vapor, most commonly, to a liquid. Water vapor condenses onto small airborne particles to form dew, fog, or clouds. The most active particles that form clouds are sea salts, atmospheric ions caused by lightning,and combustion products containing sulfurous and nitrous acids. Condensation is brought about by cooling of the air or by increasing the amount of vapor in the air to its saturation point. When water vapor condenses back into a liquid state, the same large amount of heat ( 600 calories of energy per gram) that was needed to make it a vapor is released to the environment.
Condensation is the process by which water vapor changes it's physical state from a vapor, most commonly, to a liquid. Water vapor condenses onto small airborne particles to form dew, fog, or clouds. The most active particles that form clouds are sea salts, atmospheric ions caused by lightning,and combustion products containing sulfurous and nitrous acids. Condensation is brought about by cooling of the air or by increasing the amount of vapor in the air to its saturation point. When water vapor condenses back into a liquid state, the same large amount of heat ( 600 calories of energy per gram) that was needed to make it a vapor is released to the environment.
PRECIPITATION
Precipitation is the process that occurs when any and all forms of water particles fall from the atmosphere and reach the ground. There are two sub-processes that cause clouds to release precipitation, the coalescence process and the ice-crystal process. As water drops reach a critical size, the drop is exposed to gravity and frictional drag. A falling drop leaves a turbulent wake behind which allows smaller drops to fall faster and to be overtaken to join and combine with the lead drop. The other sub-process that can occur is the ice-crystal formation process. It occurs when ice develops in cold clouds or in cloud formations high in the atmosphere where freezing temperatures occur. When nearby water droplets approach the crystals some droplets evaporate and condense on the crystals. The crystals grow to a critical size and drop as snow or ice pellets. Sometimes, as the pellets fall through lower elevation air, they melt and change into raindrops.
Precipitation is the process that occurs when any and all forms of water particles fall from the atmosphere and reach the ground. There are two sub-processes that cause clouds to release precipitation, the coalescence process and the ice-crystal process. As water drops reach a critical size, the drop is exposed to gravity and frictional drag. A falling drop leaves a turbulent wake behind which allows smaller drops to fall faster and to be overtaken to join and combine with the lead drop. The other sub-process that can occur is the ice-crystal formation process. It occurs when ice develops in cold clouds or in cloud formations high in the atmosphere where freezing temperatures occur. When nearby water droplets approach the crystals some droplets evaporate and condense on the crystals. The crystals grow to a critical size and drop as snow or ice pellets. Sometimes, as the pellets fall through lower elevation air, they melt and change into raindrops.
Precipitated water
may fall into a waterbody or it may fall onto land. It is then dispersed
several ways. The water can adhere to objects on or near the planet surface or
it can be carried over and through the land into stream channels, or it may
penetrate into the soil, or it may be intercepted by plants.
When rainfall is
small and infrequent, a high percentage of precipitation is returned to the
atmosphere by evaporation.
The portion of precipitation that appears in surface
streams is called runoff. Runoff may consist of component contributions from
such sources as surface runoff, subsurface runoff, or ground water runoff.
Surface runoff travels over the ground surface and through surface channels to
leave a catchment area called a drainage basin or watershed. The portion of the
surface runoff that flows over the land surface towards the stream channels is
called overland flow. The total runoff confined in the stream channels is
called the streamflow.
INTERCEPTION
Interception is the process of interrupting the movement of water in the chain of transportation events leading to streams. The interception can take place by vegetal cover or depression storage in puddles and in land formations such as rills and furrows.
Interception is the process of interrupting the movement of water in the chain of transportation events leading to streams. The interception can take place by vegetal cover or depression storage in puddles and in land formations such as rills and furrows.
When rain first
begins, the water striking leaves and other organic materials spreads over the
surfaces in a thin layer or it collects at points or edges. When the maximum
surface storage capability on the surface of the material is exceeded, the
material stores additional water in growing drops along its edges. Eventually
the weight of the drops exceed the surface tension and water falls to the
ground. Wind and the impact of rain drops can also release the water from the
organic material. The water layer on organic surfaces and the drops of water
along the edges are also freely exposed to evaporation.
Additionally, interception of water on the ground
surface during freezing and sub-freezing conditions can be substantial. The
interception of falling snow and ice on vegetation also occurs. The highest
level of interception occurs when it snows on conifer forests and hardwood
forests that have not yet lost their leaves.
INFILTRATION
Infiltration is the physical process involving movement of water through the boundary area where the atmosphere interfaces with the soil. The surface phenomenon is governed by soil surface conditions. Water transfer is related to the porosity of the soil and the permeability of the soil profile. Typically, the infiltration rate depends on the puddling of the water at the soil surface by the impact of raindrops, the texture and structure of the soil, the initial soil moisture content, the decreasing water concentration as the water moves deeper into the soil filling of the pores in the soil matrices, changes in the soil composition, and to the swelling of the wetted soils that in turn close cracks in the soil.
Water that is infiltrated and stored in the soil can
also become the water that later is evapotranspired or becomes subsurface
runoff. Infiltration is the physical process involving movement of water through the boundary area where the atmosphere interfaces with the soil. The surface phenomenon is governed by soil surface conditions. Water transfer is related to the porosity of the soil and the permeability of the soil profile. Typically, the infiltration rate depends on the puddling of the water at the soil surface by the impact of raindrops, the texture and structure of the soil, the initial soil moisture content, the decreasing water concentration as the water moves deeper into the soil filling of the pores in the soil matrices, changes in the soil composition, and to the swelling of the wetted soils that in turn close cracks in the soil.
PERCOLATION
Percolation is the movement of water though the soil, and it's layers, by gravity and capillary forces. The prime moving force of groundwater is gravity. Water that is in the zone of aeration where air exists is called vadose water. Water that is in the zone of saturation is called groundwater. For all practical purposes, all groundwater originates as surface water. Once underground, the water is moved by gravity. The boundary that separates the vadose and the saturation zones is called the water table. Usually the direction of water movement is changed from downward and a horizontal component to the movement is added that is based on the geologic boundary conditions.
Percolation is the movement of water though the soil, and it's layers, by gravity and capillary forces. The prime moving force of groundwater is gravity. Water that is in the zone of aeration where air exists is called vadose water. Water that is in the zone of saturation is called groundwater. For all practical purposes, all groundwater originates as surface water. Once underground, the water is moved by gravity. The boundary that separates the vadose and the saturation zones is called the water table. Usually the direction of water movement is changed from downward and a horizontal component to the movement is added that is based on the geologic boundary conditions.
Geologic formations in the earth's crust serve as
natural subterranean reservoirs for storing water. Others can also serve as
conduits for the movement of water. Essentially, all groundwater is in motion.
Some of it, however, moves extremely slowly. A geologic formation which
transmits water from one location to another in sufficient quantity for
economic development is called an aquifer. The movement of water is possible
because of the voids or pores in the geologic formations. Some formations
conduct water back to the ground surface. A spring is a place where the water
table reaches the ground surface. Stream channels can be in contact with an
unconfined aquifer that approach the ground surface. Water may move from the
ground into the stream, or visa versa, depending on the relative water level.
Groundwater discharges into a stream forms the base flow of the stream during
dry periods, especially during droughts. An influent stream supplies water to
an aquifer while and effluent stream receives water from the aquifer.
TRANSPIRATION
Transpiration is the biological process that occurs mostly in the day. Water inside of plants is transferred from the plant to the atmosphere as water vapor through numerous individual leave openings. Plants transpire to move nutrients to the upper portion of the plants and to cool the leaves exposed to the sun. Leaves undergoing rapid transpiration can be significantly cooler than the surrounding air. Transpiration is greatly affected by the species of plants that are in the soil and it is strongly affected by the amount of light to which the plants are exposed. Water can be transpired freely by plants until a water deficit develops in the plant and it water-releasing cells (stomata) begin to close. Transpiration then continues at a must slower rate. Only a small portion of the water that plants absorb are retained in the plants.
Transpiration is the biological process that occurs mostly in the day. Water inside of plants is transferred from the plant to the atmosphere as water vapor through numerous individual leave openings. Plants transpire to move nutrients to the upper portion of the plants and to cool the leaves exposed to the sun. Leaves undergoing rapid transpiration can be significantly cooler than the surrounding air. Transpiration is greatly affected by the species of plants that are in the soil and it is strongly affected by the amount of light to which the plants are exposed. Water can be transpired freely by plants until a water deficit develops in the plant and it water-releasing cells (stomata) begin to close. Transpiration then continues at a must slower rate. Only a small portion of the water that plants absorb are retained in the plants.
Vegetation generally retards evaporation from the
soil. Vegetation that is shading the soil, reduces the wind velocity. Also,
releasing water vapor to the atmosphere reduces the amount of direct
evaporation from the soil or from snow or ice cover. The absorption of water
into plant roots, along with interception that occurs on plant surfaces offsets
the general effects that vegetation has in retarding evaporation from the soil.
The forest vegetation tends to have more moisture than the soil beneath the
trees.
RUNOFF
Runoff is flow from a drainage basin or watershed that appears in surface streams. It generally consists of the flow that is unaffected by artificial diversions, storages or other works that society might have on or in a stream channel. The flow is made up partly of precipitation that falls directly on the stream , surface runoff that flows over the land surface and through channels, subsurface runoff that infiltrates the surface soils and moves laterally towards the stream, and groundwater runoff from deep percolation through the soil horizons. Part of the subsurface flow enters the stream quickly, while the remaining portion may take a longer period before joining the water in the stream. When each of the component flows enter the stream, they form the total runoff. The total runoff in the stream channels is called streamflow and it is generally regarded as direct runoff or base flow.
Runoff is flow from a drainage basin or watershed that appears in surface streams. It generally consists of the flow that is unaffected by artificial diversions, storages or other works that society might have on or in a stream channel. The flow is made up partly of precipitation that falls directly on the stream , surface runoff that flows over the land surface and through channels, subsurface runoff that infiltrates the surface soils and moves laterally towards the stream, and groundwater runoff from deep percolation through the soil horizons. Part of the subsurface flow enters the stream quickly, while the remaining portion may take a longer period before joining the water in the stream. When each of the component flows enter the stream, they form the total runoff. The total runoff in the stream channels is called streamflow and it is generally regarded as direct runoff or base flow.
STORAGE
There are three basic locations of water storage that occur in the planetary water cycle. Water is stored in the atmosphere; water is stored on the surface of the earth, and water stored in the ground.
There are three basic locations of water storage that occur in the planetary water cycle. Water is stored in the atmosphere; water is stored on the surface of the earth, and water stored in the ground.
Water stored in the atmosphere can be moved relatively
quickly from one part of the planet to another part of the planet. The type of
storage that occurs on the land surface and under the ground largely depend on
the geologic features related to the types of soil and the types of rocks
present at the storage locations. Storage occurs as surface storage in oceans,
lakes, reservoirs, and glaciers; underground storage occurs in the soil, in
aquifers, and in the crevices of rock formations.
The movement of
water through the eight other major physical processes of the water cycle can
be erratic. On average, water the atmosphere is renewed every 16 days. Soil
moisture is replaced about every year. Globally, waters in wetlands are
replaced about every 5 years while the residence time of lake water is about 17
years. In areas of low development by society, groundwater renewal can exceed
1,400 years. The uneven distribution and movement of water over time, and the
spatial distribution of water in both geographic and geologic areas, can cause
extreme phenomena such as floods and droughts to occur.
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