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Chapter 14 – FRESHWATER RESOURCES
Of all the water on the planet, only 2.5% is considered freshwater. The hydrologic cycle interacts with all other biogeochemical cycles, and the hydrosphere is intimately linked with the biosphere, lithosphere, and atmosphere. Water is unevenly distributed in time as well as space, and one challenge for human societies has been to transport freshwater from where it occurs to where it is needed. Massive projects that transfer water between major watersheds are effectively "replumbing the planet'. We are depleting many sources of surface water and groundwater, so that almost 2 billion people live in water-scarce areas. Because aquifers recharge very slowly or not at all, groundwater is at a greater risk of depletion than surface water. Water withdrawal is occurring much more rapidly than aquifers can be replenished, and the amounts being withdrawn are increasing every year. To address the problem of depletion, we can try to increase the supply of water or to reduce the demand for it. Although supply-side solutions are worth pursuing, we are better off looking for ways to reduce demand. Safeguarding the quality of water involves another batch of environmental and human health dilemmas. Pollution can be categorized into several types, and includes biological, natural and artificial chemical, physical, and thermal pollution. Although a change ill our attitudes, behavior, or technology may be necessary to clean water completely, there are many things ordinary people can do to help minimize freshwater pollution. Citizens can decrease pollution through purchasing "environmentally friendly" products and becoming involved in protecting local waterways.
Plumbing the Colorado River
On January 1, 2003, the U.S. government cut off 15% of California's water supply from the Colorado River that serves 1.6 million households in Los Angeles and San Diego.
The Colorado River provides water for irrigation of the desert, drives one of the world's biggest economies, and allows southern California as we know it to exist.
The Colorado River begins in the Rocky Mountains, crosses the border into Mexico, and dumps into the Sea of Cortez.
Its raging waters once created the Grand Canyon, but modern, massive dams have been erected to manage the flow and irrigate farm fields.
Today, the waters of the Colorado River irrigate cropland, serve over 20 million people, water golf courses, and fill swimming pools.
For 80 years, Colorado, Wyoming, Utah, Nevada, New Mexico, Arizona, and California divided the river's water among themselves, guided by the Colorado River Compact of 1922.
California has long exceeded its allotted annual amount of 4.4 million acre-feet (an acre-foot is the volume of water that would cover one acre to a depth of one foot).
However, the other states' populations are booming, and they will soon need their full shares of water, which means that California will have to decrease what it uses.
Although California worked hard to get agricultural districts, which control most of the water distribution in the state-to agree to sell part of their shares, at the last minute the Imperial Irrigation District in California refused to sell its water to San Diego.
When the water was shut off in 2003, the irrigation district sued the government, the rest of California was angry with the irrigation district, and the Interior Department would not renew the water deliveries until the state got its house in order.
Fresh water’s Movement and Distribution on Earth
Water may seem abundant to us, but in the global perspective it is quite rare and limited, because only 2.5% is considered freshwater, water that is relatively pure and contains few dissolved salts.
Only 1 % of the freshwater resides in surface reservoirs such as lakes, soil, and atmospheric moisture, while the rest is tied up in glaciers, ice caps, and underground aquifers.
Only half of this 1 % is in lakes, rivers, and streams.
Water moves in the hydrologic cycle
The hydrologic, or water, cycle interacts with all other biogeochemical cycles, and the hydrosphere is intimately linked with the biosphere, lithosphere, and atmosphere.
As water moves it redistributes heat, erodes mountain ranges, builds river deltas, maintains organisms and ecosystems, shapes civilizations, and gives rise to political conflicts.
Many rivers originate high in the mountains, from snowmelt and glaciers in the high peaks that run downhill in the spring.
Mountain slopes receive precipitation when warm moist winds on the upwind side of the mountain range are forced upward by the rugged topography.
As the air rises into regions of lower pressure, it expands, cools, and increases in relative humidity, forming clouds and creating rain or snow. As the air mass passes over the crest of the mountains and sinks down the opposite slope (the downwind side of the mountain range), it contains less moisture and lower relative humidity, and temperature and pressure increase, forming a rain shadow, an arid area on the downwind side of a mountain.
The rain, snow, and glacial ice on the slopes of mountains create thousands of rivulets and creeks that flow downhill into small rivers, which flow into larger rivers, which in turn join the world's largest rivers.
Some water is held in alpine lakes and wet meadows, or in marshes at lower elevations, but water also infiltrates into the soil, reaching the water table, or is taken up by the roots of plants, and then transpired through their leaves, returning to the atmosphere.
Water in the world's mightiest rivers used to rage downstream through ferocious rapids, but modern gigantic dams create huge lakes and remove water by pipes and canals for drinking supplies, industrial use, and agricultural irrigation.
In the past, rivers used to flow into great estuaries, but today the mighty rivers have been turned into small streams that barely reach the sea.
As ocean water evaporates into the atmosphere, water vapor gets blown inland, eventually being pushed up the west slope of mountains, to fall again as precipitation.
Ground Water Plays a Key Role in the Hydrologic Cycle
Any precipitation reaching Earth's land surface that does not evaporate, flow into waterways, or get taken up by organisms infiltrates the surface, percolates into the soil layers, and moves downward to become ground- water.
Aquifers are porous, spongelike layers of rock, sand, or gravel that can hold water.
An aquifer's upper layer, or zone of aeration, has open spaces that contain both water and air.
In the lower layer, or zone of saturation, the spaces contain nearly all water.
The boundary between these two zones is the water table.
An aquifer recharge zone is a geographic area where water infiltrates Earth's surface and reaches an aquifer below.
In a confined, or artesian, aquifer, a water-bearing porous layer of rock, sand, or gravel is trapped between an upper and a lower layer of less permeable substrate, and water may be under great pressure because it is trapped between these layers.
Unconfined aquifers are underlain by a layer of less permeable substrate, but are not confined by an upper one, and the water is under much less pressure than in confined aquifers.
Groundwater becomes surface water through springs and human-drilled wells.
Groundwater flows, very slowly, downhill and from areas of high pressure to areas of low pressure but may remain in an aquifer for thousands of years.
Each day in the United States aquifers release 492 billion gallons of groundwater into bodies of surface water, almost as much as the daily flow of the Mississippi River.
The world's largest known aquifer, the Ogallala Aquifer, which underlies the Great Plains of the United States, is 1,200 feet deep at its thickest point.
Water is unequally distributed across Earth's surface
Different regions around the world possess vastly different amounts of groundwater, surface water, and precipitation.
Precipitation ranges from about 470 inches per year in Hawaii to almost zero in the Atacama Desert of Chile.
Since people are not distributed across the globe according to water avail- ability, areas that have lots of people are often water poor, leading to inequalities in per capita water resources; for example, Canada has 20 times more water for each of its citizens than does China.
One challenge for human societies has been to transport freshwater from where it occurs to where it is needed.
The natural distribution of freshwater is uneven across time as well as space, because rain does not always fall when humans need it, and in some countries, storms bring half the country's annual rain during just a few hours, but not when the crops need it.
Humans have constructed dams to store water, to make up for this . unequal timing of natural distribution.
Freshwater Ecosystems
Although bodies of freshwater cover only a tiny portion of Earth, they host a huge amount of living organisms.
There are several aquatic ecosystems, which are the equivalent of terrestrial biomes.
Rivers and streams wind through the landscape
Surface water, which consists of bodies of actively flowing water, is from rain, snowmelt, or springs and runs downhill into small rivulets.
Eventually, the rivulets flow together to form streams, creeks, or brooks.
The larvae of many insects, as well as aquatic insects such as water striders, live in streams.
The speed of the flow of water determines the types of plants and animals that live in streams.
Streams also provide habitat for fish, and drinking and bathing water for terrestrial animals.
Streams join one another to form larger rivers that eventually flow into the ocean.
The area of land drained by a river and all its tributaries (a smaller river that flows into a larger one) is called the river's watershed.
Because of the force of moving water, rivers can shape the landscape, eroding soil around the outside shore of a bend and depositing soil on the inner shore of a bend.
An oxbow lake, an isolated V-shaped water body, results when a bend becomes so extreme that the bend is cut off from the river.
A floodplain, the flat region where a river moves over a large area, contains fertile soils deposited by the river.
Lakes and ponds change with time
Ponds are small bodies of water that do not actively flow, while lakes are larger bodies of nonflowing water.
The depth of ponds and lakes varies widely, and the organisms within them vary with depth.
Shallow waters allow sunlight to pass through so plants can photosynthesize, while deeper water is cooler and lacks plants.
Clear water allows sunlight to penetrate, while turbid water limits sunlight penetration.
The concentration of dissolved oxygen depends on the amount of photosynthesis and organismal respiration.
If too many nutrients flow into a lake or pond, eutrophication may result, in which plant material is increased and dissolved oxygen is decreased. Oligotrophic lakes contain few nutrients and high oxygen conditions, while eutrophic lakes contain high nutrients and low oxygen conditions.
Inland seas are the largest freshwater bodies
The largest lakes, called seas, may take many days to cross by boat and contain biota that are adapted to open water.
The Great Lakes contain lake sturgeon, bass, and walleye.
Wetlands include marshes, swamps, and bogs
Wetlands are ecosystems that combine elements of freshwater and terrestrial systems.
Freshwater marshes are shallow enough so that plants can grow from the bottom and rise above the water's surface.
Swamps are also shallow areas rich in vegetation but they occur more in forested areas.
Bogs are ponds that are covered with thick, floating mats of vegetation.
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All wetlands are extremely valuable to wildlife but have been extensively drained for agriculture.
How We Use Water
Humans use freshwater for drinking, washing, recreation, growing food, powering industry, generating electricity, disposing of waste, and for countless other purposes.
It is estimated that 60% of the world's largest 227 rivers have been strongly or moderately affected ,by artificial dams, canals, and diversions.
Water supplies our households, agriculture, and industry
Humans use water for drinking, cooking, and cleaning, irrigating crops, watering livestock, and in manufacturing and industrial processes.
Globally, most freshwater is used for irrigation and other agricultural purposes.
Consumptive use of water, such as irrigation and industrial and residential uses, removes water from an aquifer or surface water body and does not return it.
Nonconsumptive use of water does not remove, or only temporarily removes, water from an aquifer or surface water body, such as using water to generate electricity in hydroelectric dams by, passing it through the dam machinery to turn turbines.
Agriculture accounts for 87% of all consumptive use of fresh water particularly by irrigation
Proper irrigation can more than double crop yields because farmers can control the application, of water when and where it is needed.
However, when water becomes scarce and more valuable, economic considerations favor using it in industry instead of agriculture, where water is 70 times more valuable.
We extract water from aquifers
One-third of Earth's human population relies directly on groundwater for its water needs.
Groundwater is obtained from local village wells in developing countries and from electrically powered personal wells in developed countries.
Urban and suburban homes and businesses receive water from municipal governments.
Today, extraction of water from aquifers is increasing as developing countries boost their agricultural productivity.
We divert surface water to suit our needs
Where surface water is available, it is very easy to move water from rivers, streams, lakes, and ponds to farm fields and houses.
Archaeological evidence shows that many ancient civilizations depended on complex irrigation systems.
More large-scale diversion projects are in store, such as the proposed aqueduct that will bring water from southern China to the arid, highly populated northern parts of China.
We try to control floods with dikes and levees
Preventing floods is a major reason humans control the movement of freshwater.
Towns, cities, and farms are built on the banks of rivers because of the water supply, flat topography, and fertile soil.