The water cycle see Section 6.3 continually recycles water all around the globe, which means that the water we drink today was once used by our predecessors, in near time and very ancien
Trang 1Our planet is shrouded in water, and yet 8 million children under the age of five will die this year from lack of safe water The same irony will see 800 mil- lion people at risk from drought Two-thirds of the world's rural poor have
no access to safe drinking water, and while millions are made homeless from floods, hundreds of millions are coping with
6.1 WHERE IS EARTH'S WATER LOCATED?
W ATER continually moves around and above the earth as water vapor, liquid
water, and ice Moreover, water is continually changing its form The earth is almost a "closed system," like a terrarium That means that the earth, as a whole, neither gains nor loses much matter, including water The water cycle (see Section 6.3) continually recycles water all around the globe, which means that the water we drink today was once used by our predecessors, in near time and very ancient time.87
Water covers three-quarters of the earth's surface Most of this water is salt- water (about 97% of the earth's water) in the oceans, and less than 3% is fresh- water Of the latter, 77% is frozen in polar ice caps and glaciers, 22% is ground- water, and the remaining small fraction is in lakes, rivers, streams, plants, and animals
From Figure 6.1, it is apparent that we generally only make use of a tiny por- tion of the available water supplies The right-side pie shows that the vast ma- jority of the freshwater available for our uses is stored in the ground
Look at the data provided in Table 6.1 Surface-water sources (such as streams and rivers) only constitute about 300 cubic miles (about 1/10,00Oth of one percent) of the world's total water supply of about 326 million cubic miles
of water; yet streams and rivers are the source of most of the water used
8 6 ~ h a t Water Shortage? New York: United Nations Environment Program (UNEP), pp 1-5, undated
8 7 ~ h e r e is Earth's Water Located? USGS Water Resources: http://www.ga.usga.gov/edu/earthwherewater.html,
pp 1-2, February 2000
Trang 2STREAM WATER
Figure 6.1 Earth's water available for our uses, and the forms in which it exists (Source: USGS Wa- ter Resources, hperlman@usgs.gov, p 2,2000.)
Through history, the world's surface waters, i.e., lakes, streams, and rivers, have provided important resources and services (water available for human use), including water for drinking, washing, agriculture, energy production, transportation, recreation, and waste disposal
Humans cannot live without water Consider the 5-5-5 Rule-humans can live about five minutes without air, about five days without water, and about five weeks without food And, water has several uses All essential "biochemi- cal processes occur in water."88 Moreover, water has limitations It is very un- evenly distributed around the world Much of the Middle East, most of Africa, parts of Central America, and the western United States are already short of wa- ter.89 Unfortunately, water has been treated as an unlimited resource This atti-
TABLE 6.1 Earth's Water Supply
Water Volume, Percent of Water Source in Cubic Miles Total Water
Oceans
Ice caps, glaciers
Groundwater
Freshwater lakes
Inland seas
Soil moisture
Atmosphere
Rivers (streams)
Total water volume
Source: Where is Earth's Water Located? USGS Water Resources: http:llwww.ga.usgs.gov/edu/ earthwherewater.htm1, p 2, February 2000
88~radbury, I., The Biosphere New York: Belhaven Press, p 16, 1991
8 9 ~ o r l d ~esources 1986 New York: World Resources Institute (WRI) and International Institute of Environment and Development (IIED), pp 113-1 16, 1986
Trang 3TABLE 6.2 U.S Water Withdrawals per Day, 1940-1 985
I Year Billion Gallons
Source: U.S Bureau of Census (phone query, 1996)
tude, if continued, could lead to critical shortages in quantity and critical defi- ciencies in quality of available water
Another water limitation concerns usage; that is, the trend toward overusage For example, from Table 6.2, it can be seen that between 1950 and
1980, the amount of water drawn from lakes, rivers, streams, reservoirs, and underground aquifers in the United States increased by 150%, while the popu- lation increased by only half.90
Another water limitation is water quality Data compiled by WRI and IIED indicate that the world's lakes and rivers receive enormous quantities of munic- ipal sewage, industrial discharges, and surface runoff from agricultural areas
on a continuous basis If the discharge of pollutants into our groundwater and surface waters is not stopped, the survival of future generations is at risk.g1
To this point in this discussion, we have covered the fundamentals of ecol- ogy in limited and very basic form The intent has been to lay a foundation upon which the focus of this text, stream ecology and self-purification, can be built Beginning with an introduction to water and its importance, the rest of this dis- cussion focuses on the main topic area
Water is the life blood of the universe Living organisms, themselves about 70% water, depend upon water as a medium and reactant for biochemical reac- tions, for circulation, and for support Water covers about 75% of the earth's surface Life on the earth probably originated in water More than half of the world's animals and plant species live in water Most of our food is water: to- matoes (95%), spinach (91%), milk (go%), apples (85%), potatoes (go%), beef (61%), and hot dogs (56%) Water is at once simple and complex It is so com- plex that we can't define, beyond its chemical formula, what water really ismg2
goweber, S (Ed.), USA by Numbers Washington, DC: Zero Population Growth, p 17, 1988
9 ' ~ R I and IIED World Resources 1988-1989 New York: World Resources Institute (WRI) and International In-
stitute of Environment and Development (IIED), p 133, 1988
92~pellman, F R., The Science of Water: Concepts and Applications Lancaster, PA: Technomic Publishing Com- pany, Inc., p 13, 1998
Trang 468 STREAM WATER
Whether we can define it or not, however, humans know water not so much for what it is, but instead for its use(s) Water is used to maintain health and for industrial and home use, sanitation, agriculture, power production, recreation, and much else
Water is a stable molecule composed of one atom of oxygen and two atoms
of hydrogen Water is unique in that it is the only material found on earth in the three basic states at standard temperatures These states are liquid (water), solid (ice), and gas (water vapor) At sea level water vaporizes at 100°C (212°F) and freezes at 0°C (32°F) At 4°C (39.2"F), water is most dense In a body of water,
as water approaches this temperature during the spring and fall, a mixing action takes place whereby the denser water displaces water at lower levels This mix- ing action aerates the water and brings nutrients to the surface
6.3 WATER, OR HYDROLOGIC, CYCLE
The water, or hydrologic, cycle is shown in Figure 6.2 As illustrated, the water cycle depicts the ongoing natural circulation of water through the bio- sphere Water is taken from the earth's surface to the atmosphere by evapora- tion from the surface of lakes, rivers, streams, and oceans This evaporation process occurs when water is heated by the sun The sun's heat energizes sur- face molecules, allowing them to break free of the attractive force binding them together, and then evaporate and rise as invisible vapor in the atmosphere Wa-
ter vapor is also emitted from plant leaves by a process called transpiration Ev-
ery day, an actively growing plant transpires five to ten times as much water as
it can hold at once As water vapor rises, it cools and eventually condenses, usu- ally on tiny particles of dust in the air When it condenses, it becomes a liquid again or turns directly into a solid (ice, hail, or snow) These water particles then collect and form clouds The atmospheric water formed in clouds eventually falls to earth as precipitation Most precipitation falls in coastal areas or in high elevations Some of the water that falls in high elevations becomes runoff wa- ter, the water that runs over the ground (sometimes collecting nutrients from the soil) to lower elevations to form streams, lakes, and fertile valleys
The water that we see is known as suqace water Surface water can be bro-
ken down into five categories: oceans, lakes, rivers and streams, estuaries, and wetlands
6.4 STREAM WATER
As Allen points out, we all have an intuitive appreciation that flowing wa- ters, such as streams, contain a variety of dissolved and suspended constituents
(see Figure 6.3).93 Further, a stream is a carrier of those constituents it is ex-
posed to as it makes its inexorable journey over land to the sea
Trang 5Figure 6.3 Stream water constituents
Trang 670 STREAM WATER
Many factors influence the composition of stream water, causing variations from place to place Precipitation is, of course, one major source of chemical in- puts to streams Most streams contain much more suspended and dissolved ma- terials than found in rainwater, snow, or hail, however Ultimately, all of the constituents of stream water originate from dissolution of the earth's rocks The dissolving of rocks is commonly the major determinant of stream water chem- istry locally as well These "determinants" of stream water chemistry are actu- ally the stream's "DNA." More specifically, if we were to analyze a sample of stream water, at any given time or place, the dissolved and suspended constitu- ents found in the stream water sample would literally provide us with a sort of
"DNA" footprint, or signature, of where the stream has been It is this "DNA" footprint that allows us to determine and often pinpoint, in many cases, the ori- gin of point- and non-point source pollution
In our pursuit of presenting a logical discussion of stream ecology and self-purification, we share the view of Dr David M Rosenberg: "Chemical measurements are like taking snapshots of the ecosystem, whereas biological measurements are like making a videotape "94 Thus, in this discussion of
stream ecology and self-purification, it is the videotape (the biological) that we focus on
Hydrologic Cycle (Water Cycle)-involves the circulation of water be- tween the earth's surface and its atmosphere
world's water supply
6.3 The earth is almost a closed system Explain
94~osenberg, D M., "A National Aquatic ecosystem health program for Canada: We should go against the flow."
Bull Entomolo Soc Can., Winnipeg Freshwater Institute, 30(4):144-152, 1998
Trang 76.4 Explain the 5-5-5 rule
6.5 Humans know water not so much for what it is, but instead for its 6.6 Explain the hydrologic cycle
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