1. Chapter 19: Water Use and Management
In this chapter, the following topics will be covered:
The hydrologic cycle and how it delivers fresh water to terrestrial ecosystems
The volume and residence time of water in the earth’s major compartments
The ways we use water (withdrawal, consumption and degradation)
The consequences of water shortages
Managing water demand

2. Water Resources
Water is essential for life and is the medium in which all living
processes (including nutrient transportation, temperature regulation, and waste disposal) occur.
As far as we know, earth is the only place in the universe where
liquid water exists in great quantities.
Water covers about 70% of the earth's surface.

3. The Hydrologic Cycle
The hydrologic cycle describes the circulation of water as it evaporates from land, water, and
organisms; enters the atmosphere; condenses and precipitates; and moves into larger bodies of water.
Plants play an important role in the hydrologic process, absorbing groundwater and pumping
it into the atmosphere by transpiration.
Evaporation is the process in which a liquid is changed to vapor at temperatures well below
its boiling point.
Sublimation is the process by which water moves from solid to gaseous phase without ever
becoming a liquid.
When a volume of air contains as much water vapor as it can at a given temperature, it has
reached its saturation point.
Relative humidity is the amount of water vapor in the air expressed as a percentage of the
saturation point.
Condensation occurs when the saturation point is exceeded.
For a given amount of water vapor, the temperature at which condensation occurs is the dew
point.

5. Table 19.1 -- Some units of water measurement
One cubic kilometer (km3) equals one billion cubic meters (m3),
one trillion liters, or 264 billion gallons.
One acre-foot is the amount of water required to cover an acre of
ground one foot deep. This is equivalent to 325,821 gallons, or 1.2 million liters, or 1,234 m3, about the amount consumed annually by a family of four in the United States.
Once cubic foot per second of river flow equals 28.3 liters per
second or 449 gallons per minute.

6. Rainfall and Topography
Rain falls unevenly over the planet so some places receive more than
others.
Very heavy rainfall is typical of tropical areas, especially where
monsoon winds carry moisture-laden sea air onshore.
Mountains act as cloud formers and rain catchers. As air sweeps up
one side of a mountain, it cools and condensation and precipitation occur. As the drier air sweeps down the other side, it warms, reducing its relative humidity and preventing rainfall. This creates a rain shadow on the drier side.

9. Desert Belts
Rising and falling air masses that result from global circulation
patterns also help create deserts in two broad belts on either side of the equator around the world.
Humans and domestic animals have expanded many of these deserts
by destroying forests and stripping away protective vegetation.
Balancing the Water Budget
The global water budget is balanced by circulation systems on land,
in the atmosphere, and in the oceans that move water from areas of excess to areas of deficit.
The redistribution of heat that results from the massive evaporation,
precipitation, and transport of water is a major factor in keeping world temperatures relatively constant and making the world habitable.

10. Major Water Compartments
The distribution of water is described in terms of interacting
compartments in which water resides for short or long times.

11. Table Earth's water compartments-estimated volume of water in storage, percent of total, and average residence time.
Category Volume % Total Water Average Residence Time (thousands of km3)
Total 1,403, ,800 years
Ocean 1,370, ,000 to 30,000 years
Ice and snow 29, to 16,000 years
Groundwater down to 1 km 4, From days to thousands of years
Lakes and reservoirs to 100 years
Saline lakes to 1,000 years
Soil moisture weeks to 1 year
Biological moisture in plants week
and animals
Atmosphere to 10 days
Swamps and marshes From months to years
Rivers and streams to 30 days

12. Oceans
The oceans contain more than 97% of all the liquid water in the
world.
Over 90% of the world's living biomass resides in the oceans.
The average residence time of water in the ocean (the length of time
that an individual molecule spends circulating in the ocean before it evaporates and starts through the hydrolic cycle again) is about three thousand years.
Glaciers, Ice, and Snow
Nearly 90% of all freshwater on earth is tied up in glaciers, ice caps,
and snow fields.
The Antarctic glaciers (largest in the world) contain nearly 85% of
all the ice in the world.
There is no landmass at the North Pole, only a permanent sheet of
ice.

13. Groundwater
After glaciers, the next largest reservoir of freshwater is
groundwater.
Precipitation that does not evaporate percolates through the soil and
into permeable rock in a process called infiltration.
Lower soil layers where all spaces are filled with water make up the
zone of saturation. The top of this zone is the water table.
Porous layers of sand, gravel, and rock below the water table are
called aquifers.
Areas in which infiltration of water into an aquifer occurs are called
recharge zones.

16. Rivers and Streams
Precipitation that does not evaporate or inflitrate into the ground runs
off over the surface back to the sea.
Rivulets join to form streams, and streams join to form rivers.
These surface waters are vitally important to humans and most other
organisms.
Discharge is the amount of water in a river that passes a fixed point
in a given amount of time. This is the best measure of the volume of a river.

17. Lakes and Ponds
Ponds are generally considered to be small bodies of water shallow
enough for plants to grow over most of the bottom.
Lakes are inland depressions that hold standing water year round.
Lakes and ponds are also vitally important for many organisms.
Wetlands
Bogs, swamps, wet meadows, and marshes play a vital role in the
hydrologic cycle. Their plant growth holds back surface runoff, allowing for infiltration to take place.
In the U.S., about 20% of its area was once wetlands. Over the past
200 years, approximately half of those wetlands have been drained or degraded.
The Atmosphere
The atmosphere is among the smallest of the major water reservoirs
in terms of water volume, containing less than 0.001% of the total water supply.
Residence time of a molecule of water in the atmosphere is
approximately 10 days.
The atmosphere provides the mechanism for distributing water over
the landmasses.

18. Water Availability and Use
Clean, fresh water is essential for nearly every human endeavor.
Renewable water supplies are made up of surface runoff and
infiltration into accessible aquifers.
Water-rich and Water-poor Countries
South America, West Central Africa, and South and Southeast Asia
all have areas of very high rainfall.
The highest per capita water supplies generally occur in areas with
moist climates and low population densities, such as Iceland.

19. Drought Cycles
Rainfall is never uniform in either geographical distribution of yearly
amount.
Cycles of wet/dry season occur in many areas. In the United States,
the cycle of drought appears to be every 30 years.

20. Types of Water Use
Withdrawal is the total amount of water taken from a lake, river, or
aquifer for any purpose.
Consumption is the fraction of withdrawn water that is lost in
transmission, evaporation, absorption, chemical transformation, or otherwise made unavailable for other purposes as a result of human use.
Degradation is a change in water quality due to contamination or
pollution.
Worldwide, humans withdraw about 10% of the total annual
renewable supply.

21. Quantities of Water Used
Human water use has been increasing about twice as fast a
population growth over the past century.

23. Use by Sector
Water can be analyzed by identifying three major kinds of use or
sectors; domestic, industry, and agriculture.
Agriculture claims about 69% of total water withdrawal.
In many developing countries, water use is notoriously inefficient
and highly consumptive. Typically 60-70% of the water withdrawn for agriculture never reaches the crops it was intended for.
Industry accounts for about 25% of all water use, most going to
cooling water in power plants.

25. Freshwater Shortages
An estimated 1.5 billion people lack access to adequate quantity and
quality of drinking water.
A country in which consumption exceeds more than 20% of the
available, renewable supply is considered to be vulnerable to water stress.
Water consumption worldwide has increased about twice as fast as
population growth.

26. A Precious Resource
The WHO considers 264,000 gal of water per person per year to be
the minimum level below which most countries are likely to experience chronic shortages.
In many countries, the problem is not total water supply but access to
clean water.
More than 2/3 the world's households have to fetch water from
outside the home.
There are many reasons for water shortages including natural sources
(dry winds, rain fall) and human sources (overgrazing, urbanization, poor agricultural practices).
Pure water is available in most countries but at a price. Many times,
the poor cannot afford to pay fees for clean, sanitary water.
Improvements have been made over the past decade. Since 1990, 800
million people have gained access to clean water.

27. Depleting Groundwater
Groundwater is the source of nearly 40% of the fresh water for
domestic and agricultural use in the United States.
In many areas, groundwater is being withdrawn from aquifers and
springs faster than it is naturally replenished.
Many aquifers have slow recharge rates, so it will take thousands of
years to refill them once they are emptied.
Withdrawal of large amounts of groundwater causes porous
formations to collapse, resulting in subsidence or settling of the surface above.
Sinkholes form when the roof of an underground channel or cavern
collapses, creating a large surface crater. Drawing water from caverns and aquifers accelerates this process.
Saltwater intrusion may occur along coastal areas where pumped out
freshwater in aquifers is replaced with salty seawater.

30. Increasing Water Supplies
Although there is no way for us to increase the amount of water on
earth, there are ways for us to increase water supplies.
Seeding Clouds and Towing Icebergs
Seeding clouds with dry ice or potassium iodide particles has been
tried with mixed results to produce rain.
It has not been demonstrated that towing icebergs from arctic areas
is feasible, either in terms of energetics or economically.

31. Desalination
Desalination is a process in which salt is removed from seawater
to produce fresh water.
In countries such as Kuwait and Saudi Arabia, this is a very
important method of freshwater production.
Although desalination is still about 3 to 4 times more expensive
than other methods of fresh water production, it is the one which holds the most promise.

32. Dams, Reservoirs, Canals, and Aqueducts
Dams, reservoirs, canals, and aqueducts are used to trap and
move large amounts of runoff to areas where they are needed.
Many problems are associated with this type of water transfer,
however. By trapping and diverting water to one location, you are diverting it away from another location. Many ecosystems and ecological balances have been upset by the construction of dams.

33. Environmental Costs
Although they help one area, the construction of dams may be
detrimental to other areas.
The main problem with dams is inefficiency. Some dams lose so
much water through evaporation and seepage into porous rock that they waste more water than they help make available.
Many resorvoirs are being filled in with silt by the rivers which
supply them.
Flooding land by building dams can also be culturally devastating to
indigenous peoples.
The construction of dams also causes a loss of free-flowing rivers.

34. Water Management and Conservation
Watershed management and conservation are often more economical
and environmentally sound ways to prevent flood damage and store water for future use than building huge dams and reservoirs.
Watershed Management
A watershed is all the land drained by a stream or river. Retaining
vegetation and ground cover in a watershed helps hold back rainwater and lessens downstream floods.

35. Domestic Conservation
We could probably save as much as half of the water we now use for
domestic purposes without great sacrifice or serious changes in our lifestyles.
Shorter showers, fixing leaks, and using conserving appliances all
may forestall the coming water shortages that many experts predict.
Our largest domestic use of water is toilet flushing. Many new, more
efficient methods of treating sewage waste are being researched.

37. Recycling and Water
In many developing countries, as much as 70% of all agricultural
water is lost to leaks in irrigation canals, evaporation, or application to areas where plants don't grow. Better farming techniques could reduce these water losses dramatically.
Nearly half of all industrial water is used for cooling power plants
and industrial facilities. Better cooling systems could significantly lessen the amount of water used for this purpose.
The fastest growing water supply in California comes from human
waste. Water recovered from sewage treatment plants is an important part of the state's water budget.

38. Price Mechanisms and Water Policy
Throughout most U.S. history, water policies have worked against
conservation.
In the west, water law is primarily based on the "first in time are
first in right". However, it has to be shown that appropriated water
is being put to beneficial use.
Growing recognition that water is a precious and finite resource
has changed policies and encouraged conservation across the
United States. Although there are 37 million more people in the
United States than in 1980, we get by with 10% less water.
Charging higher proportions of real costs to users of public water
projects, more efficient agricultural techniques, and a more
environmentally aware public have all made better water
conservation possible.