Synopsis
Throughout history, the availability of water has been a vital
factor in the rise and fall of human cultures. This is largely because water is
a limiting factor for the carrying capacity for human activities in any region.
It is crucial that humans learn to live within the limits of available natural
resources, including the supply of fresh water. Because the supply of usable
water is finite, the consumption per person must be reduced in regions that are
using this resource excessively.
Water conservation is the use and management of water for the
good of all consumers. It is used in agriculture, industry, and the home. Human
requirements for agricultural production, flood control, fish and wildlife management,
navigation, industrial production, and many other uses have amended natural
hydrologic processes.
The hydrosphere refers to that portion of Earth that is made of water, including all oceans,
lakes, rivers, streams, glaciers, and underground water. Less than 3% of the water of Earth is freshwater, an amount that
includes polar ice caps, glaciers, groundwater, surface water of rivers and
freshwater lakes, and even atmospheric water. However, the amount of freshwater
useable by people and other members of the biosphere is less than 0.7% of the
total (this is water in rivers and lakes, and in the ground). This relatively
small amount of available freshwater is recycled and purified by the action of
processes within the hydrologic cycle, including evaporation, condensation,
precipitation, and percolation through the ground. All life depends on the
availability of freshwater.
Of all the freshwater used directly by humans, agricultural
irrigation accounts for about 70% of the total. The remainder is used for
industrial and domestic purposes. However, these proportions vary widely due to
the climatic and economic conditions of the particular locality. Within this
century, one-third of the countries situated in areas of water scarcity may
encounter severe water shortages. By 2025, two thirds of the world’s population is likely to live in areas of
moderate or severe water shortage. The need for more effective conservation of
the limited supplies of water that are available for use by people and required
by natural ecosystems will intensify as water stress grows.
Freshwater resources
Available freshwater resources are either ground-water or
surface water (rivers and lakes). Water that flows on the surface of the land
is surface runoff. The relationship among surface runoff, precipitation,
evaporation, and percolation is summarized in the following equation:
Surface runoff = precipitation– (evaporation+ percolation)
When surface runoff resulting from rainfall or snowmelt is
confined to a relatively narrow, well-defined channel, it is called a river or
stream.
Groundwater is that water that has percolated downward through
the soil and is present within porous spaces in soil and bedrock. It has been
estimated that the global groundwater resource is equivalent to about 34 times
the volume of all surface waters (i.e., rivers and lakes) of the world. This
resource is present nearly everywhere and has the additional advantages of
typically needing no storage or treatment. Utilization does require the
construction of a well, sometimes presenting a problem in the most needy
locations.
Water utilization efficiency is measured by the ratio of water
withdrawal and its subsequent consumption. Water withdrawal is water pumped
from rivers, reservoirs, or groundwater wells, and is then transported for use.
Water consumption is water that is withdrawn and actually used for some
specific purpose. It is then returned to the environment through evaporation,
transpiration, discharge to a river or lake, or in some other way.
Water consumption
Water consumption varies greatly among regions due to
differences in economic development. The average municipal use in the United
States is about 150 gal (568 l) per person per day, though the rate can be
higher than 350 gal (1324 l) in some locations. This includes home use for
bathing, waste disposal, and gardening, as well as institutional and commercial
usage. Per capita (per person) water usage in Asia is only 22 gal (85 l) per
day, and just 12 gal (47 l) in Africa.
According to the World Health Organization (WHO) of the United
Nations, people have a minimum water requirement of about 5 gal (20 l) per
person per day. This is the minimum amount needed for physiological
rehydration, cooking, washing, and other subsistence requirements. However, the
WHO estimates that nearly two billion people consume contaminated water. This
carries a significant risk of developing such water-borne diseases as cholera,
dysentery, polio, or typhoid, which kill about 25 million people per year. Both
conservation and sanitation are obvious necessities in meeting the huge demand
for freshwater.
Because irrigation accounts for 70% of the water used by humans
worldwide, achieving a better efficiency of agricultural use is a logical step
in advancing water conservation. This can be accomplished by lining water
delivery systems with concrete or other impervious materials to minimize loss
by leaking during transport, and by using drip-irrigation systems to minimize losses
by evaporation. Drip-irrigation systems have been successfully used on fruit
trees, certain row-crops, and horticultural plants. Conservation can also be
accomplished by improving the efficiency of utilization of water by crops,
including the cultivation of plants that are less demanding of moisture.
Efficient water utilization efforts
Subsurface irrigation is an emerging technology with high
water-utilization efficiency. Subsurface irrigation uses a drip-irrigation
tubing buried 6 to 8 in (15 to 20 cm) underground, with a spacing of 12 to 24
in (30 to 60 cm) between parallel lines. The tubing contains drip outlets that
deliver water and nutrients within the root zone at a desired rate. In addition
to water conservation, subsurface irrigation has other advantages that overhead
sprinklers do not: minimal over watering, fewer disease and aeration problems,
less runoff and erosion, fewer weeds, and better protection from vandalism.
However, this system is relatively expensive to install. In California, subsurface
irrigation has been used on fruit trees, field crops, and lawns, and has
achieved water-use savings of about 50%. However, this methodology can, in arid
environments, lead to the buildup of soil salinity levels, damaging plants and
reducing crop yields. Balancing the water needs of the plant with maintenance
of soil quality is an important component of water conservation measures.
Technologically advanced irrigation systems now incorporate climate-based
controls. These systems utilize meteorological information to determine the
need for irrigation and modify the length and duration of irrigation to match
the plant’s requirements. Though these systems are
currently used primarily on large-scale applications, development of economical
models for the small-scale user is underway.
Xeriscaping, or the cultivation of plants requiring little
water, is an especially suitable horticultural practice for conserving water in
regions with a dry, hot climate. For example, over much of the southwestern
United States, more than 50% of the domestic water consumption may be used to
irrigate lawns and other horticultural plants that are intolerant of drought.
Xeriscaping uses plants such as cacti, succulents, and shrubs of semi-desert
habitat (such as trailing rosemary Rosemarinus officinale and rock rose Cistus cobariensis ), which are well-adapted to a hot, dry climate and need little
water.
Water conservation can also be advanced by improving other
domestic uses of water. One simple conservation practice is to install ultra-low-flush
(ULF) toilets and low-flow showerheads in homes and other buildings. A ULF
toilet uses only 1.6 gal (6.1 l) per flush, compared to 5 to 7 gal by a
standard toilet. Replacing a standard toilet with an ULF saves about 30 to 40
gal (114 to 151 l) of water per day, equivalent to 10,000 to 16,000 gal (37,850
to 60,560 l) per year. More recently, advanced toilets and urinals requiring no
water have been developed and are beginning to be utilized on a limited basis.
Another way to conserve the freshwater supply is to desalinize
seawater. Desalinization is the removal of salts and other impurities from
seawater by either distillation or reverse osmosis (RO), and this method is
being increasingly used to provide high-quality water for drinking, cooking,
and other domestic uses. In 2004, the world production of desalinated water was
at least 40 billion gallons per day (150 billion liters), most of which was
produced in Saudi Arabia and other nations of the Gulf of Arabia, where energy
costs are relatively low (the cost of desalinated water is highly sensitive to
the cost of energy). The largest desalination plant in the world (Shoaiba
Desalination Plant) is located in Saudi Arabia, and it uses reverse osmosis to
produce half of its country’s drinking water. Saudi Arabia is the
largest producer of desalinated water in the world with desalination providing
70% of the country’s drinking water. Desalinization is also
practiced in California and Florida, where the cost is about three dollars per
thousand gallons, which is four to five times the cost paid for domestic water
by typical urban consumers in the United States, and more than 100 times the
cost paid by farmers for water for irrigation. The process is also gaining
popularity in Spain, Australia, and China.
Widespread recognition of the importance of reusing water has
begun to change traditional water use methods. As the value of water increases,
users are willing to employ methods that may increase the initial cost of a
project, with the hope of regaining those costs through water savings in the
future. One of the first of these reuse applications was the irrigation of golf
courses and landscaping. In many areas, treated waste-water is diverted from
its normal disposal path to be reused in irrigation. This has gained in
popularity and is also utilized in small artificial ponds for decorative
purposes. Graywater systems capture water that drains from sinks, tubs,
laundry, and dishwashers for reuse in irrigation. Graywater systems do not
incorporate toilet wastes because of the potential health threat. Dual plumbing
is required for such a system and some treatment is required prior to reuse.
Though home construction costs are obviously increased by including a graywater
system, many have become dedicated believers in the benefits of water reuse,
while others question the economic benefit of small-scale systems.
KEY TERMS
Drip irrigation —A method of irrigation utilizing small,
low-flow emitters that are located at or above the plant root zone. Designed to
reduce the quantity of water lost to evaporation.
Graywater —Used wash water collected from sinks,
laundry, etc. that is reused for irrigation. Graywater does not include toilet
wastes.
Per capita usage —The amount used by one person in a given
amount of time.
Reverse osmosis —A process for purification of water in
which water is forced through a semi-permeable membrane, retaining most ions
while transmitting the water.
Tiered pricing —A system of pricing in which unit
quantities of a commodity are priced with increasingly higher rates, such that,
higher rates of usage result in rapidly increasing costs for the consumer.
The widespread application of graywater systems has, however,
been hampered by codes and laws that make such systems illegal in many
locations.
Economic incentives for water conservation
As the availability of water becomes more restricted, the costs
to both the provider and consumer are increased. In a situation unique to the
water supply industry, providers are frequently placed in the position of
trying to convince consumers to use less of the commodity that they supply.
Most large water providers have departments dedicated to education of the
public with regard to conservation. In general, these education efforts have
been largely ineffective and conservation of freshwater resources has been best
achieved through economic incentives. Water providers frequently provide
rebates for those consumers that are willing to change from older technology to
newer, such as low-flush toilets and modern washing machines, convert to water
efficient landscaping, or otherwise demonstrate lower water usage. Greatest
effect has been achieved through tiered pricing. In this pricing structure,
users are charged higher rates for each successive unit, or block, of water
used. The rate structure penalizes heavy users with greatly increased rates.
This technique has been shown to be highly effective in reducing overall usage.
In Tucson, Arizona, for example, an increasing tiered price structure resulted
in decreased usage of 26% over a three-year period. Additionally, some
communities have implemented the use of water conservation monitors and water
waste hotlines to penalize those that continue to waste the resource. Many
communities currently limit the type and size of landscaping, the time and
nature of outdoor water use, and in extreme cases, have completely banned
outdoor water use during crisis periods.
