|| World's Fresh Water Supply
The Water Resources of Earth
Over 70% of our Earth's
surface is covered by water ( we should really call our planet "Ocean"
instead of "Earth"). Although water is seemingly abundant, the real issue
is the amount of fresh
97.5% of all water on Earth is salt
water, leaving only 2.5% as fresh water
Nearly 70% of that fresh water is frozen
in the icecaps of Antarctica
most of the remainder is present as soil moisture, or lies in deep underground
aquifers as groundwater not accessible to human use.
Only ~1% of the world's fresh water is accessible for direct human uses. This is the
water found in lakes, rivers, reservoirs and those underground sources
that are shallow enough to be tapped at an affordable cost. Only this amount
is regularly renewed by rain and snowfall, and is therefore available on
a sustainable basis.
Water as a Resource
Since antiquity, irrigation, drainage,
been the three types of water control having a major impact on landscapes
and water flows. Since the dawn of irrigated agriculture at least 5000
years ago, controlling water to grow crops has been the primary motivation
for human alteration of freshwater supplies. Today, principal demands for
fresh water are for irrigation,
household and municipal water use, and industrial
uses. Most supplies come from surface runoff, although mining of "fossil
water" from underground aquifers is an important source in some areas.
The pattern of water withdrawal over the past 300 years shows the dramatic
increases in this century.
A timeline of human water use:
12,000 yrs. ago: hunter-gatherers continually
return to fertile river valleys
7,000 yrs. ago: water shortages spur
humans to invent irrigation
1,100 yrs ago: collapse of Mayan civilization
due to drought
Mid 1800's: fecal contamination of surface
water causes severe health problems (typhoid, cholera) in some major North
American cities, notably Chicago
1858: "Year of the Great Stink" in London,
due to sewage and wastes in Thames
Late 1800s-early 1900: Dams became popular
as a water management tool
1900s: The green revolution strengthens
human dependency on irrigation for agriculture
World War II: water quality impacted
by industrial and agricultural chemicals
Water Act passed; humans recognize need to protect water
Figure 1: The water usage of different
regions of the world per capita in cubic meters.
Consumptive and Non-Consumptive Water Use
Consumptive water use refers to water
that is not returned to streams after use. For the most part, this is water
that enters the atmospheric pool of water via evaporation (from reservoirs
in arid areas) and from plant transpiration (especially from "thirsty"
crops such as cotton and alfalfa). Irrigated agriculture is responsible
for most consumptive water use, and decreases surface run-off. An extreme
example is the Colorado River,
which has most of its water diverted to irrigated agriculture, so that
in a normal year, no water at all reaches the river’s mouth.
Agriculture is responsible for 87
% of the total water used globally. In Asia it accounts for 86% of total
annual water withdrawal, compared with 49% in North and Central America
and 38% in Europe. Rice growing, in particular, is a heavy consumer of
water: it takes some 5000 liters of water to produce 1 kg of rice. Compared
with other crops, rice production is less efficient in the way it uses
water. Wheat, for example, consumes 4000 m3/ha, while rice consumes 7650
A great deal of water use is non-consumptive,
which means that the water is returned to surface runoff. Usually that
water is contaminated however, whether used for agriculture, domestic consumption,
or industry. The WHO estimates that more than 5 million people die each
year from diseases caused by unsafe drinking water, and lack of sanitation
and water for hygiene. This has economic effects as well: an outbreak of
cholera in Latin America killed hundreds of people, and cost hundreds of
millions of dollars.
Some believe that fresh water will
be a critical limiting resource for many regions in the near future. About
one-third of the world's population lives in countries that are experiencing
water stress. In Asia, where water has always been regarded as an abundant
resource, per capita availability declined by 40-60% between 1955 and 1990.
Projections suggest that most Asian countries will have severe water problems
by the year 2025. Most of Africa historically has been water-poor.
What's the problem?:
The population is growing rapidly, putting
more pressure on our water supply (demand is increasing)
The amount of water is effectively reduced
by pollution and contamination (supply is decreasing)
What does the future hold? We can best
explore this question by looking carefully at the world's water resources.
|Figure 2: This picture depicts
the global hydrological cycle adapted from Gleick. Flows are approximate
estimates and are in cubic kilometers per year.
Human Appropriation of Renewable Fresh Water
The hydrological cycle:
The water cycle on Earth is essentially
a closed system – we always have the same amount of water.
The only parts of this cycle appropriated
by humans is water held as surface water or shallow aquifers. Let us try
to quantify present use.
Available renewable fresh water:
Because it is difficult to separate
evaporation from transpiration, they are combined as evapotranspiration.
Evapotranspiration represents the water supply for all non-irrigated vegetation,
both natural and crops.
Fossil ground water can be tapped but
Terrestrial replenishable fw supply
RFWSland = ppte on land.
Pland = evapotranspiration
from the land (ETland) and runoff to sea (R).
Estimates of annual runoff range from
33,500 to 47, 000 km3 (Postel uses 40,000 km3).
Runoff is the source for all human
diversions or withdrawals for irrigation, industry, municipal uses, navigation,
dilution, hydropower, and maintenance of aquatic life including fisheries.
Estimates Appropriated for Human Dominated Land Uses.
Human occupied areas
(lawns, parks, etc.)
Table 1: A total of 26.2%
of terrestrial ET is appropriated (18,200 cubic km/69,600 cubic km). *
Assumes 2 g of biomass produced for each liter of water evapotranspired.
** Adjusts for share of ET requirement me through irrigation.
appropriation of evapotranspiration
Vitousek et al. (1986) estimated
the human co-option of terrestrial NPP at 40.6 billion metric tons, or
more than 30% of terrestrial NPP. This includes cropland, grazing land,
and trees harvested for fuelwood and timber. You can review Net
Primary Production (NPP) from a lecture in Global Change I.
The volume of ET required to produce
a unit of biomass =total terrestrial NPP (132 billion metric tons) divided
by terrestrial ET (70,00 km3) = 1.9 kg of biomass per ton of
The final estimate of appropriated
ET is downward corrected for irrigation (approx. 16% of world’s cropland
is irrigated ) and a rough estimate of irrigation of lawns, parks, and
Some 18,200 km3 (26%)
of total terrestrial ET is appropriated for human use (see table 1). The
remaining 74% must meet the needs of remaining terrestrial ecosystems.
Breakdown of Share of Global Runoff and Population
of global river runoff
of global population
|N & C
Human appropriation of runoff
Distribution of global runoff is
highly uneven and corresponds poorly to the distribution of the world population
(see table 2). Asia has 69% of world population but 36% of global runoff.
South America has 5% of world population, 25% of runoff.
Much of runoff is inaccessible. Amazon
River accounts for 15% of runoff and is currently accessible to 25 million
people (0.4% of world’s pop). Estimate it to be 95% inaccessible. Zaire
may be 50% inaccessible. The mostly untapped northern rivers have an average
annual flow of 1815 km3/yr, consider 95% to be inaccessible.
Together, this amounts to 7774 km3
or 19% of total annual runoff, leaving 32,900 km3 geographically
accessible. Does not correct for many northern rivers with large flows
relative to their pop sizes.
Temporal availability: about 27%
of global runoff (11,100 km3) is renewable ground water and
base river flow. Remainder is flood water and harder to capture (table
3). Present storage capacity of large dams totals 5500 km3,
of which 3500 km3 is used to regulate river runoff. Adding together
base flow and surface runoff controlled by dams gives total stable flow.
Correct for spatially inaccessible flows yields and estimate of available
runoff (AR) as 12,500 km3/yr.
of inaccessible runoff of selected remote rivers.
basin or region
|Amazon (95% of total flow)
|Zaire-Congo (50% of total)
|Remote undammed northern
rivers (95% of totals)
Use and Consumption Estimates on a Global Scale, 1990
|Instream flow needs
|Total as a
of AR (12,500
Table 4: *Assumes average
applied water use of 12,000 (m cubed/ha) and consumption equal to ~65%
of withdrawls. #Assumes evaporation loss equal to 5% of gross reservoir
What fraction of AR is used
Withdrawals: agricultural withdrawals
= average water application rate (12,000 m3/ha) x world irrigated
area (240 x 10^6 ha in 1990) = 2880 km3. Assuming 65% is consumed,
Industrial water use is estimated at
975 km3 and roughly 9% (90km3) is consumed. Remainder
is discharged back into environment, often polluted.
Municipal use is estimated at 300 km3
per year, of which 50 km3 (17%) is consumed.
Evaporation from reservoirs is estimated
to average 5% of gross storage capacity of reservoirs (5500 km3)
or 275 km3/yr.
Instream flow needs are estimated from
pollution dilution, assuming that this suffices to meet instream needs.
A common dilution term is 28.3 liters per second per 1000 population. Using
the 1990 population yields a dilution requirement of 4700 km3.
If half of water received adequate treatment, the dilution requirement
is reduced to 2350 km3/hr.
Combining these estimates (see table
4) indicate that humans appropriate 54% of AR. Human use of ET (18,200
km3) plus runoff (6780 km3) constitutes 30% of total
accessible RFWS and 23% of unadjusted RFWS.
How much can AR be increased?
Principal options are to capture
and store more flood runoff or desalinate sea water. Latter is too energy-intensive
for near future.
Worldwide, new dams (> 15 m ht) were
constructed at rate of 885 per year during 1950-80, present rate is 500/yr,
and future rate is estimated at 350/yr. Over next 30 years, assuming size
of reservoirs is unchanged, new construction adds 1200 km3 to accessible
supply, and raises total AR in 2025 to 13,700 km3/hr. Assuming
average per capita water demand stays unchanged, but adjusting the pollution
dilution for additional population, the total human appropriation in 2025
would be 9830 km3/yr, or 70% of estimated AR (compared to current
54%). Clearly we are approaching the limit of available fresh water supply.
What are the Solutions?
Improvements in the efficiency of
water use (ex: irrigation systems often perform poorly, wasting as much
as 60 percent of the total water pumped before it reaches the intended
Efficient management and modern technology
can stretch even scarce water supplies much further. Israel, for example,
supports its population, its growing industrial base, and intensive irrigation
with less than 500 cubic meters per person per year.
Water is often wasted because it
is underpriced. Direct and indirect subsidies (especially for agricultural
use) are still common in both developed and developing countries. Removing
such subsidies and letting water prices rise can provide incentives for
conservation and for the investments needed to spread more efficient technologies.
Water Sustainability, Water Security
The six billion people of Planet
Earth use nearly 30% of the world’s total accessible renewal supply of
water. By 2025, that value may reach 70%. Yet billions of people
lack basic water services, and millions die each year from water-related
diseases. Water is a basis of international conflict. What
is involved in achieving water sustainability and water security?
The following lists some of the criteria that should help us chart our
Basic human needs for water should be
fully acknowledged as a top international priority.
Water-related diseases, including Guinea
worm, diarrhea, onchocerciasis, malaria and typhoid should be brought under
Agricultural water should be efficiently
used and allocated.
Basic ecosystem water needs should be
identified and met.
Serious water-related conflicts should
be resolved through formal negotiations.
Water conservation through better
planning, management, and technologies offers great promise.
Fig. 4 shows per capita water withdrawals in the U.S. from 1900 to 1995.
Per capita water withdrawals began to decline in 1985, despite continued
population growth. More efficient agricultural and industrial water
use accounts for this trend.
Figure 3. Trends in freshwater
withdrawals, and population, for the United States, 1950-1995. Note
that per capita water use peaked in 1985, and has since declined.
Source: USGS Circ. 1200)
Figure 4 shows the projected
global water withdrawals for the year 2000. Note that estimates made
10, 20 or 30 years ago substantially over-estimated year 2000 withdrawals.
Less water demand actually materialized, reflecting the considerable improvements
in water use over this time period. Pricing water to its real cost
will achieve further gains. Both graphs provide a basis for cautious
Figure 4. Actual
water global withdrawals, 1960-200, and past forecasts of year 2000 water
withdrawal. Past estimates have been high, because they failed to
incorporate improved efficiencies in industrial and agricultural water
use. Source: P. Gleick, The World’s Water, 1998 Island Press).
|Figure 5. Global Water Scarcity
Map. Source: World Resources Institute (WRI).
Postel, S.L., G.C. Daily and P.R. Ehrlich.
1996. Human appropriation of renewable fresh water. Science
Gleick, P. 2000. The World's
water. Island Press.
Vitousek, P.M., P.R. Ehrlich, A.H. Ehrlich
and P.A. Matson. 1986. Human appropriation of the products of
photosynthesis. BioScience 36:368-373.