4.4.2. World Water Use
Figure 4-3: Relative change of annual net irrigation requirement between
present time (19611990) and 2025 as a result of climate change (MPI
climate scenario; areas equipped for irrigation in 1995 shown) (Döll
and Siebert, 2001).
Figure 4-2 shows estimated
total water withdrawals, by sector, from 1900 to 1998 (Shiklomanov, 1998; Shiklomanov
et al., 2000). Agricultural useprimarily for irrigationis by far
the largest proportion, accounting in 1995 for 67% of all withdrawals and 79%
of all water consumed. Municipal, or domestic, use represents only about 9%
of withdrawals. There are large differences, of course, between continents,
with the greatest absolute volume of irrgation withdrawals in Asia.
Over the past few years there have been many projections of future water withdrawals;
virtually all have overestimated the actual rate of increase (Shiklomanov, 1998).
Figure 4-2 also shows projected total global water
withdrawals estimates made for the UN Comprehensive Assessment of the Freshwater
Resources of the World (Raskin et al., 1997). The central projection represents
a Conventional Development Scenario (CDS), with best-guess
estimates of future population growth, economic development, and water-use intensity.
The upper and lower lines represent high and low cases, where the assumed rates
of growth are altered. Under the core CDS, global water withdrawals would increase
by about 35% over 1995 values by 2025, with low and high estimates of 23 and
49%, respectively. The greatest rates of growth are projected to be in Africa
and the Middle East, with the lowest growth in developed economies. These projections
are very dependent not only on the assumed rate of population growth but also
on the different assumed rates of water usage. They do not take potential climate
change into account.
The amount of municipal water withdrawals is dependent on the number of urban
dwellers, the level of development (related to the availability of a piped distribution
network and sewage systems, amongst other things), income levels, and price
where actually applied to water. Per capita withdrawals in many developed countries
are falling, and this may more than offset an increasing population: Total municipal
withdrawals in the United States, for example, are falling largely as a result
of increasing prices, conservation education provided by water utilities, and
increasing use of water-efficient appliances. However, municipal withdrawals
in developing countries can be expected to increase with living standards; under
the CDS, per capita withdrawals in these countries are projected to rise toward
those in more economically developed countries. Rapid urbanization in developing
countries is expected to lead to very substantial increases in total municipal
The industrial sector currently accounts for approximately 20% of current withdrawals.
This water is used primarily either in processing (food processing or heavy
industry) or for cooling; the relative proportions vary between countries. Future
industrial water use is expected to rise substantially as industrial development
continues, but the increase probably will be less than the increase in industrial
production as water is used with greater efficiency (using less per unit of
production, for example, or relocating power generation plants to coastal areas
to use seawater for cooling). Under the CDS, total industrial water use increases;
this increase is concentrated largely in Asia and Latin America (Raskin et al.,
The amount of water used for agriculture is dependent primarily on the level
of irrigation development, the pricing of water, and the reliability of supply.
Future irrigation uses are a function of the rate of expansion of irrigated
land, irrigation efficiency, and pricing practices. Efficiency is projected
to improvepartly as a result of changes in the cost of water and partly
as a result of technological developmentsbut this may be more than offset
by increases in the area under irrigation. The amount of increase, however,
is very uncertain; it depends on, among other things, assumed rates of population
growth (increasing demand) and assumed changes in world agricultural markets.
Previous estimates of future water use have tended to overestimate the rate
of increase because the rate of expansion of irrigated land has been overestimated
(Shiklomanov, 1998). The expanded use of pricing to reflect water scarcity is
being strongly promoted by international organizations and will affect the quantities
used. However, many regions that historically have been dependent on rainfall
are using supplemental irrigationa factor that will increase irrigation
Estimates of future water withdrawals are notoriously uncertain, largely reflecting
uncertainties in the future rate of population and economic growth. There is
an analogy here with GHG emissions scenarios. Also important, however, are possible
changes in the way water is priced. Much agricultural water, for example, currently
is heavily subsidized, and a shift toward a more economic price
for water is likely to have a very significant effect on use. The World Bank
and other economic development agencies are pushing for major reforms in the
way water is priced and sold, as well as the use of water markets as an efficient
way of reallocating existing supplies and motivating efficiency.