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dam-l Hopkins report excerpt/LS
Here is an excerpt from the new J. Hopkins report on water (as described in
the previous listserv message.)
Agriculture: Producing More from Less
Since agriculture accounts for nearly 70%
of all water withdrawn from
rivers, lakes, and underground aquifers
for human use, the greatest potential
for conservation lies with increasing
irrigation efficiencies. For example,
increasing irrigation efficiency on the
Indus Plains of Pakistan by just 10%
would allow an estimated 2 million more
hectares of farm land to be irrigated
(30).
Most irrigation systems waste water.
Typically, only between 35% and 50%
of water withdrawn for irrigated
agriculture ever reaches the crops. Most
soaks into unlined canals, leaks out of
pipes, or evaporates before reaching
the fields (139). Although some of the
water "lost" in inefficient irrigation
systems returns to streams or aquifers,
where it can be tapped again, water
quality is invariably degraded by
pesticides, fertilizers, and salts that run off
the land. Poorly planned and poorly built
irrigation systems have limited the
yields on one-half of all irrigated land
(185).
Paradoxically, even when sufficient
irrigation water reaches agricultural
fields, it can spoil much of the land
unless drained properly. Particularly in
many arid areas, salts that occur
naturally in the soil must be drained away
with irrigation runoff. If left to
accumulate in the soil, they eventually work
their way to the surface, poisoning the
land (102). Also, poorly drained
irrigation water can raise the
groundwater table until it reaches the root zone,
waterlogging and drowning crops.
Globally, some 80 million hectares of
farmland have been degraded by a
combination of salinization and
waterlogging (185).
Improving irrigation efficiency. A number
of countries are working to
improve irrigation efficiencies, thus
saving water and protecting the land.
Drip irrigation is one technique. Drip
irrigation consists of a network of
porous or perforated piping, usually
installed on the surface or below
ground, which delivers water directly to
the root zones of the crops. This
technique keeps evaporation losses low,
at an efficiency rate of 95%. Drip
irrigation systems cut water use by an
estimated 40% to 60% compared with
gravity systems (139, 142).
In the 1970s drip irrigation systems were
used on only 56,000 hectares
worldwide, mostly to supply water for
vegetables and fruit orchards in
Australia, Israel, Mexico, New Zealand,
South Africa, and the US. By 1991
this figure had grown to 1.6 million
hectares. Although this area constitutes
less than 1% of all irrigated land
worldwide, drip irrigation is widely used in
some countries. Israel, for instance,
uses drip irrigation on 50% of its total
irrigated area (142).
Another promising conservation
method-low-energy precision application
(LEPA)-offers substantial improvements
over conventional sprinkler
systems that spray water into the air.
Instead, the LEPA method delivers
water to the crops from drop tubes that
extend from the sprinkler's arm.
When applied together with appropriate
water-saving farming techniques,
this method also can achieve efficiencies
as high as 95%. Since this method
operates at low pressure, energy costs
drop by 20% to 50% compared with
conventional systems. Farmers in Texas
who have retrofitted conventional
sprinkler systems with LEPA have reported
that their yields have increased
by as much as 20% and that their
investment costs have been recouped
within one or two years (139, 142).
Irrigation options for the rural poor.
Many developing countries
cannot afford to invest in such
techniques as drip irrigation and LEPA. Yet
pressures to feed rapidly growing rural
populations require making better use
of scarce freshwater resources. In many
regions that face acute seasonal
water shortages, conservation projects
are working with rural farmers to
build small reservoirs that collect and
store water from the rainy season for
use in the dry season (35).
Also, in parts of East Africa subsistence
farmers use "water
harvesting"-ancient techniques that
consist mainly of digging deep holes
near each plant to collect and store
water from the wet season for use during
the dry season (35, 186). Another
traditional method involves placing long
lines of stones along the contours of
gently sloping ground to slow runoff
and spread the water across a wider area.
Developed in the Yatenga region of
Burkina Faso, this method is now being
used on over 8,000 hectares in 400
villages throughout the country. It is
also used in Kenya and Niger.
Combined with the practice of deep-hole
water harvesting, this practice has
increased crop production by about 50%
(30, 35).
Reusing urban wastewater. A number of
countries channel treated
urban wastewater from towns and cities
onto nearby farms growing
vegetables and fruits. Today at least
half a million hectares in 15 countries are
being irrigated with treated urban
wastewater, often referred to as "brown
water." Israel has the most ambitious
brown-water program of any country.
Most of Israel's sewage is purified and
reused to irrigate 20,000 hectares of
farm land (139).
Some developing countries also use this
technique (177):
In Mexico City treated urban
wastewater irrigates and fertilizes
alfalfa fields. The alfalfa in turn
is sold as feed to small-scale farmers
who raise guinea pigs and rabbits.
One-third of the vegetables grown
in Asmara, Eritrea, are irrigated
with treated urban wastewater.
In Lusaka, Zambia, one of the
city's biggest squatter settlements
irrigates its vegetable crops with
sewage water from nearby settling
ponds.
Some places apply the same concept
differently. For example, Calcutta,
India, channels much of its raw sewage
into a system of natural lagoons,
where fish are raised. The city's 3,000
hectares of lagoons produce about
6,000 metric tons of fish a year for
urban consumers (177). The fish are safe
to eat because the lagoons soak up and
clean the sewage. Unless urban
wastewater receives some pretreatment,
either from natural wetlands or
sewage treatment plants, however, it can
transfer disease organisms to
vegetables and fruits and endanger human
health.
Such natural water treatment technologies
as using wetlands often can be an
alternative to modern water treatment
systems that are too costly for poor
urban areas of developing countries.
Recycling waste for agricultural
purposes by using oxidation ponds and
aerated lagoons does not require as
much land as is often assumed. Moreover,
it decreases pollution, reduces the
need for fertilizers, and often can be
accomplished with "small-scale,
low-cost technology that is based in
local traditions, decentralized, and
ecologically sound," according to water
resource engineer Janus
Niemczynowicz (124).
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Lori Pottinger, Director, Southern Africa Program,
and Editor, World Rivers Review
International Rivers Network
1847 Berkeley Way, Berkeley, California 94703, USA
Tel. (510) 848 1155 Fax (510) 848 1008
http://www.irn.org
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