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-
- A class project by students in
- International
- Environmental
-
Problems & Policy
- (Geography 378, Spring 2004, University
of Wisconsin-Eau Claire)
-
- Assistant Professor
- of Geography Zoltan
Grossman grossmzc@uwec.edu
- (715) 836-4471
- P.O. Box 4004,
- Eau Claire,
WI 54702 USA
-
-
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Water
harvesting can be traced back through human history almost as far as
the origins of agriculture. Water harvesting is defined as the redirection
and productive use of rainfall. In some incarnations it resembles a
primitive type of irrigation. Unlike conventional irrigation, however,
rainwater harvesting does not depend on a constant flow of water; it
is totally dependent on rain. Basically, harvesting involves a variety
of methods used to get as much water as possible out of each rainfall.
Uses were nearly limitless when rain could assumed to be clean. These
ancient practices sustained ancient people when conditions would have
otherwise totally prevented agriculture. Many peoples in the world have
continued to rely on water harvesting practices. Others have returned
to it in order to relieve pressure on overburdened undergroundwater
tables or municipal water systems. Some Western groups are discovering
ways that rainwater harvesting can be used in our society to relieve
stress on the environment, save money, and recharge groundwater tables.
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Drinking
Water: Traditionally rain water has been
one of the few water sources known to usually be clean and safe
to drink. Today water harvesters must be wary of pesticide contamination,
high mineral levels, bacteria and other impurities in their runoff
water. Filtering technologies have been used in the past and today
to purify it by passing it through a series of rocks, gravels,
and sands to scrub out contaminants. These methods have proven
to be very effective. |
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Irrigation: Rainwater is the primary source of irrigation for
crops around the world. Water harvesting techniques have been
employed for thousands of years to get more water to the fields
in order to improve crop production. This is the primary traditional
use of rainwater harvesting. |
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Sustaining
Animals: Because of animals’ higher tolerance for bacteria
and other impurities, harvested rainwater is often used as the
primary source of water for livestock. Since cattle and sheep
are well adapted to drinking rainwater, safer groundwater is often
saved for human drinking. |
Groundwater Recharge: This is one of the more modern applications for water
harvesting. Rain runoff from buildings, parking lots and other man-made
structures is funneled into seepage ponds or deep wells in order to
directly recharge aquifers.
Case Studies
The people of Jharbeda Mundatola, in
the Sundergarh district of India, recently revived
their ancient water harvesting tradition in light of a shrinking water
supply. Years of dwindling streams and dried up water tables stirred
memories of a technology used by their forebearers to control rainwater
runoff from the hills above. In 1999 the village restored and reconstructed
the local ghaghra, a sort of mountain water reservoir. Older
members of the village remembered swimming and playing in the pond
created behind the dam of stone and mud which collected rain runoff
coming down the slope. The water is collected in behind one or more
of these barriers, also known as bunds,
and allowed down the hill at a steady rate. This provides the village
with drinking, cooking and bathing water in addition to sustaining
animals and watering terraced crops and lowland fields. The water
gathered is enough to provide for the village even through the worst
of dry season droughts. Most of the ghaghra disapeared when
British colonial authorities frustrated or dismantled traditional
water harvesting techniques by village institutions in order to take
full control of their colony. This tribal public works project rescued
not only their own village from drying out, but also revived streams
and wells in fields and villages in the surrounding valley. More information
at humanscapeindia.net
The Zuni Nation of the Colorado
Plateau employed a system of water harvesting methods to survive in
the dry climate. These people nestled their fields in vast valleys between
enormous hills. The Native Americans would build a series of stone and
mud walls between the hills to slow the movement of water through the
valley. The walls did not hold standing water, but instead prevented
soil erosion and slowed runoff long enough for it to permeate the dirt;
creating moist rich soil in which to grow crops. This technique is also
known as flood water harvesting, and has been used in traditional agriculture
all over the world. Many decades of disrepair eventually lead to the
total failure of this system and the eventual desertification of many
of these valleys. Find out more about the Zuni at Arid
Lands
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“Zuni
agriculture represents a long-term successful land conservation
strategy with a 100-year hiatus. For over 1,000 years Zuni farmers
utilized the exodus of eroded soil and water resources from
pinyon-juniper uplands to grow food that sustained their people.
As Zunis abandoned traditional farming methods to cope with
repression and misguided development strategies of the recent
century's dominant culture, the nutrient-laden waters that previously
supported them have become an expensive liability.” (Jay B.
Norton and Jonathan A. Sandor) |
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Agricultural water harvesting has come in many shapes
and sizes. This Table, provided by the FOOD
AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS gives examples
of different agricultural water harvesting systems. They are all dependent
on capturing water flowing down hill in what are called catchments.
They move the water to the cultivated area and then
allow it to permeate the earth.
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Classification |
Main
Uses |
Description |
Where
Appropriate |
Limitations |
Rough Diagram |
|
negarim
microcatchments |
microcatchment
(short slope catchment) technique |
trees
& grass |
Closed
grid of diamond shapes or open-ended "V" s formed by
small earth ridges, with infiltration pits |
For
tree planting in situations where land is uneven or only a few
tree are planted |
Not
easily mechanised therefore limited to small scale. Not easy to
cultivate between tree lines |
|
|
contour
bunds |
micro
catchment (short slope catchment) technique |
trees
& grass |
Earth
bunds on contour spaced at 5-10 metres apart with furrow upslope
and cross-ties |
For
tree planting on a large scale especially when mechanised |
Not
suitable for uneven terrain |
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|
semi
circular bunds |
micro
catchment (short slope catchment) technique |
rangeland
& fodder(also trees) |
Semi-circular
shaped earth bunds with tips on contour. In a series with bunds
in staggered formation |
Useful
for grass reseeding, fodder or tree planting in degraded rangeland |
Cannot
be mechanised therefore limited to areas with available hand labour |
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|
contour
ridges |
microcatchment
(short slope catchment) technique |
crops |
Small
earth ridges on contour at 1.5m -5m apart with furrow upslope
and cross-ties Uncultivated catchment between ridges |
For
crop production in semi-arid areas especially where soil fertile
and easy to work |
Requires
new technique of land preparation and planting, therefore may
be problem with acceptance |
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|
trapezoidal
bunds |
external
catchment (long slope catchment) technique |
crops |
Trapezoidal
shaped earth bunds capturing runoff from external catchment and
overflowing around wingtips |
Widely
suitable (in a variety of designs) for crop production in arid
and semi-arid areas |
Labour-intensive
and uneven depth of runoff within plot. |
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|
contour
stone bunds |
external
catchment (long slope catchment) technique |
crops |
Small
stone bunds constructed on the contour at spacing of 15-35 metres
apart slowing and filtering runoff |
Versatile
system for crop production in a wide variety of situations. Easily
constructed by resouce-poor farmers |
Only
possible where abundant loose stone available |
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|
permeable
rock dams |
floodwater
farming technique |
crops |
Long
low rock dams across valleys slowing and spreading floodwater
as well as healing gullies |
Suitable
for situation where gently sloping valleys are becoming gullies
and better water spreading is required |
Very
site-specific and needs considerable stone as well as provision
of transport |
|
|
water
spreading bunds |
floodwater
farming technique |
crops
& rangeland |
Earth
bunds set at a gradient, with a "dogleg" shape, spreading
diverted floodwater |
For
arid areas where water is diverted from watercourse onto crop
or fodder block |
Does
not impound much water and maintenance high in early stages after
construction |
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Water Harvesting Today
Modernized versions of water harvesting
are making a comeback in North America with some environmentally conscious
individuals and businesses. Small companies are popping up, especially
in the Pacific Northwest, which offers to install water harvesting system
in your own home. These rainwater storage systems modify the rain gutters
that are already on many homes. The gutters are led to a large storage
tank where water is held for future use. Water from the tank is filtered
first for large objects such as twigs, leaves, shingle dirt, and insects.
Next it passes through two micron filters witch eliminate tiny contaminants
down to the size of many bacteria. Finally it passes under an intense
ultraviolet light to finish sterilizing it. Then the water is ready
for almost any home use. The Ersson family in
Portland, Oregon installed the water harvesting system shown above.
They use reclaimed water for nearly all of their domestic needs. Showering,
laundry, dishes, and cooking all use reclaimed water. They do not yet
use it for drinking water, but speculate that they easily could in the
near future.
A second interesting modern use of water
harvesting is known as harvesting for groundwater recharge.
This system is similar to the previous system in that it gathers the
water that would otherwise simply run off of your roof. It is different,
however, in that instead of saving the water in a container it redirects
back into the aquifer. This may sound overly simple or useless, but
this is not just putting the water back onto the ground. Water that
falls to the ground may make it into the undergroundwater reserves,
but it will very likely be redirected downstream to a river or pond
or (more likely in urban areas) enter the sewer system and be transported
far away. Surface water is also much more likely to cause erosion
as well as pick up runoff contaminants such as fertilizer and chemicals.
The recharge process actually puts the water through a dirt and gravel
filter with biological components to clean it and then directs the
water down a deep well. This directly recharges the clean cool aquifers
without the long delay it otherwise takes water to reach these depths.
Water customers in arid and semi-arid climates are already being encouraged
to use this technology. The Town of Panchayats Tennessee, for
example, has provided the diagram above and encourages its citizens
to use this technology.
The Future of Water Harvesting
Many communities in the past and present have effectively
employed water harvesting to meet many of their water needs. Ancient
technologies have shown themselves to be useful today, and modern
water harvesting technologies are improving in leaps and bounds. If
we are facing a future of increasing water demand, and possible water
conflict, it may prove to be very wise to look to the past for a solution.
All the water we need is out there. We simply need to cherish every
drop, as our ancestors did.
Sources
For more information:
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