FOR RURAL DEVELOPMENT
INTRODUCTION
Throughout history, man has been dependent on an
adequate water supply for his food, security and well being. Water is a
universal need and is considered the principal limiting factor for human life.
Destruction of natural watersheds has caused critical water shortages affecting
vast areas and populations. Ways to help insure adequate water supplies for
household, agricultural and other uses are available to farms and communities.
The technology is called water harvesting.
Water harvesting is the
practice of collecting and storing water from various sources for beneficial
use. Water harvested from a watershed and conducted to ponds for storage can
substantially increase available water for garden irrigation, livestock
watering, aquaculture and other domestic needs.
THE HYDROLOGIC
CYCLE
The circulation of water in its various forms around the earth is called the
hydrologic cycle. Water may be harvested effectively by man at certain points in
the hydrologic cycle. An understanding of how water circulates around the earth
will assist in the selection of an appropriate technology for harvesting
it.
The sun heats water causing it to evaporate from the earth's surface.
Water returns to the earth in the form of rainfall, snowfall, sleet, dew and
hail. The higher the temperature of an air mass, the more water vapor it can
carry. As air masses cool, water vapor changes to liquid forming droplets that
fall of their own weight. Air cools due to expansion as it is lifted over
mountains by collision with warm air masses and the heating of moisture-laden
air close to the earth's surface (convection cooling).
The most important
source of air moisture is water which evaporates from the oceans, but water may
also evaporate from other open bodies of water and from the ground.
Transpiration from plants (evapotranspiration) is another source of atmospheric
moisture as water moves through plant roots, up the stem, through the leaves and
to the atmosphere. For example, a corn field may transpire up to 7,000 to 10,000
gallons of water per hectare per day into the atmosphere.
SURFACE WATER
Some precipitation runs overland by natural
drainage channels where it eventually enters rivers, lakes and oceans. Most
precipitation infiltrates the soil and becomes "ground water". Harvesting water
on the earth's surface may be done only before it evaporates. Surface runoff,
for example, can be intercepted and stored in impoundments for later
use.
Water shortages in many areas of the world can be alleviated by
harvesting surface runoff water. Criteria used to determine an appropriate
harvesting method for a given location include: 1) the purpose for which the
water will be harvested; 2) land slope; 3) soil properties; 4) construction
costs; 5) amount, intensity and seasonal distribution of rainfall; 6) social
factors such as land tenure and traditional water use practices. The following
figures illustrate practical devices and systems used to harvest water.
Figure 3: Terraces are effective in harvesting
surface runoff for cultivating rice and other crops.
Figure 4: Gabions constructed of wire mesh and fashioned like a cage are gaining wide use in Africa.
Figure 5: Gabions are filled with rock and placed across small valleys to act as barrages to retard runoff. Runoff water collected behind the barrage seeps into the ground. Resulting high soil moisture allows farmers to plant crops behind the barrage after rainy season.
Figure 6: Large jars made of ferrocement are used in Thailand to harvest rainwater from roofs. These roof catchment devices may provide a family with enough drinking water to last through the dry season.
Figure 7: Small ponds provide opportunities for agricultural diversification.
They are suitable for harvesting runoff water in rural areas, thus storing it
for many purposes which include small scale irrigation, household uses,
livestock watering and aquaculture. Ponds properly built and maintained have an
indefinite life.
Figure 8: Microcatchment farming concentrates runoff water from a large area
into a small basin. Fruit trees or other crops are planted in the
basins.
Figure 9: A modification of
microcatchment farming involves watershed modification to direct and concentrate
runoff water into a designated area. Collecting ditches or low stone walls built
on land contours can channel water to the point of use.
SUBSURFACE OR
GROUND-WATER
Some rainwater infiltrates into the soil where living
plants can take it up through their roots. Structures such as gabions and ponds,
which harvest rainfall runoff, increase the amount of subsurface water available
to plants. Water infiltrating into deeper soil layers beyond the reach of plant
roots is called ground water. Ground water may be widely dispersed among
particles of soil, sand, gravel or rock and be unharvestable. Soil type and
moisture content determine the rate and amount of soil infiltration, which may
vary from a fraction of a centimeter to several centimeters per hour.
Eventually, water may reach a porous soil layer saturated with water. These
porous soil layers are called aquifers. Water is typically harvested from
aquifers by pumping or lifting from wells. The top layer of saturation is called
the water table. Wells must be dug or bored down into this zone before water can
be withdrawn from the aquifer.
An artesian aquifer is under natural
pressure due to confinement between upper and lower impervious soil layers. (See
Figure 10). At the lower elevation, pressure will push water upward if a well
shaft penetrates the upper confining layer. Water in the shaft may rise
considerably above the normal water table and even flow freely from the well due
to pressure from the confining beds below. Figures 10 and 11 illustrate how
water tables and aquifers are positioned relative to various soil layers.
Aquifers can become depleted by withdrawing water faster than the rate
of recharge. When this happens, wells go dry and are often dug deeper as a
temporary remedy. If a depleted aquifer is not recharged with new water,
deepening wells only makes the problem worse. Aquifers are recharged as rain
water infiltrates to the water table. Slow runoff rates enhance infiltration.
High runoff rates and low infiltration typically occur in deforested areas. Many
water harvesting practices retard runoff and encourage water to infiltrate
deeply into the soil, thus aiding aquifer recharge. For example, a well dug
downslope from a pond will be charged by seepage through the pond bottom and may
never go dry.
Water seeping from springs may be harvested, and can be a source of good
quality drinking water. Springs are often found in areas where a rock or clay
layer surfaces on a hillside, as shown in Figure 11. Water that is perched above
this impervious layer may flow from the hillside as a spring.
Springs may
dry up when the watershed or recharge area is cleared of vegetation. Springs can
be prevented from drying up by building water harvesting structures on the
watershed to collect rainfall runoff and increase infiltration rates. Such water
conservation measures help maintain an adequate water supply to rural families
and villages.
GLOSSARY OF TERMS
aquifer - a soil layer saturated with
water.
artesian aquifer - an aquifer under pressure due to
confinement between upper and lower impervious strata.
evaporation
- the process by which water is changed from a liquid to a gas or
vapor.
evapotranspiration - the passage of water through a plant
from the roots, through the vascular system and to the
atmosphere.
gabion - a wire cage that is filled with rock and
earth and used to construct barrages for temporarily impounding water or
preventing soil erosion.
ground water - water that has infiltrated
soil beyond the root zone of plants.
hydrologic cycle - the
natural sequence through which water cycles from the atmosphere to the earth and
back to the atmosphere.
infiltration - seepage of water downward
into the soil.
perched water table - a water table lying above a
layer of impervious soil or rock which surfaces on a hillside as a
spring.
permeability - the property of soil or rock which allows
the passage of water through it.
roof catchment device - a device,
such as a cement tank or cistern, that collects rain water falling from the roof
of a building.
root zone - the depth to which the roots of plants
penetrate the soil.
runoff - water that flows over the ground
surface after a rain.
spring - a water source which flows up
freely from the ground.
surface water - water, such as runoff,
that stays on the ground surface and can be collected in ponds or other
impounding structures.
subsurface water - water that has
infiltrated soil to the root zone.
water harvesting - the practice
of collecting and storing water from a variety of sources for beneficial
use.
watershed - a region or area from which water flows to a
single point.
water table - the top zone of water saturation in
the ground.
zone of saturation - the layer or depth of soil which
has become saturated with water that has infiltrated down through surface soil
layers.
Funding for this series was provided by the United States Agency
for International Development. Communications regarding this and other technical
brochures on water harvesting and aquaculture should be sent to:
Information contained herein is available to all persons regardless of race,
color, sex or national origin.