BRE 211: Principles of
Agriculture and Forestry
Lecture 7
WATER SUPPLY AND
IRRIGATION
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Sources of Water include:
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Surface: rivers, streams, lakes and dams.
Ground: springs, wells and boreholes
Rain.
River, streams and dams
Rivers and streams originate from water catchment
areas such as mountains, hills and forested areas.
They are either temporary or permanent.
 Most farmers depend on rivers and streams for their
water supply.
 Water from streams and rivers has to be collected
and distributed to farms.
 To facilitate this, weirs and dams are constructed to
raise the water level. This makes it easy to pump or
channel it through pipes or canals.
Rivers Streams and Dams
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A weir is a barrier constructed across the river to
raise the water level and still allow the water to flow
over it.
It can be constructed of stone or concrete or loose
stones held by wire mesh.
A dam is a barrier constructed across a river or a dry
valley to hold water and raise its level to form a
reservoir or lake.
A dam differs from a weir in that it is designed to
impede the flow of water and store as much of it as
possible. It has, however, a spillway provision to
allow excess water to flow away.
The accumulated water can be pumped to farms.
Lakes
 Lakes
are huge collections of water on
land depressions.
 They are fed with water from river,
streams, spring and rain.
 Fresh water lakes such as Victoria and
Naivasha supply water to farms by
means of pumps and pipes.
 Such water requires treatment before
use for domestic purposes.
Ground Water: springs, wells
and boreholes
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Springs
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Water comes out of the ground in small or large
amounts as a result of an impervious layer
meeting the ground surface.
Later it collects into a stream.
A low wall is constructed around the spring to
increase the water volume for easier pumping.
It is also necessary to fence the spring to prevent
contamination by animals.
Water from a spring is on high ground water
can be conveyed to a lower ground by
gravitational flow.
 Spring water is less contaminated than river
or lake water.
Wells
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Wells are holes dug in the ground until the water
table is reached. Some can be as deep as 15
metres.
 Wells should be dug during the dry season when the
water table is very low to ensure that the water level
does not go below the bottom of the well during the
dry season.
 There should be a fence around the well to prevent
accidents.
 A reinforced concrete slab with a lockable lid should
also be constructed to prevent
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Accident
Contamination and
Wearing of the topsides of the well.
Water from the well is usually lifted by use of simple
lift hand pumps or buckets.
Boreholes
 These
are deep holes drilled into the
ground by the use of machines.
 They are usually small in diameter.
 It is advisable to line the hole with a
special metal casing perforated at the
bottom end to allow the water to rise up.
 Special pumps operated by either
electricity or engines are used to lift the
water out of the borehole.
 Boreholes are common in areas where
the streams and rivers are scarce.
Rain Water
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Rain-water is collected from roof tops and
stored in tanks during the rainy season.
 Ponds can also be dug to collect the runoff.
The stored water can be used to domestic
and livestock purposes.
 Rain-water is the purest compared to water
from sources mentioned above. However, its
purity will depend on the method of collection
and storage.
 Rain Water can be collected and stored in
dams or in water tanks.
Water Collection and Storage
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Water can be collected and stored in:
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Dams
Weirs
Tanks
Dams
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These are earthen embankment constructed
across rivers.
The bottom part of the area behind the dam
should have an impervious layer such as clay or
rock to prevent water seepage.
The embankment should be free from trees and
bushes to prevent the roots from damaging it and
giving rise to water seepage.
Grass should be planted on the embankment to
prevent erosion.
Dams
 In
the dry areas where streams are
seasonal, dams are constructed across
the river- beds to form sub-surface
reservoirs on a rock foundation.
 When rain falls, sand and water collect
behind the dam.
 When the streams dry up, the water is
stored in the sand, hence preventing a
lot of evaporation.
 Water is collected, by making shallow
holes in the sand to reach it.
Weirs and Water Tanks
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Weirs
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Water Tanks
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Are basically used to raise the water level in a river to
facilitated pumping.
Water is usually supplied to farms by gravitational flow.
Are made of concrete, stone, metal sheets, plastics or
rubber.
They store water from rain or water pumped in from rivers,
streams, lakes, wells and borehole.
Tanks should be covered to prevent contamination
from dust.
They should also be cleaned when dirt has
accumulated. Stone and concrete tanks need to be
reinforced with wire-mesh or barbed wire during
construction so they can resist the pressure of water.
Water Tanks
 Iron
sheets or aluminum tanks should
be placed on a strong base.
 Underground concrete tanks are used
to collect and store water from runoff
and roof tops.
 Runoff water requires filtering before
storage.
 Although drums are used to collect and
store water, care should be taken to
establish what these drums contained
initially to avoid possible poisoning.
Water Treatment
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Water is treated for the following reasons:
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To kill disease-causing micro-organisms such as
cholera and typhoid and bacteria that thrive in dirty
water.
To remove chemical impurities such as excess
fluoride, which may be harmful to humans.
To remove smell and bad taste. Clean and pure
water has no smell, but should be of good
taste.Smells and bad taste in water make it unfit
for human consumption.
To remove sediments of solid particles such as
soil, sand, sticks etc. Removal of these particles
makes the water clear.
There are several stages involved in treating
water for human consumption.
Stage 1: Filtration at water
intake
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At the point where the water leaves the
source e.g. river or reservoir, it passes
through a series of sieves before entering the
intake pipe.
 These sieves have different sizes of holes.
 The first sieve has larger holes, the second
medium and the last, near the pipe, tiny
holes.
 As raw water enters the pipe, large particles
of impurities will have been trapped by these
sieves.
Stage 2: Softening of Water
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The water in the pipe flows into a mixing
chamber.
 This is a small tank where water circulates
and is mixed with soda ash (sodium
bicarbonate) and alum (aluminium sulphate).
 These chemicals are added in to the water in
equal proportions. Soda ash softens the
water, while alum helps to coagulate sold
particles, which finally settle down to the
bottom.
Stage 3: Coagulation and
Sedimentation
 The
softened water moves to the
coagulation tank, which is circular and
large.
 Sold particles such as silt, sand
coagulate and settle down.
 The tank is also open to allow in fresh
air into water.
 Water stays in this tank for about 36
hours to kill bilharzia which cannot
survive in water stored that long.
Stage 4: Filtration
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Water with very few impurities passes into a filtration
tank where all the remaining sold particles, such as
silt, are removed.
 The filtration tank has layers of different sizes of
gravel and a top layer of sand.
 At its bottom is a layer of large pieces of gravel. This
is followed by another layer of gravel but of medium
size.
 Then there is another layer of gravel of fine textures
(small). A layer of fine sand is placed on top of this
fine gravel.
 These layers allow water to seep through very slowly
leaving all the solid particles behind.
 When water leaves this tank, it is clean.
Stage 5: Chlorination
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The filtered water enters the Chlorination
tank.
 In this tank, a small amount of chlorine
solution is added.
 The flow of the chlorine solution is controlled
by a doser and the amount added will depend
on the volume of water to be treated and the
outbreak of water-borne diseases.
 The chlorine is used to kill micro-organisms in
water.
Stage 6: Storage
 This
is the final stage where treated
water is stored in a large tank before
distribution.
 This tank should be out of bounds to
unauthorized persons.
 It should be properly covered and the
area around it well fenced.
 After the treatment, water is then
distributed to consumers either by
pumping or gravity through pipes.
Irrigation
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Irrigation is the artificial application of water to soil for
the purpose of supplying sufficient moisture to crops.
It is practiced in the following circumstances:
In dry areas with low amounts of rainfall insufficient
for crop production.
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During dry periods
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If such areas irrigated, crop production can be practiced.
Irrigation in dry areas is used as a method of land
reclamation.
Some crops such as coffee, citrus, pineapple and other
horticultural crops require irrigation during the dry period to
meet their moisture requirements for high production.
During the dry period, farmers can grow off-season crops,
especially horticultural crops aimed at fetching high market
prices and also ensure a continuous supply of such crops.
In the growing of paddy rice because it requires
flooded fields.
Types of Irrigation
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There are four types of irrigation namely:
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Surface
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sub-surface
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Overhead and
drip/trickle.
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Surface Irrigation
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Water is brought to crop fields from its source through canals
or furrows.
Includes flood, furrow and basin irrigation.
Surface irrigation method used is influenced by:
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Topography:
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The land must be fairly level.
Amount of water supply:
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There must be a lot of water for this type of irrigation.
Soil type:
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Soil must be able to hold a lot of water and allow it to stand for a long
period, as in rice fields.
Flood Irrigation
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In this method, water is allowed to flow into the field
from a source such as a river or lake through furrows
or canals.
These are fitted with sluice gates to control the flow
of water. When they are opened the water flows
freely into the fields.
The land to be irrigated by flooding should be level.
There must be plenty of water for this method of
irrigation to succeed.
Advantage
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It is relatively inexpensive to establish and maintain.
Disadvantage
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There is an uneven distribution of water to crops and
Plenty of water is wasted.
Furrow Irrigation
 Water
flows from irrigation canals
through gates into furrows.
 These furrows should be dug along the
contours to reduce erosion.
 When water flows through these
contour furrows, it wets the soil.
 Crops are planted on the ridges of the
furrows.
 The land to be irrigated should be fairly
level.
 The spacing of the furrows depends on
the spacing of the crop.
Furrow Irrigation
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The furrows should be maintained by
repairing when eroded or worn out, removing
the weeds and accumulated silt.
 The main canal should be maintained in the
same manner.
 Advantage:
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Reduces fungal diseases such as blight and is
also cheap to maintain and establish, requiring
little skill.
Disadvantage:
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A lot of water is wasted and
Soil erosion may occur where the furrows are not
properly planned or maintained.
Basin irrigation
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This involves flooding the whole area enclosed by
dykes.
 The depth of water is controlled by dykes or the
levees.
 The soil should be made level and earth
embankments called dykes or levees constructed
around each levelled ground.
 This leveled ground surrounded by dykes is called
the level basin.
 Water is allowed into each level basin through an
inlet on the levees to make it easy to get water into
the basins.
 Heavy soils such as clay soils are most suitable for
this method of irrigation as they are able to hold
water for a long time with little infiltration.
Basin Irrigation
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The size of basins vary from large rice fields
to small plots for one or a few plants.
 The method is common in rice fields, which
are flooded throughout the growing period.
 Some of the areas where basin irrigation is
practiced in Kenya include Mwea Tabere,
Ahero, Bunyala, West Kano-Kabonyo, etc in
the rice growing projects.
Basin Irrigation
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Disadvantages:
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Method is not suitable for land that is not level
since it becomes very expensive to construct level
basins.
Maintenance cost involving rebuilding the levees if
broken, removal of weeds from canals, basin
inlets and outlets is high.
Basin irrigation may result in the accumulation of a
lot of salts in the soil.
Subsurface Irrigation
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Involves laying perforated pipes underground to allow water to
pass out through tiny holes and wet the soil around the root
zones of the crop.
It requires soils of high capillarity and water holding capacity.
Advantages
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Minimizes labour requirement, especially in changing water pipes.
No need of constructing dykes, leveling or making level basins.
It can be practiced on sloppy or flat land.
Water does not cause erosion because it comes out in small
amounts.
Fungal diseases such as blight are reduced, because water does
not accumulate on leaves.
Less water is required in comparison to sprinkler and surface
irrigation.
Disadvantages
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It is an expensive undertaking i.e. in buying pipes and laying them,
especially when irrigating large areas.
Pipes can be broken during weeding or land preparation.
Nozzles can block and make irrigation inefficient.
Overhead/Sprinkler Irrigation
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This is the application of water to crops by means of
sprinklers or watering cans.
 Irrigation water is applied in form of rain by rotating
sprinklers mounted on vertical pipes.
 In this method, water must pass through the
sprinklers under high pressure.
 Moreover the wind may misdirect the water and, to
prevent this, a windbreak should be established.
 Advantages
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Water is evenly distributed over the required area.
There is less water wastage than in furrow irrigation.
It can be practiced on sloppy grounds.
Soluble fertilizers can be applied together with irrigation
water e.g. nitrogenous fertilizers.
Sprinklers can easily be moved from plot to plot.
Overhead/Sprinkler Irrigation
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Disadvantages
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Expensive installation i.e. buying pipes, sprinklers, and
sometimes a water pump.
Encouraging fungal diseases like blight, CBD etc due to
water accumulating on the leaves.
Causing soil erosion if it is not properly controlled and
especially on sloppy ground.
It may require the establishment of a windbreak.
Maintenance of the system requires a lot of skill and it is also
expensive.
Two types of sprinklers are commonly used in
overhead irrigation
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The continuous rotating type: Suitable for small-scale
irrigation
The spring loaded: Suitable for large-scale irrigation.
Drip/Trickle Irrigation
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Uses polythene pipes with tiny perforations.
As water passes through them, it comes out through the holes in
small quantities and drips to the ground, wetting the soil around
the base of the crop.
The pipes should be laid along the rows of the crops.
Bottles are also used in this irrigation.
Advantages
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Little amount of water is required.
Water under low pressure can be used so long as it can flow along
the pipe.
It discourages diseases such as blight, CBD etc. as it does not wet
the leaves of the crop.
It does not encourage the growth of weeds between the rows.
Disadvantages
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Pipes are expensive to buy and, therefore, irrigation is practiced on
a limited scale.
When using perforated pipes, only clean water should be used to
avoid blockage of the perforations.
Pumping and Piping Of Water
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Pumping is lifting water from one point to another by
use of mechanical force.
Water is pumped from rivers, reservoirs, lakes, wells
and springs and piped to where it is required for use
or storage.
Piping water is the conveyance of water through
pipes from one place to another. The water is forced
through them by pumps or gravity.
Types of Water Pumps
Several types of pumps can be used to lift water from
its source. They include:
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Centrifugal pumps
Piston pumps
Semi-rotary pumps
Hydram
Centrifugal pumps
 These
have metal discs with blades that
rotate at high speed.
 The water is driven out by centrifugal
force to the outlet.
 These pumps are powerful and can
pump large quantities of water; hence
they are suitable for pumping water for
irrigation.
 Diesel/petrol/engines or electrical
motors are used to operate them.
Piston pumps
 These
have pistons that move back and
forth, thus pushing water through the
pipes.
 However, they do not pump a lot of
water and are, therefore, suitable to
pump water for domestic and livestock
use.
 Most of them are petrol/diesel engine
operated.
Semi-rotary pumps
 These
are used to pump water from
wells by hand.
 The quantity of water pumped is enough
to use domestically and for livestock
use.
 They are operated by hand.
Hydram
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The pumps are operated by the force of
flowing water.
 The higher the speed of water, the greater
the pressure created in the pump and this
results in more water being pumped.
 This means that Hydram pumps cannot pump
stationary water and are, therefore, suited to
sloppy areas where water flows and high
speeds.
 They pump little quantities of water but to
considerable heights.
Types of Water Pipes
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They include:
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Metal Pipes
Plastic Pipes
Hose pipes
Metal pipes
There are two types of metal pipes commonly used on the farm;
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Galvanized iron
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Aluminum pipes
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Are heavy and suitable for permanent installation of a water system
and
Are light and are used in irrigation systems. Metal pipes are expensive
but durable.
Plastic Pipes
These are synthetic.
They are cheap to buy and easy to install.
They are durable when properly installed.
They have some limitations in that they can bust under high
pressure, become brittle is exposed to the sun and can be
gnawed by rodents e.g. moles.
Types of Water Pipes
 Hose
pipes
 Hose pipes are used to convey water
from taps to various areas e.g. irrigation
areas or washing places.
 There are two types of hoses
 Rubber
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hoses
Usually more expensive but more durable
 Plastic
Cheaper ones.