BRE 211: Principles of Agriculture and Forestry

Lecture 7

WATER SUPPLY AND IRRIGATION Sources of Water include:

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 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.

LakesLakes 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 Springs

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 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 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.

BoreholesThese 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 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 Water can be collected and stored in:

Dams Weirs Tanks

Dams 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 Weirs

Are basically used to raise the water level in a river to facilitated pumping.

Water is usually supplied to farms by gravitational flow. Water Tanks

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 Water is treated for the following reasons:

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 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 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 SedimentationThe 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 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 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: StorageThis 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 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. If such areas irrigated, crop production can be practiced. Irrigation in dry areas is used as a method of land

reclamation. During dry periods

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 There are four types of irrigation namely:

Surface sub-surface Overhead and drip/trickle.

Surface Irrigation 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: Topography:

The land must be fairly level. Amount of water supply:

There must be a lot of water for this type of irrigation.

Soil type: 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 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 It is relatively inexpensive to establish and maintain.

Disadvantage There is an uneven distribution of water to crops and Plenty of water is wasted.

Furrow IrrigationWater 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 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: Reduces fungal diseases such as blight and is

also cheap to maintain and establish, requiring little skill.

Disadvantage: A lot of water is wasted and Soil erosion may occur where the furrows are not

properly planned or maintained.

Basin irrigation 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 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 Disadvantages:

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 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

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

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 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

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 Disadvantages

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 The continuous rotating type: Suitable for small-scale

irrigation The spring loaded: Suitable for large-scale irrigation.

Drip/Trickle Irrigation 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

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 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 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: Centrifugal pumps Piston pumps Semi-rotary pumps Hydram

Centrifugal pumpsThese 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 pumpsThese 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 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 They include:

Metal Pipes Plastic Pipes Hose pipes

Metal pipes There are two types of metal pipes commonly used on the farm;

Galvanized iron Are heavy and suitable for permanent installation of a water system and

Aluminum pipes 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 PipesHose pipesHose 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 hoses

Usually more expensive but more durable Plastic Cheaper ones.