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UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION

REVITALISATION PROJECT-PHASE II

YEAR II- SE MESTER II

THEORY Version 1: December 2008

NATIONAL DIPLOMA IN

CIVIL ENGINEERING TECHNOLOGY

SOIL SCIENCE AND IRRIGATION ENGINEERING

COURSE CODE: CEC208

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WEEK 1. 1.0 INTRODUCTION

1.1 FORMATION OF SOIL

1.2 CAUSES OF WEATHERING

1.3 FUNCTIONS OF SOIL

1.4 IMPORTANCE IN SOIL STUDIES

1.5 BRANCHES OF SOIL SCIENCE

1.6 DIFFERENCES BETWEEN SOIL AND ROCK

1.7 COMPONENTS OF SOIL

1.8 TYPES OF PORE SPACES

WEEK 2. 2.0 SOIL PHYSICAL PROPERTIES

2.1 SOIL STRUCTURE

2.3 SOIL CONSISTENCE

2.4 SOIL CONSISTENCE TERMS

2.5 SOIL COLOUR

2.6 FACTORS AFFECTING SOIL COLOUR

WEEK 3. 3.0 VOLUME AND MASS RELATIONSHIP OF SOIL

CONTITUENTS

3.1 SOIL WETNESS

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WEEK 4. 4.0 PERMEABILITY

4.1 INFILTRATION

4.2 FACTOR AFFECTING INFILTRATION

WEEK 5. 5.0 SOURCES OF IRRIGATION WATER

5.1 STANDARDS FOR IRRIGATION WATER

5.2 PROBLEMS OF USING POOR QUALITY

IRRIGATION WATER

5.3 QUALITY OF WATER FOR IRRIGATION

WEEK 6. 6.0 INTERRELATION OF SOIL – MOISTURE AND PLANT

6.1 MOVEMENT OF WATER IN SOILS

6.3 MEASUREMENT OF SOIL MOISTURE

6.4 CLASSES AND AVAILABLE OF SOIL WATER

6.5 GRAVITATION WATER

6.6 CAPILLARY WATER

6.7 HYGROSCOPIC WATER

WEEK 7. 7.0 CROP WATER REQUIREMENTS

7.1 FUNCTION OF IRRIGATION WATER

7.2 WATER REQUIREMENTS (WR) OF CROPS

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7.3 FIELD WATER BALANCE

7.4 EFFECTIVE RAINFALL

7.5 FACTORS INFLUENCING EFFECTIVE RAINFALL

WEEK 8. 8.0 IRRIGATION

8.1 NECESSITY OF IRRIGATION

8.2 BENEFITS OR ADVANTAGES OF IRRIGATION

8.3 METHOD OF IRRIGATION

8.4 BORDER IRRIGATION

8.5 ADVANTAGES OF BORDER METHOD

8.6 CHECK BASIN IRRIGATION

8.7 THE COMPONENTS AND CONTROLS OF

CHECCK BASIN

8.8 ADVANTAGES OF CHECK BASIN

8.9 DISADVANTAGES OF CHECK BASIN

8.10 FURROW IRRIGATION

8.11 COMPONENTS AND ONTROLS OF FURROWS

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8.12 ADVANTAGES OF FURROW IRRIGATION

WEEK 9. 9.0 SUB-SURFACE IRRIGATION

9.1 SPRINKLER IRRIGATION

9.2 ADVANTAGE OF SPRINKLER IRRIGATION

9.3 DISADVANTAGE OF SPRINKLES IRRIGATION

9.4 DRIP IRRIGATION

9.5 ADVANTAGE OF DRIP IRRIGATION

9.6 DISADVANTAGES OF DRIP IRRIGATION

WEEK 10. 10.0 FACTORS THAT AFFECT THE CHOICE OF

IRRIGATION METHOD

WEEK 11. 11.0 IRRIGATION EFFICIENCIES

11.1 WORK EXAMPLES ON IRRIGATION EFFICIENCY

WEEK 12. 12.0 WATER LOGGING

12.1 CAUSES OF WATERLOGGING

WEEK 13. 13.0 REMEDIAL MEASURES

13.1 DESIGN OF DRAINAGE

WEEK 14. 14.0 DRAINAGE

14.1 BENEFITS OF DRAINAGE

14.2 ESSENTIAL REQUIREMENTS OF A DRAIN

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14.3 CLASSIFICATION OF DRAINS

14.4 CLASSIFICATION ACCORDING TO ONSTRUCTION

14.5 CLASSIFICATION ACCORDING TO FUNCTION

SERVED

14.6 DISPOSAL METHODS OF DRAINAGE WATER

WEEK 15. 15.0 FLOOD

15.1 THE PROBLEMS OF FLOOD AND THE NEED

TO

FIND SOLUTION

15.2 TYPES OF FLOOD CONTROL STRUCTURE

15.3 FLOOD MITIGATION RESERVOIRS

15.4 LEVEES AND FLOOD WALLS

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WEEK ONE

1.0 INTRODUCTION Soils is that thin layer of the earth made up of a mixture of mineral and organic materials,

water and air formed from the underlying rocks plant and animal material by various

physical, chemical and Biological processes.

1.1 FORMATION OF SOIL

Most soil begins to form when big rocks break up. The breaking up of rocks is called

weathering. Weathering makes pieces of rocks smaller and smaller. There are two types

of weathering, physical and chemical weathering. After weathering breaks up rocks, a

process called Erosion spreads the bite about.

1.2 CAUSES OF WEATHERING

Most physical weathering is caused by ice. Ice is frost water, and water expands when it

freezes.

Freezing water makes a powerful force. When water seeps into cracks in rocks and

freezes, it can split the rock apart; strong winds and growing tree roots can also break up

rocks.

Water causes most chemical Weathering .Chemical weathering changes the materials that

make up rocks. Rain pours down on rocks, rivers flow over rocks, and waves

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pound rocks along beaches. The water takes certain minerals out of rocks. For example,

grains of sand form after water takes mineral called feldspar out of granite rock. Erosion

also makes soil. Erosion can help break up rocks. Water, wind and glaciers cause erosion.

1.3 FUNCTIONS OF SOIL

� Soil provides anchorage to roots enabling plants to stand erect.

� It acts as an abode of flora and fauna which suitably transform nutrients for up

take by plant roots.

� It acts as a store house of water and nutrients for plant growth.

� It provides space for air and aeration which create a healthy environment for the

biological activity of soil organisms.

1.4 IMPORTANCE IN SOIL STUDIES

� Soil physical properties

� Soil chemistry and nutrient availability

� Soil management – Management of irrigation drainage and conservation

� Soil pesticide interaction

� Weathering and soil formation

� Soil classification.

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1.5 BRANCHES OF SOIL SCIENCE

a. Soil physics – is that branch of soil science which deals with the mechanical

behaviours of soil mass, i.e. the physical properties of soils as well as the

measurement and control of physical processes.

b. Soil chemistry – deals with chemical opposition and properties of soil and

describe the chemical processes taking place in the soil.

c. Soil biology – deals with ecology the organism and their role in biological

transformation in the soil.

d. Soil mineralogy – deals with the minerals, (primary rock mineral and

secondary minerals) present in soil and their contribution to the chemistry,

physics and biology and also fertility of soil in relation to the genesis of soil.

e. Soil fertility – deals with the nutrient status or ability of soil to supply

nutrients for plant growth under favourable condition.

f. Soil genesis and classification (pedology) – deals with weathering of rocks

and minerals factors and processes of formation of soils, and classification of

soils in a recognized system.

g. Soil survey – I the systematic examination of soil in the field and laboratories,

their descriptions and classification, the mapping of kinds of an area and also

interpretation of soils according to adaptability to various

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h. plants and their productivity under different management systems.

i. Soil technology – is an applied science and deal with the principles and

practices of soil erosion and conservation and management of problem soil.

1.6 DIFFERENCES BETWEEN SOIL AND ROCK

Although soils are mainly formed from rocks, they differ from rocks in three main ways.

� Unlike rocks, soil are made up mostly of secondary minerals which are

formed from the products of the weathering of primary rock minerals

� Unlike rocks, soils contain active organic matter in the form of humus,

plant roots and tiny plants and animals called micro-organisms.

� Unlike rocks, soils are distributed in regular fashion over the earth’s

surface in accord with the variations in climate, rocks, vegetation and

relief.

1.7 COMPONENTS OF SOIL

The soil body may be thought of as consisting of two main components; Solid materials

and pore spaces. The solid material fall into two main categories, that is mineral matter

and organic matter.

1.8 TYPES OF PORE SPACES

There are two types of pore space, the macro pores or large open space which are

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normally occupied by air and the micro pore or small spaces which normally contain

water.

Thus the soil is made up of four main constituents

(a) Mineral matter (b) Organic matter (c) Water and (d) Air

K-soil phase pore space

Min

eral

m

atte

r

Org

anic

m

atte

r

Wat

er

Air

The mineral matter consists of all solid in organic material in the soil and they include:-

(i) Rock fragments which are un decomposed reminants of the original rock material

from which the soil is formed.

ii. Sand

iii. Silt

iv. Clay

These are differentiated on the basis of the sizes of the particles.

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WEEK TWO

2.0 SOIL PHYSICAL PROPERTIES

Physically, soils are composed of mineral and organic particle of varying sizes. The

particles are arranged in a matrix that results in about 50% pore space. The pore space

and filled with water and air. The physical properties include: - texture, structure,

consistence, porosity, density, color etc.

The soil texture:- The physical and chemical weathering of rocks and minerals results

in a wide range in size of particles from stones, to gravel, to sand, to silt, and to very

small day particles. The particle size distribution determines the soils coarseness or

fineness, or the soil’s texture. Texture is therefore the relative proportions of sand, silt

and clay in soil. Texture is designated by using the names of predominant size fraction

and the word “loam” when ever all three major size fractions occur in sizable

proportions. Thus the term “siltyclay describes a soil in which the day characteristics are

outstanding and which also contains a substantial quantity of silt. A silty clay loam is

similar to silty cl ay except that it contains sand in a sizable proportion sandy. Soils are

classified as coarse – textured, loam soils are medium. Textured and day soils are fine

textured. The least complex textured group is sand which contains less than 15% silt and

day sandy soils are relatively inert chemically, are loose and non cohesive, and has a low

water holding

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capacity. The textural classification has only on approximate relation ship to the behavior

of a soil as a medium for plant growth.

The textural properties may be modified appreciably by organic matter content. The kind

of day minerals present and kinds of iron associated with them. Example, aggregation

effects of organic matter tend to give a fine textured soil high in days some of the pore

space properties of a coarser – textured soil. Similarly, colloid all effect of organic

additions to coarse textured sandy soil give it some of the moisture and cation retention

characteristic of a fine – textured soil. The figure below shows the textured triangle of the

limited of sand, day and silt.

Soil therefore can be describe by the following

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Problem: - calculate the percentage of sand; day and silt when the 40 second and 8 hour

by diameter reading are 30 and 12 respectively assume a 50 gram oil sample is used.

Sample weight – 40 second reading x 100 = % sand Sample weight

50g – 30g x 100 = 40% sand 50g

Clay

SiltyClay

Clay loam Silty Clay Loam

Silt Loam Silt Loam Sandy Loam

Sandy clay loam

Sandy loam

Sand Loam sand 100% Silt 100% Sand

Per

cent

age

clay

Percentage silt

Percentage sand

10 20 30 40 50 60 70 80 90

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8hr reading x 100 = % day Sample weight

12g x 100 = 24% day 50g

100 – (40 + 24%) = 36% silt.

2.1 SOIL STRUCTURE

Texture is used in reference to the size of soil particle, where as structure is used in

reference to arrangement of the oil particles, and silt and day particles are typically

arranged into secondary particles called peds or aggregate. The shape and size of the peds

determine the soil structure.

The structure modifies the influence of texture with regard to water and air relationships

and the ease of root penetration. The macroscopic size of most peds results in the

existence of interped pace that much larger than the spaces existing between adjacent

sand, silt, and day particles. Grouping of particles into structural units occurs in all soils.

However, the strength of the bond, the size and shape of the structural units and the

proportion of the soil particles involved in the unit differ considerably among soils.

2.2 SOIL CONSISTENCE

Consistence is the resistance of the soil to deformation or rupture. It is determined by

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the cohesive and adhesive properties of the entire soil mass. Where as structure deals

with the shape, size and distinctiveness of natural soil aggregate, consistence deals with

the strength and nature of the forces consistence is important for tillage and traffic

considerations.

2.3 SOIL CONSISTENCE TERMS

Consistence is described for three moisture levels wet, moist, and dry. A given soil may

be sticky when wet, firm when moist, and hard when dry. A partial list of term used to

describe consistence include:-

1. Wet soil – non sticky , sticky, non plastic, plastic

2.

3.

4. Moist soil- loose, friable, firm

5. Dry soil – Loose, soft, hard

Plastic soil is capable of being molded or deformed continuously and permanently, by

relatively moderate pressure, into various shapes when wet.

Friable soils readily break apart and are not sticky when moist. Two additional

consistence terms for special situations are cemented and indurated.

2.4 SOIL COLOUR

Colour is about the most obvious and easily determined soil property. Soil colour is

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important because it is an indirect measure of other important characteristics such as

water drainage, elevation, and the organic matter content. Thus colour is used with other

characteristics to make many important references regarding soil formation and land use.

The soil colours are determined by matching the colour of a soil sample with colour chip

in a munsells oil colour Brok. The books consist of pages, each having colour chip

arranged systematically according to their hue, value and chroma. The three variables that

combine to give colours. Hue refers to wave length or colour of the light. Value refers to

the quantity of light and it increases from dark to light colours.

Chroma- refers to the relative purity of the dominant wave length of the light. The three

properties are always given in the order of hue, value and chroma. In the notation, 10

year before, 10 year the hue, 6 is the value and 4 is the chroma. This colour is light-

yellowish brown. This colour system enables a person to communicate accurately the

colour of a soil to any one in the world.

2.5 FACTORS AFFECTING SOIL COLOUR

1. Organic Matter- is a major clouring agent that affect soil colour, depending on

its nature, amount, and distribution in the soil profile.

2. Iron compounds- The major colouring agents of most horizons are non

compounds in various states of oxidation and hydration.

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WEEK THREE

3.0 VOLUME AND MASS RELATIONSHIP OF SOIL CONTITUENT S

The constituent of the oil are the solids, liquids (water) and air. The diagram below show

the volume and mass relationships of the three soil phases.

Volume relative Mass relative

Ma= Mass of air (negligible) 4 ma Mw=Mass of water Ms=Mass of solid

Mt=Total mass(Ma+Mw+Ms) mw m Va=Volume of air Vw=volume of water Vs=Volume of solids Vf=Volume of pores ms Vt=Total volume (Vf+Vs)

The diagram above shows the presence of the three phases in relative proportion

both in masses and volume. Density of solids of soil is the ratio of mass of solid to it

volume.

ρs =Ms Vsρw.

The ratio of mass of solid to its volume in which ρw= density of water @ 40c. Soil density

is the mass per unit volume of the soil particles. Dry bulk density ρb is the ratio of the

mass of dried particles to the total volume of solid (including particles and pores).

Air

Water

Solids

va

vw

vs

Vt mt

ma

mw

ms

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ρb = Mt = Ms Vt Vs + Vw + Va

Total (wet) bulk density I the mass of moist soil per unit volume

ρt = Mt = M + Mw Vt Vs + Vw + Va

Porosity – is the ratio of the volume of pores (voids) to the total oil volume.

n = Vf = Va + Vw Vt V a + Va + Vw

Porosity is an index of the relative volume of pores. It is influenced by the textural and

structural characteristic of the oil. The more finely divided are the individual soil particle,

the greater is the porosity.

Void ratio- The quantity expressing the ratio of the volume of pores to the volume of

solid is term the as void ratio or relative porosity.

e = Vf = Va + Vw Vf Vs

This index has certain advantages over porosity. In the case of void ratio. Total volume

changes with volume change of voids, where as in case of porosity the volume of pores

may change without change in the volume of solids. The term is commonly used in

engineering works relating to the compaction of foundation, Embankment etc. The

following relationships exit between porosity and volume ratio to apparent and true

specific gravity.

ρb = ρs (1 – n) 100

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Where ρs = ρb (1 + e)

3.1 SOIL WETNESS

Moisture content – Degree of saturation should not be confused with moisture content,

which is the ratio of the weight of water in the sample to weight of solids. Moisture

content

= m = Ww Ws

Volume of wetness;- is the relative water content of soil expressed on volume basis of

water and soil volume of wetness in the ratio of the volume of water to the volume of

total soil.

Vwt = Vw = Vw Vt Vs + Vf

Degree of saturation: – refers to the volume of water, present in the total pore volume.

Degree of saturation = Vw = Vw Vf V a + Vw.

Question 1:- A 500 m3 oven dry core has a bulk density of 1.1g/ cm3. The soil core is

placed in a pan of water and becomes water saturated. The oven dry soil and water at

saturation weight 825 grams. Calculate the total soil porosity.

Weight of oven dry soil = 500m3 x 1.1g/cm3

= 550g

Weight of water in saturated core = 825 – 550g

= 275g

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275m3 pore space x 100 = 55% 500m3 soil volume

Questions 2: A sample of soil weighing 30.6kg had a volume of 0.0183m3, when dried

out in an oven its weight was reduced to 27.2kg. The specific gravity of the solids was

found to be 2.65. Determine the dry density, Bulk density, percentage of moisture cont.

The Saturated density and the percentage of air voids.

Solution: - (a) Bulk density(α) = w = 30.6 = 1672kg/m3 V 0.0183 (b) Dry density αd = Ws = 27.2 = 1486kg/m3 V 0.0183

(c) Percentage of moisture content = Ww Ws

But weight of water in sample = 30.6 – 27.2 = 3.4

Moisture content = 3.4 = 0.125 27.2 Or percentage moisture content = 12.5%.

(d) Density of particles = αs = Ws = GsVw Vs Vs = Ws = 27.2 = 0.0103m3 GsVw 2.65x1000

VV = V-Vs

= 0.0183 – 0.0103 = 0.008m3

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If soil is saturated then voids will be all water

Saturated density = Ws + Vv αw V = 27.2 + 0.008 x 1000 0.0083

= 1923kg/m3

3.2 ASSIGNMENT

The following data were obtained in determining the soil moisture content at successive

depth in the root zone prior to applying irrigation water.

DEPTH OF SAMPLING cm

Wt OF MOIT SOIL SAMPLES gm

OVEN DRY wt OF SOILS 5M gm

0-25 134.60 126.82 25-50 136.28 127.95 50-75 122.95 115.32 75-100 110.92 102.64

The bulk density of the soil in the root zones was 1.50gm/c. The available moisture

holding capacity of the oil was 17.8/m depth. Determine

(i) The moisture content at the different depth in the root zone.

(ii) Moisture content in the root zone at the time of irrigation.

(iii) Net depth of water to be applied to bring the moisture content to

Field capacity.

(iv) Gross irrigation requirement at an estimated field irrigation

Efficiency of 70%.

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WEEK THREE

3.0 VOLUME AND MASS RELATIONSHIP OF SOIL CONTITUENT S

The constituent of the oil are the solids, liquids (water) and air. The diagram below show

the volume and mass relationships of the three soil phases.

Volume relative Mass relative

Ma= Mass of air (negligible) 4 ma Mw=Mass of water Ms=Mass of solid

Mt=Total mass(Ma+Mw+Ms) mw m Va=Volume of air Vw=volume of water Vs=Volume of solids Vf=Volume of pores ms Vt=Total volume (Vf+Vs)

The diagram above shows the presence of the three phases in relative proportion

both in masses and volume. Density of solids of soil is the ratio of mass of solid to it

volume.

ρs =Ms Vsρw.

The ratio of mass of solid to its volume in which ρw= density of water @ 40c. Soil density

is the mass per unit volume of the soil particles. Dry bulk density ρb is the ratio of the

mass of dried particles to the total volume of solid (including particles and pores).

ρb = Mt = Ms Vt Vs + Vw + Va

Air

Water

Solids

va

vw

vs

Vt mt

ma

mw

ms

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Total (wet) bulk density I the mass of moist soil per unit volume

ρt = Mt = M + Mw Vt Vs + Vw + Va

Porosity – is the ratio of the volume of pores (voids) to the total oil volume.

n = Vf = Va + Vw Vt V a + Va + Vw

Porosity is an index of the relative volume of pores. It is influenced by the textural and

structural characteristic of the oil. The more finely divided are the individual soil particle,

the greater is the porosity.

Void ratio- The quantity expressing the ratio of the volume of pores to the volume of

solid is term the as void ratio or relative porosity.

e = Vf = Va + Vw Vf Vs

This index has certain advantages over porosity. In the case of void ratio. Total volume

changes with volume change of voids, where as in case of porosity the volume of pores

may change without change in the volume of solids. The term is commonly used in

engineering works relating to the compaction of foundation, Embankment etc. The

following relationships exit between porosity and volume ratio to apparent and true

specific gravity.

ρb = ρs (1 – n) 100

Where ρs = ρb (1 + e)

3.1 SOIL WETNESS

Moisture content – Degree of saturation should not be confused with moisture

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content, which is the ratio of the weight of water in the sample to weight of solids.

Moisture content

= m = Ww Ws

Volume of wetness;- is the relative water content of soil expressed on volume basis of

water and soil volume of wetness in the ratio of the volume of water to the volume of

total soil.

Vwt = Vw = Vw Vt Vs + Vf

Degree of saturation: – refers to the volume of water, present in the total pore volume.

Degree of saturation = Vw = Vw Vf V a + Vw.

Question 1:- A 500 m3 oven dry core has a bulk density of 1.1g/ cm3. The soil core is

placed in a pan of water and becomes water saturated. The oven dry soil and water at

saturation weight 825 grams. Calculate the total soil porosity.

Weight of oven dry soil = 500m3 x 1.1g/cm3

= 550g

Weight of water in saturated core = 825 – 550g

= 275g

275m3 pore space x 100 = 55% 500m3 soil volume

Questions 2: A sample of soil weighing 30.6kg had a volume of 0.0183m3, when

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dried out in an oven its weight was reduced to 27.2kg. The specific gravity of the solids

was found to be 2.65. Determine the dry density, Bulk density, percentage of moisture

cont.

The Saturated density and the percentage of air voids.

Solution: - (a) Bulk density(α) = w = 30.6 = 1672kg/m3 V 0.0183 (b) Dry density αd = Ws = 27.2 = 1486kg/m3 V 0.0183

(c) Percentage of moisture content = Ww Ws

But weight of water in sample = 30.6 – 27.2 = 3.4

Moisture content = 3.4 = 0.125 27.2 Or percentage moisture content = 12.5%.

(d) Density of particles = αs = Ws = GsVw Vs Vs = Ws = 27.2 = 0.0103m3 GsVw 2.65x1000

VV = V-Vs

= 0.0183 – 0.0103 = 0.008m3

If soil is saturated then voids will be all water

Saturated density = Ws + Vv αw V

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= 27.2 + 0.008 x 1000 0.0083

= 1923kg/m3

3.2 ASSIGNMENT

The following data were obtained in determining the soil moisture content at successive

depth in the root zone prior to applying irrigation water.

DEPTH OF SAMPLING cm

Wt OF MOIT SOIL SAMPLES gm

OVEN DRY wt OF SOILS 5M gm

0-25 134.60 126.82 25-50 136.28 127.95 50-75 122.95 115.32 75-100 110.92 102.64

The bulk density of the soil in the root zones was 1.50gm/c. The available moisture

holding capacity of the oil was 17.8/m depth. Determine

(i) The moisture content at the different depth in the root zone.

(ii) Moisture content in the root zone at the time of irrigation.

(iii) Net depth of water to be applied to bring the moisture content to

Field capacity.

(iv) Gross irrigation requirement at an estimated field irrigation

Efficiency of 70%.

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WEEK FIVE

5.0 SOURCES OF IRRIGATION WATER

Most of sources of irrigation water are from surface water or ground water which can

either be river, canal, Tank, open well or tube water well.

All the sources of water contain some soluble salt which always dissolved in them.

The main source of irrigation water includes:-

i. Rainfall

ii. Atmospheric water other than rainfall

iii. Flood water

iv. Ground water

v. Snow

vi. Waste water

The main soluble constituents in water are calcium, magnesium, sodium and sometimes

potassium as cat ions and chloride, soleplates, bicarbonate and sometimes carbonate as

anions. However, ions of some other elements such as lithium, silicon, bromide, iodine,

copper, nickel etc. and organic matters are present in minor quantities. These elements

usually do not affect the quality of irrigation water as far as the total salt concentration is

concerned.

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5.1 STANDARDS FOR IRRIGATION WATER

Irrigation water maybe said to be unsatisfactory for its intended use if it contain:-

� Chemicals toxic to plants or the person using plant as food

� Chemicals which react with the soil to produce unsatisfactory moisture

characteristics.

� Bacteria injurious to persons or animal eating plant irrigated with the water.

5.2 PROBLEMS OF USING POOR QUALITY IRRIGATION WATER

The following are the most common problems that result from using poor quality

irrigation water.

i. Salinity: - A salinity problem related to water quality occurs if the total quantity

of salts in the irrigation water is high enough for the salts to accumulate in the

crop root zone to the extent that fields are affected.

If excessive quantities of soluble salts accumulate in the root zone, the crop has

difficulty in extracting enough water from the salty soil solution.

ii. Permeability:- A permeability problem related water quality occur when the

rate of water infiltration into and through the soil is reduced by the effect of

specific salts or lack of salts in the water to such an extent that the crop is not

adequately supplied with water and yield is reduced. The poor soil

permeability makes it more difficult to supply the crop with water and may

greatly add to cropping difficulties through crushing of seed beds, water

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iii. logging of surface soil and accompanying disease, salinity, weed, oxygen and

nutritional problems.

iii. Toxicity: - A toxicity problem occurs when certain constituent in the water are

taken up by the crop and accumulate in amounts that result in reduced yield. This

is usually related to one or more specific ions in water, namely, boron, chloride

and sodium.

iv. Miscellaneous:- Various other problem related to irrigation water quality

occur with sufficient frequency and should be spastically this include

excessive vegetative growth, lodging and delayed crop maturity resulting from

excessive nitrogen in the water quality, white deposited on fruit or leaves due

to sprinkle irrigation with high bicarbonate and abnormalities by an usual pH

of the irrigation water.

5.3 QUALITY OF WATER FOR IRRIGATION

The term quality as applied to water, embraces its combined physical, chemical and

biological characteristics. The quality of water for irrigation is as important as nature of

soil.

Good quality water improves the soil because of it calcium content, conversely, if water

applied for irrigation is not of suitability quality soil deteriorate and crop yield decreases

the suitability of water for irrigating a particular crop grown on a particular soil require

consideration of

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i. Sodium and calcium salt dissolved in it

ii. Its pH

iii. Texture of soil and salts present in it.

iv. Sensitivity of crop and drainage conditions of soil.

Water in streams in humid areas is generally suitable for irrigation. Streams in industrial

locations are, however, polluted with industrial waste rendering it unsuitable for

irrigation. Irrigation water is generally obtained from rivers, canals, reservoirs, ground

water, and tanks.

Reservoirs yield better quality water than rivers because of beneficial effects of

impoundment. However, water quality characteristic depends on the source and storage.

Based on the source of the irrigation water the chemical and salts content it can be

classified as follows:-

1. Quality of the surface water for irrigation.

Class of total dissolved water salts (mg/L)

Electrical conductivity Micro ohms/cm

Na2 S04 Cl Boron Suitability excellent to good.

I 0-700 0 – 1000 0 – 192 0-142

0-0.5

II 700 – 2000 1000-3000 192-480 142-355

0.5-2.0 Good to injurious

III Over 2000 Over 3000 Over 480 Over 355

Over 2 Unfit.

1. Total Concentration of Solids-

32

Total dissolved solids in water are related to the specific conductance. Salts of

calcium, magnesium, sodium and potassium present in irrigation water may prove

detrimental to crops. Irrigation water within the zone of good or

moderate is okay. Is: 11624 – 1986 has specified classification on the basis of hazardous

effects of total salt concentration into four groups, as under.

S/no Class Range of electrical conductivity (EC)

1. Low Below 1500 2. Medium 1500-3000 3 High 3000-6000 4 Very high Above 6000

2. Electrical Conductivity-

Electrical conductance is the ability of water solutions to conduct an electric

current and is measured is ohms. It is a function of temperature, type of ions

present and concentration of various ions. The classification of irrigation water

based on electrical conductivity is shown below:

Type of water Classification Electrical conductance micro ohms/c at 25oc

Suitability for irrigation excellent to good

Fresh water 0-100 Excellent to good Low saline (G) Excellent 100-250 All ropes and all

soils except extremely low permeable soils.

33

Medium saline© Good 250-750 Normal salt tolerant plants with moderate leaching

Saline (c3) Permissible 750-2000 Only high salt tolerant plants, drainage is required.

Highly saline (c4) Doubtful 2000-3000 Bad water for irrigation

Very highly saline c5

Unsuitable Over 3000 Unsuitable.

There are several other standards of salts contents that have to be analysis and compare

before selecting the required quality of irrigation water such as the sodium, Boron etc.

The irrigation water with PH value more than 8.5 will cause sodium hazard.

3. Quality of Ground water for Irrigation

Suitability of ground water for irrigation depends upon the effect of mineral

constituents of water on both plants and soils as also on the piping system of the

tube well. Quality of ground water varies from place to place, from stratum to

stratum and fro season to season. The suitability of ground water for irrigation is

determined on the basis of chemical, physical and biological characteristics.

Chemical analysis required determination of the concentrations of in organics

constituents, measurement of pH and specific electrical conductance. Physical

analysis requires determination of the colour, odour, taste, temperature, turbidity

etc. Bacterial analysis is done to determine the presence of coli form organisms.

34

WEEK SIX

6.0 INTERRELATION OF SOIL – MOISTURE AND PLANT

MOVEMENT OF WATER IN SOILS

The movement of water in the soil controls not only the rate of infiltration but also the

supply of moisture to plants roots and the rate of underground flow to springs and

streams and recharge of ground water. Water in the liquid phase flows through the soil

filled pore space under the influence of gravity.

In the films of surrounding soil particles (under unsaturated conditions, it moves under

the influence of surface tension forces). Water also diffuses as vapour through air-filled

pore spaces along gradients of decreasing vapour pressure. In all cases, the movement is

along gradients of decreasing water potential.

In dealing with the movement of the water into the soils, the following terminologies are

very important to be considered.

1. Water Intake

The movement of irrigation water from the soil surface into and through the soil.

It is the expression of several factors including infiltration and percolation.

Percolation- is the down ward movement of water through saturated or nearly

saturated soil in response to the force of gravity. Percolation is

35

synonymous with infiltration rate with the qualitative provision of saturated or

nearly conditions.

The water intake differs from the soil type and different soils absorb water at

different rates. The soil intake is Ana logging to infiltration.

fo

Fo = Initial infiltration

Fc = Infiltration capacity

Fo = Depends on soil moisture content

F = f + (Fo - Fc) e –kt-

K = Constant

t = time.

Seepage – is the infiltration (vertically) down ward and lateral movement of water into

soil or sub strata from a source of supply such as a reservoir or irrigation.

t

f

fc

Infil

trat

ion

rate

m

m/ h

r

Time

36

Such water may reappear at the surface as wet spots or seeps or may percolate to join

ground water or may join the sub surface flow to springs or streams. Seepage rate

depends on the wetted perimeter of the reservoir or the canal and the capacity of the soil

to conduct water both vertically and horizontally.

6.1 MEASUREMENT OF SOIL MOISTURE

The importance of the moisture content in the soil in relation to plant growth has resulted

in the development of many methods for measuring soil moisture.

Soil moisture measurements are important in the suitable scheduling of irrigation and

estimating the amount of water to apply in each irrigation. Measurement of changes in

soil moisture storage with time is important in estimating evapo-transpiration. There are

many experimental situations where careful measurement and investigations on soil-plant

water relationship are to be interpreted properly. The principal methods of expressing

soil moisture are:-

a. By the amount of water in a given amount of soil

b. The stress of tension under which the water is held by the soil. The

37

c. relationship between these two properties through out the entire moisture

range gives a good deal of insight into the physical properties of a soil.

Expressing the amount of soil moisture, the amount of moisture that is held by a certain

mass or volume of soil can be expressed as weight % or volume %. Soil moisture on

weight basis is based on the dry weight of the sample.

Soil moisture, % by weight = wt of moist sample– wt of oven dry sample Wt of oven dry sample. = wt moist sample – wt of dry sample x 100 Wt of dry sample.

Expression of moisture content as a percentage of dry weight may not indicate the

amount of water, available to plants, unless the moisture characteristics curve or field

capacity and permanent wilting point are known.

Field capacity: This is the moisture content of an initially saturated soil after all the

gravitational water has drain out. It is regarded as the storage capacity of the soil for

irrigation purposes.

Wilting point - This is the maximum moisture content of the soil at which roots of

plants can no longer extract water from the soil and the plant “wilts” and may die if water

is not added to the soil.

38

Available water = (Fc – wp) % - soil moisture between field capacity and permanent

wilting point, and it is also referred to as readily available moisture. It is the moisture

available for plant use.

inflow

fc

AW

WP Overflow Out flow

Fc = field capacity

Wp = wilting point

Aw = Available water.

6.2 CLASSES AND AVAILABLE OF SOIL WATER

Water present in the soil may be classified under three heads, because some moisture is

not available to the plant.

� Hygroscopic water

� Capillary water

� Gravitational water

39

i GRAVITATION WATER

This occupies the larger pores of the soil and drains away under the influence of

gravity. The upper limit of gravitational water is when the soil is saturated, that is,

when the pores are completely filled with water.

The saturation capacity is then equal to the porosity of the soil which may be expressed as P = 100(s-v)

S Where P = Porosity %

S = density of the soil grains (gm/cc)

V = bulk density of the dry soil Mass (gm/cc)

If the porosity of the soil is 50% by volume, then the saturated capacity can be

expressed as 500mm of water per meter of soil. In other words, the amount of

water held at saturation in one metre depth of this soil is 500mm.

Gravitational water drains from the root zone unless prevented by a barrier such

as head – pan or a high water table. This process takes less than one day for

coarse sandy and three to four days for a heavy clay soil. Because of the relatively

rapid disappearance of this drainable water, it is not normally included in the

amount available to plants, but some allowance in the calculation of an irrigation

cycle should be allowed for the time taken for the soil to drain to the gravity limit.

ii. CAPILLARY WATER

Capillary water is that held by surface tension in the pores between the particles.

The upper limit is when all the gravitational water has drained away: Soil in this

40

state is said to be at field capacity, which is normally taken as the upper limit to

the water available to the plant.

iii HYGROSCOPIC WATER

This water is held as a very thin film round the soil particle, and is held so firmly

that it is unavailable to the plant except perhaps in extreme cases of drought.

The figure below shows the schematic of classes of soil water.

Gravitational water

Wilting coefficient

Hygroscopic water.

Capillary water

41

WEEK SEVEN

7.0 CROP WATER REQUIREMENTS

7.1 FUNCTION OF IRRIGATION WATER

� It acts as a solvent for the nutrients. Water forms the solution of the

nutrients and this solution is absorbed by the roots.

� The irrigation water supplies moisture which is essential for the life of

bacteria beneficial to the plant growth.

� Irrigation water supplies moisture which is essential for the chemical

action within the plant leading to its growth.

� Some salts present in soil react to produce nourishing food products only

in the presence of water.

� Water cools the soil and the atmosphere, and thus makes more favourable

environment for healthy plant growth.

� Irrigation water, with controlled supplies, washes out or dilutes salts in the

soil.

� It reduces the hazard of soil piping

� It softens the tillage pans.

7.2 WATER REQUIREMENTS (WR) OF CROPS

Having established the suitability of an area for irrigation, the next step is the

determination of water requirement. Knowledge of the rate of water use by crops and the

water retention characteristic of soils is fundamental in the design of the water supply

system and scheduling of the irrigation scheme. The pattern of crop water use, allowing

42

for rainfall and operational losses, determines the canal, pipeline, storage and pumping

capacities of the system.

The total water requirement consists of the water needed by the crop, the losses

associated with the delivery and application of the water. The best source of information

on over all water requirements is often the experience of a good irrigators operating under

conditions similar to those of the project area. Such information must be selected with

care since it is a common practice to use excessive amounts of water if abundant supply

is available.

Water requirement includes the losses due to evapo-transpiration (ET) or consumptive

use (CU) plus the losses during the application of irrigation of water (un avoidable losses)

and quantity of water required for special operations such as land preparation,

transplanting, leaching etc. it may thus be formulated as follows:

WR = ET or CU + application losses + special needs. Water requirement is therefore a

“demand” and the “supply” would consist of contributions from any of the sources of

water, the major source being the irrigation water (IR), effective Rainfall (ER) and soil

profile contributions (S) including that shallow water tables.

Numerically, therefore, water requirement of a crop is given as

WR = IR + ER +S.

The field irrigation requirement of a crop, therefore refers to the water requirement of

crops, exclusive of effective rainfall and contribution from soil profile, and is given as IR

= WR – (ER + s)

43

The farm irrigation requirement depends on the irrigation need of individual crops their

area and the losses in the farm water distribution systems, mainly by the seepage.

7.3 FIELD WATER BALANCE

The water balance of a field is an itemed statement of all gains, losses and changes of

storage of water occurring in a given field with in specified boundaries during a specified

period of time. The task of monitoring and controlling the field water balance is vital to

the efficient management of water and soil. Knowledge of the water balance is necessary

to evaluate the possible methods to minimize loss and maximize gain and utilization of

water which is so often the limiting factor in crop production.

Accordingly the water balance equation may be stated as follows:

Gains – Losses = change in Storage

P + - (R + D + E + T) = DS +DV

Where p is precipitation, I is irrigation, R is run off from the field, D is down ward

drainage out of the root zone, E evaporation from the soil, T transpiration by the crop

canopy, Ds the change in soil water content of the root zone and Dv the charge in plant

water content.

7.4 EFFECTIVE RAINFALL

In the simplest sense, effective rain fall means useful or utilizable rainfall. Rainfall is not

necessarily useful or desirable at time rate or amount in which it is received. The useful

portion of rainfall is stored and supplied to the user, the unwanted parts need to be

conveyed or removed speedily. An agriculturalist considers effective rainfall as that

44

portion of the total rainfall which directly satisfies crop water needs and also the surface

run off which can be used for crop production on their farms by being pumped from

ponds or wells.

7.5 FACTORS INFLUENCING EFFECTIVE RAINFALL

There are so any factors influencing the proportion of the effective rain fall but only very

few will be mentioned here;-

� Rain Fall Characteristic: – a soil has a definite and limited infiltration and

moisture holding capacity.

� Hence greater quantities as well as intensities of rain fall normally reduce the

effective fraction, increasing run off and lessening infiltration.

� Land Slope: - The slope of the land has profound influence on the time available

for the rain water to infiltrate into soil.

� Characteristics of the Soil: - The soil properties influencing infiltration, and

moisture retention release and movement influence the degree of effective

rainfall.

� Crop Characteristics: - Crops with high water consumption create greater deficit

of moisture in soil. The effective rain fall is directly proportional to the rate of

water up take by the plant. Crop characteristics influencing the rate of water up

take are the degree of ground cover, rooting depth and stage of growth.

� Management Practices;- Any management practice which influences run off,

information, hydraulic conductivity or evaporation separation also influences the

degree of effective rainfall.

45

WEEK EIGHT

8.0 IRRIGATION

Irrigation may be defined as the artificially supplying water to soil for raising crops.

It is the engineering of controlling and harnessing the various natural sources of water, by

the construction of dams and reservoirs, canals and head works; and finally distributing

the water to the agricultural fields. Irrigation engineering includes the study and design

of works in connection with river control, drainage of water logged areas, and generation

of hydro electric power.

8.1 NECESSITY OF IRRIGATION

� Less Rainfall: - When the total rainfall is less than needed for crop, artificial

supply is necessary. In this case irrigation work can be constructed at a place

where over water is available and convey to less disadvantage area.

� Non – Uniform: - The rainfall in a particular area may not be uniform one the

crop period. During the early stage rain may be more, but no water may be

available at the end.

� Commercial Crops with Additional Water:- The rain fall in particular area may

be sufficient to raise the usual crops, but more water may be necessary for raising

commercial and cash crops.

� Controlled Water Supply: - By the construction of proper distribution system,

the yield of the crop maybe increased.

46

8.2 BENEFITS OR ADVANTAGES OF IRRIGATION

� Increase in food production

� Protection from famine; - During the contraction of the irrigation works,

employment is carried to the people and this relief famine, and after the

construction of such works, continuous water supply is maintained during

drought.

� Cultivation of cash crops: - Irrigation makes it possible to grow cash crops such

as sugar cane, tobacco cotton, etc.

� Addition to the wealth of the country: - The water tax obtained from farmer, the

bumper crops produced due to irrigation makes country self – sufficient in food

requirements and this serves the foreign exchange and therefore increases

revenue.

� Increase in property of people: - Due to irrigation facility, the value of land is

increased. The increase in the yield of the crop, the growing of cash crops makes

the formers to prosper and the living standard also improved.

� Generation of Hydro Electric Power; - majesty of large river valley projects are

usually planned to provide hydro – electric power together with irrigation. Also,

falls on the irrigation channels can be vitalized to generate electricity which may

help in industrializing the viral area.

� Domestic and industrial water supply: – They can be use for domestic and for

industries that need water for their functions.

47

� Inland navigation: – It can be use a means of transporting the people and

agricultural products.

� Improvement in the Ground water storage; - Due to constant percolation and

seepage of water, the ground water table is raised in the area where irrigation

facilities are prevalent.

� General development of the country: – Due to the increased yield and value of

the crop, means of communication such as wad – ways, rail ways and post and

telegraph facilities are introduced. Due to the living standards, of the people,

schools, hospitable and other facilities are provided.

8.3 METHOD OF IRRIGATION

Irrigation water may be applied to crops by flooding it on the field surface, by applying it

is not the soil surface, by spraying it under pressure or by applying it in crops. The

common methods of irrigation are indicated below.

Irrigation methods

Surface sprinkles sub surface Drip

Border check basin Furrow Rotating head perforated pipe

In the surface methods of irrigation, water is applied directly to the soil surface from a

channel located at the upper reach of the field, water may be distributed to the crops in

border strips check basin or furrow. Two general requirements of prime importance to

obtain high efficiency in surface methods of irrigation are properly constructed water

48

distribution systems to provide adequate control of water to the fields and proper land

preparations to permit uniform distribution of water over the field.

8.4 BORDER IRRIGATION

In the border strip flooding method, the farm is divided into a series of strips 10 to 20

meters wide and 100 to 300 meters long.

These strips are separated by low levees or border (low flat dillies) and run down the

predominant or any other desired slope. To irrigate, water is turned from the supply ditch

on to the head of the border. Water advances – confined and guided by two borders in a

thin sheet towards the lower end of the strip. The surface is essentially level between two

borders so that the advancing sheet of water over the entire width of the strip. The length

wise slope varies from 0.5 to 1.5%. This method is especially suited to forage crops, its

advantage being that for a relatively low investment a system can be developed which

can afford the highest irrigation efficiency and lowest labour requirement. With highly

mechanized farming, large area and be irrigated within a short time by border strip

method. The length of border strip depends upon how quickly it can be wetted over its

entire length. This, however, depends upon:

i. Infiltration rate of the soil

ii. Longitudinal slope of the land

iii. Size of irrigation stream available

The following lengths are suggested for moderate conditions

Types of soil Length of border strip

i. Sandy soil or sandy loam 60 to 90m

ii. Medium silt loam 90 to 150m

49

iii. Clay loam or clay soil 150 to 300m.

The first 6 to 12m length of the strip should be made level to ensure uniform

spreading of water.

Water is diverted to the border strips from the following

a. Earth or concrete ditches (canals): These run at a flat longitudinal grade. The

water is discharge into the trip via border gates, aluminum siphon or plastic

piping.

b. Under-ground concrete pipes through risers: In this method, water is let into the

trips by concrete risers.

The figure below shows the border strip method.

SUPPLY DITCH CONCRETE RISER PIPES

8.5 ADVANTAGES OF BORDER METHOD

� Border ridges can be constructed economically with simple farm implements like

a bullock-drawn

� Labour requirement in irrigation is greatly reduced as compared to the

conventional check basin method.

300m

50

� Uniform distribution and high water application efficiencies are possible if the

system is properly designed.

� Large irrigation streams is properly efficiently used

� Operation of the system is simple and easy

� Adequate surface drainage is provided if out lets are available.

8.6 CHECK BASIN IRRIGATION

This is the simplest in principle of all methods of irrigation. There are many variations in

its use, but all involve dividing the field into smaller unit areas so that each has a nearly

level surface as shown in the figure.

Figure showing Check basin.

BASIN

Levelled Flot

Check or levee

51

Bunds or ridges are constructed around the areas forming basins within which the

irrigation water can be controlled. The basins are filled to the desired depth and the water

is retained until it infiltrates into the soil, the depth of water maybe maintained for

considerable period of time by

allowing water to continue to flow into the basins.

8.7 THE COMPONENTS AND CONTROLS OF CHECCK BASIN

The distinguishing feature of the various uses of the check basin method of irrigation

involve the size and shape of the basins and whether irrigation is accomplished by

intermittent or continues ponding of water in the basins. The ridges or bunds may be

temporary for a single irrigation as in the pre-sowing irrigation of seasonal crops or semi-

permanent for repeated use as in the case of paddy fields. The size of the ridge will

depend on the depth of water to be impounded as well as on the stability of the oil when

wet. Water is conveyed to the field by channel supply.

52

As the infiltration rate of the soil increases, the stream size must be increased or the size

of the basins reduced in order to cover the area within a short period of time.

8.8 ADVANTAGES OF CHECK BASIN

� It is suited to smooth gentle and uniform land slope and for soils having moderate

to slow infiltration rates

� Both row crops and close-growing crops are adapted to be used with basin as long

as the crop is not affected by temporary inundation or is planted in beds so that it

will remain above the water level.

� It is also suitable in very permeable soils which must be covered with water

rapidly to prevent excessive deep percolation losses at the up stream end.

� The method is especially adapted to very slowly and is required to stand for a

relatively long time to ensure adequate irrigation.

� The method enables the conservation of rainfall and reduction in soil erosion by

retaining a large part of the rain in the basin to be infiltrated gradually with out

loss due to surface run off.

8.9 DISADVANTAGES OF CHECK BASIN

� The ridges interfere with the movement of animal drawn or tractor-drawn

implement for inter-culture or harvesting of crops.

� Considerable land is occupied by ridges and lateral field channel and crop yields

are substantially low on the ridge and in the lateral channels.

� The method impedes surface drainage.

� Labour requirement in land preparation and irrigation is much higher compare to

other methods.

53

� The method is not suitable for irrigated crops which are sensitive to wet soil

conditions around the stems of plant.

8.10 FURROW IRRIGATION

The furrow method of irrigation is used in the irrigation of row crops with furrows

developed between the crop rows in the planting and cultivating process.

The size and shape of the furrow depends on the crop grown, equipment used and spacing

between crop rows. Water I applied by running small streams in furrows between the

crop rows. Water infiltrates into the soil and spreads laterally to irrigate the areas between

the furrows a shown below.

Row Furrow

Schematic sketch illustrating furrow irrigation of a vegetable crop with one

furrow for each two rows of the crop.

54

The length of time the water is to blow in the furrows depends on the amount of the water

required to replenish the root zone and the infiltration rate of the soil and the rate of

lateral spread of water in the soil. Both large

and small irrigation streams can be used by adjusting the number of furrows irrigated at

any one time to suit the available furrow.

8.11 COMPONENTS AND ONTROLS OF FURROWS

Efficient irrigation by the furrow method is obtained by selecting proper combination of

spacing length and slope of furrows and suitable size of the irrigation stream and the

duration of the water application.

j. Furrow spacing – Furrows can be spaced to fit the crops grown and the type

of machines used for planting and cultivation. Furrows should be spaced close

enough to ensure that water spreads to the sides into the ridge and root zone of

the crop to replenish the soil moisture uniformly. The spacing depends on the

type of crops e.g. potatoes, maize and cotton are planted 60 to 90cm apart and

have furrows between all rows. The lateral movement of water depends on

soil texture and depth.

55

ii Furrow length – The optimum length of a furrow in usually the longest

furrow that can be safely and efficiently irrigated. Long furrows are an

advantage in inter-cultivation. If the length is too long, water soaks in too

deep at the head of the furrow by the time the stream reaches the lower end.

This results in the over-irrigation at the upper end or under-irrigation at the

lower end.

Short furrows require field supply channels to be spaced too close with consequent

loss of land and increase in labour requirement. Proper furrow length depends

largely on the hydraulic conductivity of the soil. Furrows must be shorter on a

porous sandy soil than on a tight day soil.

iii Furrow slope – The slope or grade of the furrow is important because it controls

the speeds at which water flows down the furrow. A minimum furrow grade of

0.05 percent is needed to ensure surface drainage. In general, the ranges in slope

recommended for borders apply to furrows also. As the furrow grade increase, the

rate of infiltration slows down and the side spread of water into the crop ridge

decrease, so that, wastage may occur at the end of the furrows. With highly

permeable soils, these factors may not be limiting. However, steeper grades lead

to higher water velocities and more erosion.

iv. Furrows stream- The size of the furrow stream is the one factor which can be

varied after the furrow irrigation system has been installed. The size of the

furrow usually varies from 0.5 to 2.5 litres per second. To obtain the most

uniform irrigation, the largest stream of water that will not cause erosion is

used in each furrow at the beginning of irrigation. Its purpose is to wet the

56

entire length of each furrow as quickly as possible, thus enabling the soil to

absorb water evenly through the entire furrow length. After the water reached

the lower end of a furrow, the stream is reduced or cut back so that it will just

keep the furrow wet through out its length with a minimum waste at the end.

8.12 ADVANTAGES OF FURROW IRRIGATION

� Water in the furrows contacts only ½ to 1/5 of the land surface there by reducing

pudding, and crusting of the soil, evaporation losses.

� Earlier cultivation is possible which a distinct advantage in heavy soils.

� There is no wastage of land in field ditches.

� Labour requirements in land preparation and irrigation are very much reduced.

� It is especially suitable for those crops (like maze etc) that are injured by contact

with water.

57

WEEK NINE

9.0 SUB-SURFACE IRRIGATION

It is irrigated by water movement upward from a water table located some distance

below the soil surface. Inherent advantages make controlled sub-irrigation an

attractive proportion to the irrigate if he can device the means of execution. The

advantages are the avoidance of the evaporative losses of open water or wet soil

surfaces and the elimination of the impedance caused to cultivation by pipes and

ditches.

The sub-surface irrigation is classified into two:-

The natural sub-surface irrigation and the artificial sub-surface irrigation.

i. Natural sub- Surface irrigation is so called because the conditions which make

it possible are geological and topographical. These are near level terrain, and a

deep top soil of very high lateral permeability under laid at 2m to 7 depth by an

impermeable stratum. If the area with this soil profile is sufficiently expansive,

it constitutes a convenient under ground reservoir which can be replenished by

spreader ditchers and wells. A constant heck is kept on the water table at

representative points in the irrigation area, and losses, comprising consumptive

use by vegetation and net seepage outflow, are replaced by supply.

Since all water movement in the process of supply to the plant is upwards

from the water table, there is also an upward movement of unwanted salts

within the soil.

58

In arid climates where there is no significant rainfall to countered this, there is

a risk of a built-up of harmful salts close to or on the surface. Should this be

the case, provision is made for periodic leaching of the soil by heavy

application of water to the surface. There must be drainage for the removal of

the salts thus leached.

In humid climate where supplemental irrigation is beneficial during spring and

summer but drainage is needed during the winter, and where the soil is a

highly permeable sand or peat, water table control can be affected by parallel

deep ditches. In times of excess rain water is removed by gravity or pumping

and part is stored in reservoirs to be fed back to the field via the dither during

the dry periods.

ii. Artificial sub-surface irrigation : – Involves the use of a system of buried

perforated pipes through which water is passed at pressure to percolate into the

soils.

This method will only function effectively if the soil has high horizontal and low

vertical permeability. Systems of this type require pipes at spacing as low as

450mm and depths in the region of 500mm. There are expensive and liable to be

managed by deep cultivation. In operation they require the maintenance of

pressure by pumping or gravity form an elevated storage.

Ditch Ditch

Water Table

59

9.1 SPRINKLER IRRIGATION

Figure showing sprinkler irrigation.

The sprinkler method consists of applying the water in the form of spray,

some what as in ordinary rain, as is done in the garden lawn sprinkling. The

greatest advantage of sprinkler irrigation is it adaptabilities to use under

conditions where surface irrigation methods are not efficient. This method is

move useful where:

i. The land can not be prepared for surface methods.

ii. Slope are excessive

iii. Topography is irregular

iv. Soil is erosive

v. Soil I excessively permeable or impermeable

vi. Depth of soil is shallow over gravel or sand.

In this system, the cost of land preparation and permanent water delivery system of

channels or conduct is less. However, there is large initial investment in the purchase of

the pumping and sprinkling equipment.

Impermeable clay

60

Sprinkler system can be classified under three heads:-

i. Permanent system

ii. Semi-permanent system

iii. Portable system.

Earlier, stationary over- head perforated pipe installations were used. However, with the

introduction of light weight steel pipes and quick couplers, portable sprinkler system was

developed. In the permanent system, pipes are permanently buried in such a way that they

do not interfere with tillage operations. In the semi-permanent system, the main lines are

buried while the laterals are portable. Portable system has both portable main lines and

laterals. These systems are designed to be moved from around the farm from field to

field.

A pump usually lifts the water from the source, pushes it through the distribution system

and through the sprinkler nozzle on the sprinkler heads mounted on rising pipes attached

to the laterals. Turbine and horizontal centrifugal pumps are usually used. Sprinkler

system usually is opposed of perforated pipes or revolving head sprinklers and may be

high pressure (201/kg/c2) or low pressure (1.4kg/2) system.

Generally, perforated pipe system operates on the low pressure where as the resolving

head sprinklers operate in both ranges depending on the type of rotary head used.

9.2 ADVANTAGE OF SPRINKLER IRRIGATION

i. Erosion can be controlled.

ii. Uniform application for water is possible

61

iii. Irrigation is better controlled; light irrigation is possible for seedling and

plants which are young.

iv. Land preparation is not required, labour cost is reduced more land is available

for cropping and surface run off is eliminated.

v. Small streams of irrigation water can be use efficiently.

vi. Time and amount of fertilizers can be controlled for application.

9.3 DISADVANTAGE OF SPRINKLES IRRIGATION

i. Wind may distort sprinkling system.

ii. A constant water supply is needed for commercial use of equipment

iii. Water must be clean and free from stand.

iv. Power requirement is high.

v. Heavy soil with poor intake can not be irrigated efficiently.

9.4 DRIP IRRIGATION

In drip irrigation, also known as trickle irrigation, water is applied in the form of drops

directly near the base of the plant. Water is conveyed through a system of flexible pipe

lines, operating at low pressure, and is applied to the plant through drip nozzles. This

technique is also known as “feeding bottle” technique where by the soil is maintained in

the most congenital form by keeping the soil – water – air proportion in the optimum

range. Drip irrigation limits the water supplied for consumptive use of the plant by

maintaining minimum soil moisture, equal to the field capacity, there by maximizing the

saving.

62

Figure above showing Drip irrigation.

The system permits the fine control on the application of moisture and nutrients at stated

frequencies.

The method of drip irrigation was first introduced is Israel but is now practiced in many

countries of the world. Along with irrigation water, nutrients (fertilizer solutions) are also

fed to the system. Water is first filtered so that the impurities may not clog the fine holes

of the drippers.

9.5 ADVANTAGE OF DRIP IRRIGATION

� Less requirement of irrigation water.

� Water logging avoided

� Cultivation of cash crops

� No over irrigation

� Reduced labour cost

� Nutrients preservation

� Suitable for any topography

9.6 DISADVANTAGES OF DRIP IRRIGATION

� High initial cost

� Danger of Blockade of nozzles

� Shallow root depth of the crops, especially for fruit trees.

63

Pump.

Water source

(P.V

.C. P

IPE

S)

TR

ICK

LE L

INE

S

(P.V

.C. P

IPE

S)

TR

ICK

LE L

INE

S

OVERHEAD TANK

N

Fertilizer Tank.

Filter Unit

Pressure Regulator.

64

WEEK TEN

10.0 FACTORS THAT AFFECT THE CHOICE OF IRRIGATION

METHOD

The choice of irrigation methods is based on technical feasibility and economics. Surface

methods are generally the cheapest to install, and where conditions are suitable there is

little point in considering other methods.

However, where high value cash crop is to be grown there may be economic justification

for considering other types of irrigation, especially where conditions are not ideal for

surface irrigation.

a. Land preparation – Surface irrigation requires uniform slope which are too

steep. Unless terracing is to be carried out, an expensive process, steep slope

probably preclude surface irrigation in favour of sprinkler or trickle irrigation.

The uniformity of the land surface is also important. For efficient irrigation by a

surface method, slopes must be uniform with no high or low spots. To accomplish

this, land grading is required, the extend of which depends on the natural

topography. Land grading reduced top-soil, neither of which aid crop production.

It should be noted that land grading may be an expensive operation, and therefore

in some cases, it may be cheaper to install sprinkler irrigation at the out set.

b. Variability of soil type: - The soil types in the irrigation area also affect the

choice of method. Soils with low available water require frequent light irrigation

which is difficult with surface methods. Soils with a high infiltration rate tend to

65

waste water because of percolation below the rooting range unless surface

irrigation run are very short. The short runs increase labour costs, waste land

because of the number of canals required and produce mechanization difficulties.

Soil variability causes difficulties for engineers scheduling irrigation, especially if

over than one type of soil I present in one field. Therefore in this type of condition

sprinkler and trickle irrigation designs can easily be adapted to suit areas of

variable soil type.

c. Water quantity and quality: - The amount, quality and cost of the water supply

also have some bearing on the irrigation method. Where the flow of water is

small, surface irrigation is often uneconomic if possible at all, although the

effective flow can be increase by providing farm storage during periods when

irrigation is not being practiced e.g. at night. If the total quantity of the water is

small, then it must be used with the highest efficiency. High efficiencies are not

generally attained with surface methods unless design, operation and management

are of a high standard and distribution canal are lined. Sprinklers and trickle

irrigation generally have a much higher efficiency than surface methods.

Where sediment is in water and the water contains objectionable matter, for

example sewage, then sprinkler and trickle irrigation can not be choused.

d. Climate- Winds in excess of 15 to 20km/h generally make sprinkler unsuitable as

the smaller droplet are blown away and the water application pattern is distorted

resulting in

low efficiencies. High temperatures and low humidity reduce sprinkling efficiencies, but

sprays; by lowering the atmospheric water demand, can alleviate water stress in

66

the plant and increase growth. Heavy rain after irrigation by surface method can

result in flooding.

e. Crop: - The type of crop being irrigated has little effect technically on the choice

of a surface or sprinkler method. Tall crops are difficult to work in and thus the

movement of pipes and sprinklers can be difficult.

Surface irrigation, by its nature, has relatively long irrigation cycles, and in

extreme circumstance will cause the plants to lose more growth than they would

under short interval sprinkler or trickle method.

67

WEEK ELEVEN

11.0 IRRIGATION EFFICIENCIES

Efficient use of irrigation water is an obligation of each user as well as of the planners.

Even under the best method of irrigation, not all the water applied during irrigation is

shored in the roof zone. In general, efficiency in the ratio of water out put to the water in

put and is expressed as percentage.

The objective of efficiency concepts is to show when improvements can be made which

will result in more efficient irrigation. The following are the various types of irrigation

efficiencies.

i. Water conveyance efficiency

ii. Water application efficiency

iii. Water use efficiency

iv. Water storage efficiency

v. Water distribution efficiency

vi. Consumptive use efficiency

1. Water Conveyance Efficiency: - This term is used to measure the efficiency of

water conveyance systems associated with the canal network, water courses and field

channels. It is also applicable where the water is conveyed in channel from the well to

the individual fields.

It is expressed as Ec = Wf x 100 Wd

Where Ec = water conveyance efficiency %

Wf = water delivered to the irrigated plot (at the field supply Channel).

68

2. Water Application Efficiency : - After the water reached the field supply channel, it I measure of how efficiently this I done I the water application efficiency. And it is defined as Ea = Ws x 100

Wf

Where Ea = water application efficiency, %

Ws = water stored in the root zone of the plants.

Wf = water delivered to the field (at the field supply channel).

Water application efficiencies decreases due to seepage, percolation and run off

losses at the tail and of borders and furrows.

3. Water use efficiency – It is the ratio of water beneficially use, inducing leaching

water, to the quantity of water delivered, and is determined from the following

expression.

Eu = Wu = 100 Wd Where Eu = water use efficiency

Wu = water used beneficially or consumptively

Wa = Water delivered

4. Water surface efficiency – The concept of water storage efficiency gives an

insight to completely the required water has been stored in the root zone during irrigation. Is is determined from the following expression: Es = ws x 100

Wn Where Es = water storage efficiency, %

Ws = water stored in the root zone during irrigation

Wn = water needed in the root zone prior to irrigation

69

Water storage efficiency becomes important when water supplies are limited or

when excessive time is required to secure adequate penetration of water into the

soil.

1. Water distribution efficiency – This indicates the extent to which water is

uniformly distributed along the run. Not only the application of the right

amount of water to the field but also its uniform distribution over the field is

important permissible lengths of irrigation runs are controlled to large extent

by the uniformity of water distribution which is possible for given soil and

irrigation management practice. It is also defined mathematically as

Ed = (1 – y) x 100 d

Where Ed = water distribution efficiency, %

d = average depth of water stored along the run during the

Irrigation

y = average numerical deviation from d

6. Consumptive use efficiency (cue)

It is given by cue = wcu x 100 Wd Where wcu or cu = normal consumptive use of water

Wd = net amount of water depleted from not zone soil.

The efficiency, therefore, evaluates the loss of water by deep percolation and by

excessive surface evaporation following irrigation.

70

11.1 WORK EXAMPLES ON IRRIGATION EFFICIENCY

1. A stream of 135 liters per second was delivered from a canal and 100 litr per

second were delivered to the field. An area of 1.6 hectares was irrigated in eight

hrs. The effective depth of root zone was 1.8m. The run off lose in the field was

432m3. The depth of water penetration varied linearly from 1.8m at the head end

of the field to 1.2m at the tail end. Available moisture holding capacity of the soil

is 20cm per metre depth of soil. Determine the water conveyance efficiency,

water application efficiency, water storage efficiency and water distribution

efficiency, irrigation was started at moisture extraction level of 50% of the

available moisture.

Solution

i. Water conveyance efficiency, Ec = wt x 100 wd

= 100 x 100 = 74% 135

ii. Water application efficiency, Ea = ws x 100 wf

But water delivered to the plot = 100 x 60 x 8 = 2880m3 1000 Water stored in the root zone = 2880 – 432 = 2448m3

:. Water application efficiency = 2448 x 100 = 85%

2880

iii. Water storage efficiency, Es = ws x 100 wn

Water holding capacity of the zone = 20 x1.8 = 36cm

71

Moisture required in the root zone = 36 – 36 x 50 = 18cm 100

18/100 x 1.6 x 10,000 = 2880m3

Water storage efficiency = 2448 x 100 = 85% 2880

1v. Water distribution efficiency, Ed = 100 (1 – y) d

d = 1.8 + 1.2 = 1.5m 2 Numerical deviation from depth of penetration:

At uppers end = 1.8 – 1.5 = 0.3

At lower end = 1.5 – 1.2 = 0.3

Average numerical deviation = 0.3 + 0.3 = 0.3 2

Efficiency Ed = 100 (1 – 0.3) 1.5

= 80%

2. An area of 20 hectares is to be irrigated by a pump working for 12hrs a day. The

available moisture holding capacity of the soil is 16cm/m and the depth of root

zone is 1m. Irrigation is to be done when 50 percent of the available moisture in

the root zone is depleted. Water application efficiency is 70%. Peak rate of

moisture use by the crops is 4mm (weighted average). Losses in water

conveyance are negligible. Determine the irrigation period, net depth of water

application, depth of water application efficiency is 70%. Peak rate of moisture

use by the corps is 4mm (weighted average). Losses in water conveyance are

negligible. Determine the irrigation period, net depth of water application, depth

72

of water pumped per application, and the required capacity of the irrigation

system in hectare cm/day and litres per second.

Solution

Net depth of water application = 16 x 50 = 8cm 100

Irrigation period =net irrigation required = 8 = 20 day Peak use rate 0.4

Depth of water pumped per application = 8 = 11.4cm 0.7

Required capacity of irrigation system = 11.4cm x 20ha

20 days

= 11.4 x 10,000 x 1000 100 x 12 x 60 x 60

= 26.4 litres/sec

73

WEEK TWELVE

12.0 WATER LOGGING

An agricultural land is said to be waterlogged when its productivity or fertility is affected

by high water table. At which it tends to make the soil waterlogged and harmful to the

growth and subsistence of plants life. Depends upon the height of capillary fringe, which

is the height to which water will rise due to capillary action. The normal height of the

capillary fringe met within agricultural soils varies from 0.5m to 1.60m.the land will

therefore, be waterlogged when the water table is within 1.5m below the ground surface.

The adverse effects of high water table upon the yield of crops also depend upon the

nature of crop grown. The dept of water table which adversely affects the growth of

different crop is given below

CROPS DEPT OF WATER TABLE

� Wheat 0.9m to 1.2m

� Cotton 1.5m to 1.8m

� Rice 0.6m

� Sugar cane 0.3m

� Folder crop 1.2m

12.1 EFFECTS OF WATERLOGGING

The fertility of the soil when an area becomes waterlogged is usually due to the

following reasons.

� Inhibiting activity of soil bacteria

The liberation of plant food is independent upon the activity of soil bacteria which

require adequate amount of oxygen in the air for proper functioning. When the soil

pores within the root zones of the crops normally grown so saturated as to effectively

cut off the normal circulation of air, the land is said to be waterlogged

� decrease in available capillary water

Plant life draws its substance from the soil–solution round the soil particles which is

drawn in the plants by capillary action and osmosis. If the water table is high the roots

74

of the plants are confined to the top layers of the soil above the water table while if

the water table is low, the roots of plants have more room for growth.

� Fall in soil temperature

A waterlogged soil warms up slowly and due to lower temperature, action of soil

bacteria is sluggish and plant food available is less.

� Defective air circulation

When the water table is high, the drainage becomes impossible and the Rise of salt

The rate of water table also causes accumulation of alkali salt in the surface soil by

the upward flow of water which is established in waterlogged lands. If the underlying

layers contains alkali salt in solution. They are brought up with water which

evaporates having the salt on the surface. The alkaline deposits change the PH value

of soil. Soils with PH value 7.0 to 8.5 gives normal yields, with PH value 8.0 and 9.0.

The yield decreases when PH value rises to 11.0. The soil becomes infertile.

12.2 CAUSES OF WATERLOGGING

Water logging in any particular area is normally the result of several contributory

factors. The main factors causing water logging areas are giving below

� In adequate surface drainage

When the surface drainage is not adequate the heavy precipitations in the area is not

drained off quickly and the rain water remains stagnant over the area for considerable

time. This gives rise to heavy percolation and water table rises in the area.

� Natural obstruction to the flow of ground water

Sometimes subsoil does not permit free flow of sub soil water due to some natural

obstruction. This may accentuate the process of raising the water table. The creation

of a high false water table or parched water table also leads to water logging.

� Construction of water reservoir

Similar to the seepage from a canal, the seepage from the reservoir augments the

water table and may cause water logging.

� Obliteration of natural drainage

Sometimes the cultivator plough up and obliterates an existing natural drainage. This

results in stoppage of storm water flow, consequent flooding and water logging.

� Inadequate capacity for arterial drainage

75

This arterial drainage or nadi may not have adequate capacity to pass the

heaviest floods in the entire catchments. As such the function of all the drains

connected to the arterial drain is seriously hampered. The flood water of local

drains thus spreads over the country side for clays and heavy percolations into

the sub soil causes alarming rise in water table.

� Over irrigation of fields

When the irrigation water applied to the field is in excess of the requirement of

the crop, deep percolation takes place which is retained in the intermediate zone

augmenting the ground water storage.

� Obstruction of natural drainage

If a natural drainage is obstructed by irrigation channel, rail or road embankments,

it will not be able to pass the rain water of catchments. There will thus be flooding

of land and consequent water logging.

Carbon dioxide liberated by the plant root can not be dissolved and taken away.

� Adverse effect on community health

The climate of a waterlogged area becomes damp. Formation of stagnant pools

may become breeding places for mosquitoes. The climate does become extremely

detrimental to the health of community.

76

WEEK THIRTEEN

13.0 REMEDIAL MEASURES

In devising anti-water logging measures, the nature and magnitude of various factors,

enumerated in previous article should be correctly assessed and allow for, various

remedial measures adopted for prevention of water logging are discussed below

� Efficient surface drainage

An efficient drainage system which permits a quick flow of rain water in short period

helps to reduce the water logging.

� Under-drained by tile drains

The drainage of agricultural lands is done more satisfactorily by the drains. A suitable

tile drain can hold the water table at a pre-determined level which will be most

beneficent to the crops.

� Restriction of irrigation

a. The cultivators should be educated for economic use of water and induce

to divide his field into "Kiaries" to avoid wastage. He should also be

encouraged to supplement his water requirement from open wells and tube

wells.

b.Area with critical water table may be allowed only for kharif irrigation and

during Rabi the cultivators may irrigate from open wells and tube wells.

� Removing obstruction in natural drainage

Drainage crossing with road, railways and canals should be remodeled to

make it more efficient.

� Prevention of seepage from water reservoir

Adequate and suitably designed toe filters are provided so that seepage

ultimately fines its way into the natural stream.

� Change in crop pattern

A change in crop pattern may minimize the damage to plant line.

� Adoption of sprinkler for irrigation

This reduces the percolation losses from water causes as only predetermined

amount of water is applied to the land.

77

13.1 DESIGN OF DRAINAGE

For design, we are normally given,

1) Design capacity Qm³/s

2) Slope [longitudinal]

3) Channel section type [rectangular /trapezium]

We are required to obtain the actual size of the channel.

Example: Design a trapezoidal canal to convey, 10m³/s of water across a land slopping at

3.5m over 3000m. Take marnings n=0.015

Soln Assume Z, M and estimate B

Maximum velocity =1.75m/s

.: Area =10/1.75 =5.7m²

Normally for maximum efficiency, the depth is about half of the width (or top width for

trapezoidal).

Assuming side slope of 1:1 (i.e. 45°), Z=1, M=1

.: A=By +y²

5.7=By +y²

5.7=By +y² Try B=2, y=1

Area (A) =2x1+1²=3

Try B=2, y=2, =A=2x2+2²=8

B=2, y=1.5, A=2x1.5+1.5²=5.25

Use B=2m (Z=1/m and m=1/2)

78

Depth=y A=2y+y² P=2+2√2×Y R=A/P V=1/nR2/3

s½

Q=AV

1.5 5.25 6.245 0.840 1.983 10.408

1.45 5.003 6.104 0.820 1.995 9.981

The capacity of the drainage is determined from the velocity of flow and sectional area of

the canal and its bank/ sides are not eroded by the water, also deposition of salt or

suspension solids should be avoided.

To obtain velocity of flow, we use the manning equation

: V=1/n R2/3 √S

V=Velocity of flow m/s

S=Longitudinal slope of channel

R=Hydraulic radius =A/P

A=Area of flow m²

P=Welted perimeter m

n=Manning constant

Varies with the type of surface of the channel.

MATERIAL N

Concrete 0.015

Wood 0.012

Earth 0.032

Metal 0.011

Corrugated metal 0.024

Maximum velocity is about 1.75m/s for concrete

Normally we get the size by trial, i.e. we assume a given section of certain depth. We

then compute its capacity using manning equation. If the result is less than the given

capacity, we increase the depth. If it is more, we decrease the depth, until we get the

correct size.

79

Channel section Area of flow Welted perimeter

y ------- B------- 1 y m Z 1 ---B----

By

By +Zy

OR

By +y²/m

B +2y

B+Zy²√1+Z²

OR

B +y²√1+1/m²

80

WEEK FOURTEEN

14.0 DRAINAGE

The term drainage is applied to systems for dealing with excess water. The three primary

drainage tasks are urban storm drainage, land drainage (agriculture drainage) and high

way drainage. Our primary concern is the agricultural drainage so far we are dealing with

irrigation engineering.

Agricultural drainage is the removal of excess water from agricultural lands by means of open or covered drains, shallow surface drains, bedding and land grading or smoothing are measures use to collect and remove surface water from fields.

14.1 BENEFITS OF DRAINAGE

The benefits of drainage are:-

� Improvement of the soil structure and increase in productivity

of the soil

� Lengthening of growing season

� Facilitates early ploughing and sowing of the crop. Crop period

is thus increased resulting in higher crop yield.

� More soil moisture is made available for crop growth due to

extension of crop root zone into the soil, there by ensuring

vigorous plant growth.

� Maintains proper aeration of upper soil layers

� Maintains higher soil temperature. The soil is kept warmer.

� Reclamation of water logged lands. Harmful salts are leached

off.

� Improvement in sanitary conditions of the area. Malaria and

weed control.

� Maintenance of water table at a reasonable depth so that water

cannot rise above the natural ground by capillary action.

81

14.2 ESSENTIAL REQUIREMENTS OF A DRAIN

The essential requirements to be satisfied by a drain are:-

� Admit all the flood discharge from the catchments

� Quick and unobstructed flow towards the drain from the catchments

� Capacity to carry away the received water to the out fall

� Seepage and or low discharge does not spread thin over the entire section

� Low maintenance cost

� Low initial cost

� Stable section with non-silting tendency and capable of avoiding sloughing of

side slopes.

CLASSIFICATION OF DRAINS

________________|_________________

| |

According to construction According to functions

_________|_________ _________|_________

| | | Natural

artificial open drains closed drains

14.3 CLASSIFICATION ACCORDING TO CONSTRUCTION

1. NATURAL DRAINS: - These are the lowest valley line between two ridges.

2. ARTIFICIAL DRAINS: -These are the constructed drains generated aligned

along drainage line. Sometimes taken across the valley to reduce length of the

drain or to reduce length of the drain or to have proper out fall condition.

14.4 CLASSIFICATION ACCORDING TO FUNCTION SERVED

1. OPEN DRAINS:

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I. Surface drains: - surface drains are normally used for removal of excess

surface irrigation water or for the disposal of storm water. They remove

water before it has entered the soil.

II. Seepage: - Cater for the sub soil water. They are made deep enough to allow

water table to drop in the drain and seepage water is carried away. They are

of smaller section compared to surface drains. They help maintain aeration

of root zone depths.

III. Surface-cum-seepage drains: - They are the dual purpose of seepage and

storm water drain. During rainy season they carry storm water and seepage

water in non-monsoon months.

2. CLOSED DRAIN: - The sub-surface drains remove water which has entered the

soil. They are usually laid 1 to 1.5m below the ground surface and at a suitable spacing

and grade to lower water table to greater depths.

14.5 DISPOSAL METHODS OF DRAINAGE WATER

Sub surface drainage water in arid region is likely to be saline and disposal should be

considered with care. Return to the natural drainage channels can lead in time to serious

river water salinity. Some irrigation areas are under laid by extensive highly permeable,

volcanic deposits which if they have a suitable zone of discharge, can be used as a cheap

and convenient medium for the disposal o drainage water.

Another geological asset to drainage is a buried historical river bed. This may be a course

of very permeable sands and gravels, sealed from the surface by over laying clay layers,

which if penetrated forms an excellent and cheap drainage disposal channel. Depending

on the extent and depth of the quiver, it can be reached by a system of small ‘down’ wells

each draining its immediate vicinity, or by a small number of main wells into which the

out flow from a system of surface and under drains can be discharged.

While it is generally inadvisable for sub surface drainage water to be returned to the

irrigation supply unless monitored for undesirable solutes, excess storm rain and surface

irrigation run off are unlikely to be contaminated and can safely be re-used. Re-use of

irrigation run off in particular is practiced widely where economically justified

83

Drainage channel

Impermeable stratum

Sand and gravel

Figure shows down well.

84

WEEK FIFTEEN

15.0 FLOOD

Flood is terms used to describe the inundation of an area by water for a certain period of

time, leading to disruption of normal activities and possible lose of properties and life.

The flood could be caused by any of the following: -

i. Excessive rainfall leading to extra ordinary run off.

ii. Poor drainage system and drains of inadequate capacity.

iii. Silting up of natural drains and river beds with sediments due to erosion in the

catchments area.

iv. Encroachment of flood plain by human settlement.

v. Construction of structures on river banks and beds.

vi. Highly meandering streams.

vii. Sudden failure of water retaining structures.

15.1 THE PROBLEMS OF FLOOD AND THE NEED TO FIND

SOLUTION

Civilization has always developed along rivers, whose presence guaranteed

access to and from the sea coast, irrigation for crop water supplies for urban communities

and latterly power development and industrial water supply. The many advantages have

always been counter balanced by the dangers of floods and in the past levees or flood

banks were built along many major rivers to prevent inundation in the flood season.

Flood-damage mitigation was distinguished from drainage as embracing

methods for combating the effects of excess water in streams. More commonly called

flood control in the United States, the terminology flood-damage mitigation has been

adopted in Australian practice to emphasize that absolute control over flood is verily

feasible either physically or economically. What we seek to do is to reduce flood damage

to a minimum consistent with the cost involved. The flood is the result of run off from

rainfall in quantities too great to be confined in the low-water channels of streams. Man

85

can do little to prevent a major flood, but he may be able to minimize damage to crops

and properly with in the flood plains of the river.

The commonly accepted measures for reducing flood damage are: -

� Reduction of peak flow by reservoirs

� Confinement of the flow with in a pre-determined channel by levees, flood walls

or a closed conduit

� Reduction of peak stage by increased velocities resulting from channel

improvement.

� Diversion of flood water through by passes or flood ways to other channel or even

another water shed.

� Reduction of flood run off by land management

� Temporary evaluation of flood threatened areas on the basis of flood warnings.

15.2 TYPES OF FLOOD CONTROL STRUCTURE

The types of flood control structures usually used are: -

1. Flood mitigation reservoirs

2. Levees and flood walls.

FLOOD MITIGATION RESERVOIRS: - This is a structure used to store a portion of the

flood flow so as to minimize the flood peak at the point to be protected. The reservoir is

situated immediately upstream from the protected area and is operated to cut off the flood

peak. This is accomplished by discharging all reservoirs in flow until in flow drops below

the safe channel capacity, and the stored water is reduced to recover storage capacity for

the next flood.

If there is some distance between the reservoir and the protected area but

no local in flow between these points, the reservoir operation will be quite similar.

15.3 LEVEES AND FLOOD WALLS

One of the oldest most widely used methods of protecting land from flood water is to

erect a barrier preventing over flow levees and flood walls are essentially longitudinal

dams erected roughly parallel to a river rather than across its channel. A levee is an earth

like, while a flood wall is usually of masonry construction.

86

In general, levees and flood walls must satisfy the same structural criteria as regular

dams. Levees are most frequently used for flood mitigation because they can be built at

relatively low cost of materials available at the site. Levees are usually built of excavated

material from borrow pits paralleling the levee line. The material should be placed in

layers and compacted, with the least pervious material along the river side of the levee.

Usually there is no suitable material for a core, and most levees are homogeneous

embankments.

87