lecture # 2 - irrigation engineering by sir asfaque

8/3/2019 Lecture # 2 - Irrigation Engineering by Sir Asfaque

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Suitability of Soil for Irrigation

The soil should be carefully studied with regard to thefol1owing :

(a) Size of soil particles

(b) Compactness

(c) Depth

(d) Organic matter content

(e) Position of water table.

All the above aspects influence the depth of available water that

the irrigator can store in the root zone of soil in a single

application of water and hence influence the required frequency

of watering.


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Preparation of Land for Irrigation

The uncultivated land should be properly prepared, as following, before irrigation

water is applied upon it.

(i) Removal of thick jungle, bushes etc., from the raw land. The roots of the trees

should be extracted and burnt. The land should thereafter be properly cleaned.

(ii) The land should be made level. High patches should be scraped and depression

filled. Unless this is done, water will fill the depression and duty may be too high.

(iii) The land should be provided with regular slope in the direction of falling gradient.

(iv) The land should be divided into suitable plots by small levees according to the

method of irrigation to be practiced.

(v) Permanent supply ditches and water courses should be excavated at regular

spacing which facilitate proper distribution of the water to the entire field.

(vi) A drain ditch which carries the waste water should also be excavated. .

(vii) Proper drainage measures should be adopted where the danger of water logging

may become eminent after the introduction of canal irrigation


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Classes of Soil Water

Water present in the soil may be classified under

three heads:

1. Hygroscopic water: When an oven dried sample

is kept open in the atmosphere, it absorbs some

amount of water from the atmosphere. This is

known as hygroscopic water, and is not capable of

movement by the action of gravity or capillary

forces

2. Capillary water: Capillary water is that part, in

excess of hygroscopic water, which exists in the

pore space of the soil by molecular attraction.

3. Gravitational water: Gravitational water is that

part in excess of hygroscopic and capillary water

which will move out of the soil if favourable

drainage is provided.


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Availability of Soil Water

Soil moisture is always being subjected to pressure gradients and vapour pressure

differences that cause it to move. Certain moisture contents are of particular

significance, often called soil moisture constants, with regards to irrigation andagriculture engineering, viz:

Saturation capacity: When all the pores of the soil are filled with water, the soil is said

to be under saturation capacity or maximum water-holding capacity. The tension of

water at saturation capacity is almost zero and it is equal to free water surface.

Field capacity: The field capacity of soil is the moisture content after the drainage ofgravitational water has become very slow and the moisture content has become

relatively stable.

This situation usually exists for one to three days after the soil has been thoroughly

wetted by rain or irrigation.

At field capacity, the large soil pores are filled with air, the micro pores are filled with

water and any further drainage is slow.

The field capacity is the upper limit of available moisture range in soil moisture and

plant relations.


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Moisture equivalent:

This is an artificial moisture property of the soil and is used as an index of the

natural properties.

It is the percentage of moisture retained in a small sample of wet soil 1 cm

deep when subjected to a centrifugal force 1000 times as great as gravity,

usually for a period of 30 minutes.

Moisture equivalent is used as a single factor to which the properties of soilcan be related within reasonable limits.

The moisture equivalent roughly equals field capacity for a medium textured

soil. The relation between these are as follows:

Moisture equivalent Field capacity

= 1.8 to 2 Permanent wilting point

= 2.7 Hygroscopic coefficient


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Permanent WiltingPoint

Permanent wilting point or the wilting coefficientis that water content at which

plants can no longer extract sufficient water from the soil for its growth.

It is at the lower end of the available moisture range.

If the plant does not get sufficient water to meet its needs, it will wilt

permanently.

Temporary wilting may sometime take place during the hot windy day, but it

will recover in the cooler portion of the day.

A plant is considered to be permanently wilted when it will not recover after

being placed in a saturated atmosphere.

Recent studies indicate that the wilting point is closely indicated by themoisture retained against a tension of 15 atm.

As an approximation, the permanent wilting % can be estimated by dividing

the field capacity by a factor varying from 2.0 to 2.4, depending upon the

amount of silt in the soil. For most of the soils, the wilting coefficient is about

150% of the hygroscopic water.


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Available Moisture:

The difference in wattr content of the soil between field capacity and

permanent wilting is known as available water or available moisture.

Readily Available Moisture:

It is that portion of the available moisture that is most easily extracted by

plants, and is approximately 75% of the available moisture.


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SoilMoisture Deficiency:

Soil moisture deficiency or field moisture deficiency is the water required to bring the

soil moisture content of the soil to its field capacity.

Depth of water stored in root zone.

In order to estimate the depth of water stored in the root zone of soil containing water

up to field capacity, let,

d= depth of root zone (in metres) ; Fc = field capacity (expressed as ratio);

= unit weight of soil; and w = unit weight of water.

Considering unit area (1 sq. metre) of soil area;

This depth of water will be available for evapo-transpiration.w

c

c

dF

dF

K

K

K

..areaunitinstoredwaterofDepth

..areaunitinretainedwaterofWeight

!@

!@

? AtCoefficien

W

ilting-CapacityField

.

depthmoistureAvailable w

d

K

K

!

d1

areaunitinretainedwaterofWeight

areaunitofsoilofWeight

areaunitinretainedwaterofWeight

vv

!

!

K

cF


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Water requirements of a crop

Every crop requires a certain quantity of water after a certain fixed interval,

throughout its period of growth.

If the natural rain is sufficient and timely so as to satisfy both these

requirements, no irrigation water is required for raising that crop.

In England, for example, the natural rainfall satisfies both these requirements

for practically all crops, and, therefore, irrigation is not significantly needed inEngland.

But in a tropical country like Pakistan, the natural rainfall is either insufficient,

or the water does not fall after fixed intervals, as required by the crops.

Since the magnitude as well as the frequency of the rainfall varies throughout a

tropical country, certain crop may require irrigation in certain part of the

country, and the same crop may not require any irrigation in some other part of

the country.


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Limiting Soil Moisture Conditions


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Depth and Frequency of Irrigation


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The area where irrigation is a must for agriculture is called the

arid region, while the area in which inferior crops can be grown

without irrigation is called a semi-arid region.

The term Water requirements of a crop means the total quantity

and the way in which a crop requires water, from the time it is

sown to the time it is harvested.

water requirement varies with the crop as well as with the place.

In other words, different crops will have different water

requirements and the same crop may have different water

requirements at different places of the same country depending

upon the climate, type of soil, method of cultivation, and useful

rainfall, etc.


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Crop Period or Base Period

The time period that elapses from the instant of its sowing to the instant of its

harvesting is called the crop-period.

The time between the first watering of a crop at the time of its sowing to its

last watering before harvesting is called the Base period.

Crop period is slightly more than the base period, but for all practical purposes,

they are taken as one and the same thing, and generally expressed in B days.


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Delta

Each crop requires a certain amount of water after a certain fixed interval of time,

throughout its period of growth.

The depth of water required every time, generally varies from 5 to 10 cmdepending upon the type of the crop.

If this depth of water is required five times during the base period, then the total

water required by the crop for its full growth, will be 5 multiplied by each time

depth. The final figure will represent the total quantity of water required by the

crop for its full-fledged nourishment.The total quantity of water required by the crop for its full growth may be

expressed in hectare-metre (Acre-ft) or million cubic meters (million cubic ft).

This total depth of water (in cm) required by a crop to come to maturity is called

its delta ().

orDelta is the total depth of the water required by a crop during the entire period the

crop is in the field and it is denoted by the symbol ().


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Example 1

If rice requires about 10 cm depth of water at an average interval of about 10 days.

and the crop period for rice is 120 days, find out the delta for rice.

Solution.Water is required at an interval of 10 days for a period of 120 days.

Hence, No. of required waterings = 120/10 = 12

Therefore, Total depth of water required = No. of waterings x Depth of watering

= 12 x 10 cm = 120 cm.

Hence, for rice =120 cm. Ans.

Example 2

If wheat requires about 7.5 cm of water after every 28 days, and the base period for

wheat is 140 days, find out the value of delta for wheat.

Solution.

No. of required waterings = 140/28 = 5

The depth of water required each time = 7.5 cm.

:. Total depth of water reqd. in 140 days = 5 x 7.5 cm = 37.5 cm

Hence, for wheat = 37.5 cm. Ans.


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Average Approximate Values of for Certain Important Crops in Pakistan


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Irrigation requirements ofCertain Important crops


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Duty of Water.

The duty of water is the relationship between the volume of water and the area

of the crop it matures.

This volume of water is generally expressed as, a unit discharge flowing for a

time equal to the base period of the crop, called Base of a duty.

If water flowing at a rate of one cubic metre per second, runs continuously for

B days, and matures 200 hectares, then the duty of water for that particular

crop will be defined as 200 hectares per cumec to the base ofB days.

Hence, duty is defined as the area irrigated per cumec of discharge running for

base period B. The duty is generally represented by the letterD.

Or

Duty represents the irrigation capacity of unit water. It is the relation between

the area of the crop irrigated and the quantity of irrigation water required

during the entire period of that crop.


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Relation between Duty, Delta and Base period

Let, base period of the crop be Bdays, and

one cumec of water be applied to this crop on the field forB

days.Now, volume of water applied to this crop duringBdays

= V= (1 x 60 x 60 x 24 x B) m3

= 86,400 B m3

By definition of duty (D), one cubic meter supplied forB days matures D hectaresof land.

:. This quantity of water (V) maturesD hectares of land or 104D sq. m of area.

Total depth of water applied on this land

= Volume/area = 86400 B / 104D = 8.64 B/ D metres

By definition, this total depth of water is called delta (),

= 8.64 B/D meter

= 864B /D cm

where, is in cm, B is in days ; and D is duty in hectares/cumec.


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Example

Find the delta for a crop when its duty is 864 hectares/cumec on the field. The base

period of this crop is 120 days.

Solution.In this question, B = 120 days; and D = 864 hectares/cumec

Since, = 864 B / D cm

= 864 x 120 / 864

= 120 cm

lecture # 2 - irrigation engineering by sir asfaque

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