Irrigation water management

IRRIGATION WATER
MANAGEMENT
AS PER GTU SYLLEBUS B.E. VIII CIVIL ENGINEERING
DARSHAN INSTITUTE OF ENGINEERING & TECHNOLOGY
ASST. PROF. VIDHI KHOKHANI

IRRIGATION WATER MANAGEMENT

CHAPTER 3
IRRIGATION EFFICIENCY,& SCHEDULING
3.1 IRRIGATION EFFICIENCY
 Irrigation efficiency Ei is the ratio, usually expressed as per cent, of the amount of
water used to meet the consumptive use requirement of the crop plus that
necessary to maintain a favourable salt balance in the crop root zone to the total
volume of water diverted, stored or pumped for irrigation.
 Irrigation systems are designed and operated to supply the irrigation requirement
to each fie'd of the farm.
 The performance of an irrigation system determined by the efficiency with which
water is stored in the surface reservoir at the head works, diverted and conveyed to
the irrigated area through the conveyance system and applied to the fields and by
the adequacy and uniformity of water application in each field.
 The overall efficiency of an irrigation system is defined as the per cent of water
supplied to the farm that is beneficially used for irrigation on the farm.
 Overall system efficiency, often known as irrigation efficiency, is expressed as
follows:
Et = 100 (S-P-R-O)/S
in which,
Ei = Irrigation efficiency %
1= Irrigation requirement
L = Leaching requirement
S = Amount of water supplied to the farm
P = Total deep percolation on the farm
R - Total runoff from the farm
O = Operational losses in channels and pipelines
Water losses in an irrigation system can be considered in two groups, namely,
1. the losses in the storage reservoir and the conveyance system, and
2. the losses in the field.
 Losses in the storage reservoir are due to evaporation from the water surface in the
reservoir, seepage losses from the sides and bottom of the reservoir and spillage
from the reservoir.
 Conveyance losses include seepage losses in the canal system, spillage due to
overtopping of the canals and breaches and rat holes in the canal bunds, runoff to
drains from the canals and evaporations from the water surface in the canal
network Field losses are mainly due to deep percolation below the crop root zone
and runoff from irrigated fields in case of land served by surface irrigation methods
and evaporation during water application in case of land served by sprinkler
irrigation.

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 In evaluating the performance of irrigation system it is useful to examine the
efficiency of each component of the system. This allows identification of the
components which are not performing well and taking suitable measures to
improve them.
 The overall system efficiency may be expressed as follows –

in which,
Eo = Irrigation efficiency overall (%)
Er = Reservoir storage efficiency (%)
Ec = Water conveyance efficiency (%)
Ea - Irrigation application efficiency
Reservoir storage efficiency –
The term defines the efficiency with which water is stored in a reservoir. Reservoir
storage efficiency is expressed as

in which, Ve = Evaporation volume from the reservoir
Vs = Seepage volume from the reservoir
V, = Inflow to the reservoir during a time interval
V0 = Volume of outflow from the reservoir during time interval
S = Change in reservoir storage during the time interval.
The S term is often neglected when long time periods are considered.
Example 3.1. A Persian wheel discharges at the rate of 11,000 litres per hour and works for
eight hours each day. Estimate the area commanded by the water lift if the average depth of
irrigation is 8 cm and irrigation interval is 15 days.
Solution:
The discharge of Persian wheel in 15 days
Water required to cover one hectare to a depth of 8 cm
Area commanded by Persian wheel

Example 3.2. The following data were obtained in determining the soil moisture content at
successive depths in the root zone prior to applying irrigation water:


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Depth of sampling

Wt. of moist soil sample

Oven dry wt. of soil
sample

Cm
gm
gm
0-25
134.6
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 zone was 1.50 gm/cc. The available moisture holding
capacity of the soil was 17.8 cm/m depth. Determine
(i) the moisture content at the different depths 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 mositure content to field capacity, and
(iv) gross irrigation requirement at an estimated field irrigation efficiency of 70 per cent.
Solution:
(i)
Soil moisture content at different depths
Depth of root
Moisture content before
Depth of water required at
irrigation
root zone (omc *ϒ*d)
Cm
In %
In cm
0-25
6.13 x 1.5 x 0.25 = 2.30
25-50

6.53 x 1.5 x 0.25 = 2.44

50-75

6.60 x 1.5 x 0.25 = 2.47

75-100

8.06 x 1.5 x 0.25 = 3.02

(ii) Moisture content in the root zone at the time of irrigation
= 2.30 + 2.44 + 2.47 + 3.02 = 10.23 cm
(iii)Net irrigation requirement
= 17.80 - 10.25 = 7.55 cm
(ic)Gross irrigation requirement
=7.55/70 x 100 = 10.78 cm
Example 3.3. Compute the gross depth of water application, the total number of irrigations
and the irrigation interval of tomatoes of 5 months growing period in loamy soil. The
irrigation water need of tomatoes for the whole growing period, estimated month wise is 720
mm.
solution: Net depth of irrigation for tometoes in loamy soil is 40 mm.

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Therefore , the gross irrigation depth = 720/40 =18 mm
The growing season = 5 x 30 = 150 days.
Irrigation interval = 150/18 = 8.33 days. rounded of to 8 days.
Example 3.4. Compute the efficiency of an irrigation system based on the following data:
1800 litres / min of water is diverted to the farm each day of 24 hrs. Each day 0.8 ha of maize
and 1 ha of wheat are irrigated. The irrigation requirement of maize is 8 cm and that of
wheat 12 cm.
Solution: Assuming that the leaching requirement is nil
Efficiency of an irrigation = volume of water diverted to field/ volume of water required
Volume of water diverted = 1800 litres/min = (1800 x 60 x 24)/1000 = 2592 cubic metres
Volume of water required to irrigate maize = area x depth = 0.8 x 0.08 x 1000 = 640 cubic
metres
Volume of water required to irrigate wheat =0.12 x l x 1000 = 1200 cubic metres
Total water use by maize and wheat = 640 + 1200 = 1840 cubic metres
Efficiency of an irrigation = 1840 / 2592 = 70.99%, say 71%.
Example 3.5. Compute the reservoir storage efficiency for a 24-hr period when 3100
liters/minute of water are being diverted from the reservoir based on the following data: The
rate of inflow into the reservoir of 4420 litres/minute.
S = 410 cubic metres (the quantity of to be removed to restore the initial water level in the
reservoir.
Solution- VI = 4420 litres/minute: = 6364.8 cubic metres per day
VO = 3790 litres/minute = 5457.6 cubic metres per day
S= 410 cubic metres (given)
92.2 %

3.2 PROJECT IRRIGATION EFFICIENCY
 Irrigation efficiency is usually expressed as the percentage ratio of the amount of
water stored in crop root zone for crop use in the project command area to the
amount of water diverted from the project source. It is expressed as,

where,
Ep = project irrigation efficiency in per cent
Ws = amount of water stored in crop root zone soil
Wd = amount of water diverted or pumped from the source
 COMPONENTS OF PROJECT IRRIGATION EFFICIENCY
Irrigation efficiency may be considered in stages from the point of diversion of
water from a source to its actual use by crops.
The components are: (i) water conveyance efficiency EC and
(ii) water application efficiency Ea.

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where,
Ep = project irrigation efficiency in per cent
Ec = water conveyance efficiency in per cent
Ea = water application efficiency in per cent
(i)
Water Conveyance Efficiency(IF QUESTION CONTAIN 3 TO 4 MARKS)
Water is conveyed through canal network, watercourses and channels from sources such
as reservoirs, rivers and dams to fields or farms for crop use. Conveyance efficiency is used
to evaluate the efficiency of the system conveying water.
It is also used to measure the efficiency of channels conveying water from wells and ponds
to fields.
Water conveyance efficiency may be defined as the percentage ratio of the amount of water
delivered to fields or farms to the amount of water diverted from sources.
It is expressed as,

where,
Ec = water conveyance efficiency in per cent
Wf = amount of water delivered to fields or farms (at the head of field supply channel or
farm distribution system)
Wd = amount of water diverted from sources

(NOTE - IF THE QUESTION IS EXPLAIN IT FOR SEVEN MARKS THEN
ALSO WRITE FOLLOWING three steps shown in capital letters)
1. STAGES OF CONVEYANCE EFFICIENCY –
 The conveyance system for Water supplied to farms from sources through
irrigation agencies.
 The conveyance system within a farm for irrigating fields from outside sources
such as canal. Thus the farmer himself to evaluate the water conveyance system in
his farm.
2. FACTORS AFFECTING CONVEYANCE EFFICIENCY/effect of losses on conveyance
efficiency
 It is affected by water losses through evaporation, seepage, and transpiration by
undesired vegetation and leakage through water control structures in the
conveyance system.
 The losses may vary from 25 to 60 per cent of the water diverted for irrigation.
 In unlined canals, waterways and channels, water loss is heavy mainly due to
seepage.

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 Soil permeability, wetting perimeter and depth of water in conveyance structures,
depth of water table below the canal bed, time for which water is run, temperature
of water and types of materials used in construction of the conveyance system are
the principal factors influencing the seepage loss.
 There is often a growth of undesirable vegetation along and in canals and in channel
beds and sides. The loss through transpiration by this vegetation may sometimes be
considerable.
 Occasionally, animals like rats and rabbits make burrows in channel bunds. There
may be cracks in channel beds and bunds leading to water losses.
3. IMPROVEMENT MEASURES FOR CONVEYANCE EFFICIENCY
 The efficiency can be improved considerably by lining canals, waterways and
channels with impervious materials like bricks or stone masonry, bitumen clay
mixture, concrete slabs, asphalt membrane and so on.
 High-density polyethylene sheets may be used in irrigation channels as a cheap
lining material.
 Repairs of cracks, holes, burrows, erosion damages, leaks in water control
structures should be done as a part of continuous maintenance.
 Weed growth should not be allowed in unlined canals, waterways and field
channels.
 Pipes may be laid for water conveyance in farms or wherever feasible to cut the
water conveyance losses.
 Lining of conveyance system or laying pipes to convey water is a costly affair that
becomes prohibitive in many occasions.
 However, the extent of water losses, water scarcity and its high cost demand
particularly that the conveyance system should have lining.
(ii)
Water Application Efficiency
 Water application efficiency refers to the efficiency of water application to fields.
Water is applied to fields by various methods. Those may be surface, subsurface,
sprinkler or drip irrigation methods.
 The efficiency of those methods individually or for single irrigation or irrigation of
farm fields may be estimated.
 The water application efficiency may be defined as the percentage ratio of the
amount of water stored in the crop root zone to the amount of water delivered to
fields. It is expressed as,

where,
Ea = water application efficiency in per cent
Ws = amount of water stored in the crop root zone soil
Wf = amount of water delivered to fields.

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(NOTE - IF THE QUESTION IS EXPLAIN IT FOR SEVEN MARKS THEN
ALSO WRITE FOLLOWING two steps shown in capital letters)
1. EFFECT OF LOSSES ON APPLICATION EFFICIENCY
 This efficiency is determined to evaluate the irrigation practices in a farm. It
accounts for loss of water by seepage in the supply channel, deep percolation and
occasionally run-off occurring in fields.
 Application losses of water in crops other than rice, on an average, account for about
17 per cent of the water supply reaching the fields.
 The deep percolation loss in wet rice is exceptionally high and ranges from 38.3 per
cent to as much as 80 per cent of the water applied in various soils.
 The efficiency may be very low in a badly managed farm and 75 per cent in a wellmanaged farm. It can however be increased to approach 100 per cent if crops are
under-irrigated by applying lower amount water than needed.
 It is sometimes done because of water scarcity or high-priced water. Underirrigation may completely prevent deep percolation and run-off of water, but it is
undesirable as crops suffer from water stress and give low yields.
 Improper land levelling and grading, faulty choice of irrigation methods, application
of excess water, frequent irrigation, very small or very large stream sizes, improper
attentions during irrigation by the irrigator and faulty design of fields are the
principal factors that cause low efficiencies.
2. IMPROVEMENT MEASURES FOR APPLICATION EFFICIENCY
 Proper land levelling and grading is a prerequisite for efficient water application.
This is needed to avoid accumulation of excess water in lower spots leading to deep
percolation loss and under-irrigation of higher spots, and to achieve uniform run
and distribution of water in the field.
 Proper selection of irrigation methods according to crops, soil types, topography,
climate and stream sizes are important to secure high efficiency through uniform
application of water and preventing the deep percolation and run-off losses.
 There is a tendency of farmers to frequently irrigate and give excess water to the
crops when the supply of water is abundant.
 Besides, absence of water measuring devices and ignorance of farmers in deciding
the time of irrigation and quantity of water required for irrigation often pose
problems.
 A smaller size stream needs a longer time to irrigate a long and big strip of land and
causes greater infiltration and deep percolation of water at the upper reach of the
field.
 On the other hand, a bigger size stream makes effective control of flow difficult and
may cause surface run-off. The size of a stream requires to be decided based on soil
type, method of irrigation, length of water run and depth of water to be applied.
 The size of field and length of water run are determined according to stream sizes
available, intake rates of soils, methods of irrigation adopted, prevailing land grade

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and topography. Longer water run causes a greater intake of water and
consequently deep percolation near the entry point of the stream.

3.3EFFICIENCY OF IRRIGATION PRACTICES,
OPERATION OF IRRIGATION SYSTEM

WATER

USE

AND

3.3.1 Water Distribution Efficiency
 Water distribution efficiency measures the extent to which water is uniformly
distributed and stored in the effective root zone soil along the irrigation run.
 Not only the application of the right amount of water to the held but also its uniform
distribution over the field is important.
 Permissible lengths of irrigation runs are controlled to a large extent by the
uniformity of water distribution which is possible for a given soil and irrigation
management practice.
 Water distribution efficiency indicates the extent to which water is uniformly
distributed in the field.
It is described as.

where,
Ed = water distribution efficiency in per cent
y = average numerical deviation in depth of water stored in root zone soil along the
irrigation run from the average depth of water stored during irrigation
d = average depth of water stored during irrigation along the water run
 Water distribution efficiency dictates the permissible length of irrigation run. It
provides a measure of efficiency of an irrigation system or method over the other.
3.3.2 Water Use Efficiency
 Water use efficiency is determined to evaluate the benefit of applied water through
economic crop production. It is very important in crop production and irrigation
water management. It is described in the following two ways:
(i)
Field water use efficiency.
This may be defined as the ratio of the amount of economic crop yield to the
amount of water required for crop growing. It is obtained as follows,
where,
Eu = field water use efficiency expressed in kilogram of economic yield per hectarecm or hectare-mm of water
Y = economic crop yield in kilogram per hectare
WR = water requirement of the crop in hectare-cm or hectare-mm
(ii)
Crop water use efficiency.

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This may be defined as the ratio of the amount of economic yield of a crop to the
amount of water consumptively used by the crop. It is found out as follows,









where,
Ecu = c r o P water use efficiency in kilogram of economic yield per hectare cm or
hectare-mm of water
WUE = water use efficiency of crop in kilogram of economic yield per hectare-cm or
hectare-mm of water
Y = economic yield of crop in kilogram per hectare
CU = consumptive use of water in hectare-cm or hectare-mm
ET = evapotranspiration in hectare-cm or hectare-mm
Water use efficiency is influenced by changes in either numerator or denominator
or both. The numerator is the crop yield that depends on various factors of crop
production and losses due to pests, diseases, weather hazards and other
environmental conditions.
The denominator is influenced by various plant, soil and climatic conditions and by
soil and crop management practices.
An increase in crop yields or decrease in consumptive use and water requirement of
crops improves the water use efficiency.
Factors of crop production influencing crop yields are many. Climatic conditions,
amount, distribution and intensity of rainfall, occurrences of drought, soil
characteristics, drainage, irrigation, fertilizers use, crop varieties, crop management
practices and prevalence of weeds, pests and diseases are some very important
factors.
The consumptive use of crops is similarly influenced by climatic conditions, soil
characteristics, crop types and varieties, water supply and irrigation practices,
tillage practices, weed control, fertilizer use and use of mulch, antitranspirants and
growth retardants.

3.3.4 Increasing Water Use Efficiency by crop and Soil Management Practices
 Water use efficiency (crop yield/evapotranspiration of crop area) is influenced by
crop and soil management practices.
 The numerator of this formula, namely, crop yield can be changed appreciably by
management practices.
 The evapotranspiration or denominator of the formula is more difficult for man to
control because it is dependent on climate and the availability of water for the crop.
 Water use efficiency is not closely dependent on the water available if the supply is
within the evapotranspiration limit.
 The crop yields depend on the adequacy of the water supply. Most crops are
sensitive to water stress at the critical phases of their growth.
plant species adaptation.
One of the primary ways of increasing crop yield and water use efficiency in a particular
environment is to select plant species adapted to the total amount and distribution of

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water in an area. Plant species vary greatly in water requirement. Species selection, plant
selection and plant breeding have contributed greatly to increasing the efficiency of the
water available in an area.
Planting pattern.
Planting patterns have a direct effect on yield, solar energy capture, and evaporation and
thus an indirect effect on water use efficiency. Two important planting pattern practices
are plant density and row spacing.
Planting date.
The main reason for choosing the optimum dates for sowing is to ensure good germination,
by placing the seed in the optimum moisture zone, neither too deep nor very shallow. After
the seeds germinate, the moisture should be optimum for root growth and root penetration
and to envelope maximum soil volume for nutrient uptake. Planting dates also indicate the
right type of climate for the shoot growth and optimum utilisation of moisture by the roots
under normal rainfall conditions. Short season and early maturing crop species are often
grown to escape the draught part of the season.
Weed control.
One of the main management means of obtaining more efficient water use is the
elimination of weeds in crops. Weeds compete with crops for soil nutrients, water and light.
Technology for efficient weed control, both mechanical and chemical, have been developed
which should be availed in increasing water use efficiency of crop plants.
Disease and insect pests.
An often overlooked means of increasing water use efficiency lies in the area of disease and
pest management of crops. It is possible to select crop varieties which are resistant to pests
and diseases common to an area. Efficient crop and water management also includes the
judicious use of chemical pesticides and selecting appropriate cropping patterns and
cropping sequences. Growing the same crop year after year on a land tend to increase
disease incidences in crops. Planting dates could influence pest incidence. For example, in
North India the recommended sowing date for mustard crop is in the early part of the last
week of September. Delayed sowing will result in delayed flowering which usually
coincides with the incidence of aphids.
.
3.3.5 Economic irrigation efficiency –
Economic efficiency is ratio of total production attained with the operating
irrigation system, compared to the total production expected under ideal
condition. This parameter is a measure of overall efficiency, because it relates
the final output to input.
EXAMPLES – FROM DILIPKUMAR MAJUMDAR -


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Example 10.17. A stream of 135 litres per second was diverted from a canal and 100 litres
per second was delivered to the field. An area of 1.6 hectare was irrigated in eight hours. The
effective depth of root was 1.8 m. The runoff loss in the field was 432 cu. m. the depth of water
penetration varied linearly from 1.8 m at the head end of the field to 1.2 m at the tail end.
Available moisture holding capacity of the soil is 20 cm per meter depth of soil. Determine the
water conveyance efficiency water application efficiency, water storage efficiency and water
distribution efficiency. Irrigation was started at a moisture extraction level of 50 per cent of
the available moisture.

3.3.6 CROP - WATER RELATIONSHIP
1. Water utilization efficiency and harvest index.
Water requirement and yield of crops vary between crops and their
varieties. The total dry matter and yield produced per unit quantity of water (kg per
cubic metre) vary with the crop. This is expressed as the water utilization efficiency

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in kg per cubic metre. Usually only a part of the dry matter like grain, oil content or
sugar content is useful. The ratio between the total dry matter and the harvested
yield is known as the harvest index. The harvest index varies with the crop and its
variety.
2. Yield response factor of a crop.
The response of crop yield to water supply is quantified using the yield
response factor ky, which relates relative yield decrease (1 - Ya/Ym) to relative
evapotranspiration deficit (1 – ETa/ETm). Results of research on actual crop yield Ya
in relation to maximum potential yield Ym under different water supply regimes
have resulted in the following empirical relationship
(1 - Ya / Ym) = ky (l- ETa/ ETm ...(10.65)
in which,
Ky= Yield response factor of a crop
Ya= Actual harvested yield of the crop
Ym= Maximum (potential) harvested crop yield
ETa= Actual evapotranspiration
ETm= Maximum potential evaporation
In addition to the availability of water in terms of the amount, time, and crop
growth stage, crop yields are influenced by its variety, density of plants, nutrient
availability, soil salinity, and control of crop pests and diseases.

3.3.7 Crop response to Water at Different stages of Growth
 It has been observed that water requirement of crops varies with the stage of its
growth. When water supply is limited, it is necessary to take into account the critical
stages of crop growth with respect to moisture.
 The term critical stage is commonly used to define the stage of growth when plants
are most sensitive to shortage of water.
 Each crop has certain critical stages at which if there is shortage of moisture, yield is
reduced.
 When there is shortage of water, it is better take care of the critical stages first to
obtain increased water use efficiency.
 The critical stages of crops growth are required to be studied for different crops.
Growth stages of cereals in relation to irrigation.
The following terms are used to describe the growth and developmental stages of grain
crop in relation to irrigation.
Stages
Details
Germination
the appearance of the radicle,
Tillering
the formation of tillers, i.e., branches produced from the base of the stem.
Jointing
the stage when 2 nods can be seen, i.e., the beginning of shooting.
(During this stage the internodes of the stem are elongating).
Shooting
the stage of elongation of internodes.
Booting
end of shooting stage and just prior to the emergence of ears.
Heading
the emergence of the ear from the tube formed by the leaf sheaths,
Flowering
the opening of the flowers.

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Grain
formation

the period of grain development from fertilization until maturity,
which can be further subdivided into:
'milk stage'-grain contents have a milky consistency,
'dough stage'-grain contents have a doughy consistency,
'waxy-ripe'-grain contents have a waxy appearance,
'full-ripe'-grain contents are hard,
'dead-ripe'-grain ripe for harvesting.

3.4 IRRIGATION SCHEDULING (IF QUESTION OF 7 MARKS THEN ALSO WRITE
3.4.1,3.4.2,3.4.3 and list out irrigation indicies and explain each shortly in one to two
line)
Irrigation scheduling is “the process of determining the time to irrigate and how much
water is to be applied (irrigation depth) in each irrigation”. Proper scheduling is essential
for the efficient use of water and other inputs in crop production. Irrigation schedules are
planned to either fully or partially provide the estimated water requirement of the crop.
3.4.1 Amount of water to apply per irrigation. (water requirement of
irrigation)
With ‘full irrigation’ the amount of water required per irrigation is computed as follows:
1 = Dr(fc-fm)/irrigation efficiency
in which,
Dr = Depth of root zone
fc = Soil moisture content at field capacit
fm = Soil moisture content prior to irrigat
1. Full irrigation provides adequate water to meet the entire irrigation requirement
and is aimed at achieving the maximum production potential of the crop. Excess
irrigation, however, reduces crop yields by adversely influencing soil physical
properties like soil aeration soil temperature and the microbial activities in the soil.
Full irrigation is justified when there is no scarcity of water and the cost of irrigation
is low.
2. Deficit irrigation meets the water requirement of crops only partially. It is
economically justified when reducing the amount of irrigation supply below the full
level causes production costs to decrease faster than the decline in the value of crop
harvest. Deficit irrigation is practiced when there is water scarcity or when the
irrigation system capacity is limited.
With ‘deficit’ irrigation the crop root zone is not always filled to the field capacity
moisture content level. In locations with appreciable amounts of precipitation
during the irrigation season, it is possible to fill the root zone only partially so that
some precipitation can be stored in the crop root zone.
Many research studies indicate that with increasing water shortage globally it may
be more economical to plan irrigation system to obtain economical crop production
per unit of land. However, care should be taken to ensure that there is no water
shortage during the 'critical stages' in the growth of the crop.

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3.4.2 Irrigation schedules based on prediction of ET.
Irrigations can be suitably scheduled on a farm if the allowable maximum water depletion
in the crop root zone and the evapotranspiration (ET) for short periods during the growing
period are known.
3.4.3 Irrigation scheduling based on pan evaporation.
Prihar et al. (1974) suggested a 'practical approach' relating irrigation requirement to the
cumulative evaporation during a time period as measured in a standard open pan. The
relationship is expressed as IW/Epan, in which IW is the depth of irrigation water and Epan
is the cumulative open pan evaporation. The rainfall during the period, if any, is subtracted
from the value of Epan. The validity of the procedure will depend on the proper installation
of the evaporation pan and the rain gauge and the precision in measurements of pan
evaporation and rainfall. Further, the suitability of the method is situation-specific and
limited to the particular variety of a crops.

3.5 Irrigation Indices
Scientific irrigation must be based on an understanding of the soil-water-plantatmospheric relationship. Irrigation needs of crops depend on the evaporation demand of
the ambient atmosphere, soil-water regime in the crop root zone and the plant foliage,
1. plant symptoms.
Visual symptoms in plants to decide on the time to irrigate are changes in the colour
of plants curling of the leaves and tendencies of wilting, these are observed by
looking at the crop as a whole and not individual plants. When the crop comes under
water stress, appearance changes from vigorous growth to retarded growth and
often there is marked change in the colour of the leaf which gets darker and
sometimes grey.
However, symptoms may sometimes be misleading due to changes in crop varieties
and nutritional disorders as well as insect and pest incidence.
2. water content and water potential.
The status of water in the plant is generally indexed from the measurement of leafwater content and leaf-water potential. Their values at any time of the day will
depend on the lag between the evaporative demand of the atmosphere and the
moisture uptake rate by the crop. When their values fall below certain critical limits
specific to the plant species and their growth stages, important physiological and
growth factors are adversely affected. Hence, these values can serve as reliable
indicators for irrigating crops,
3. plant temperature.
Almost the entire radiation energy received on the leaf surface during the daylight
hours is utilized for evapotranspiration (ET). The remaining energy, if any, is used to
heat the leaf tissues and the ambient air. When ET reduces due to water deficit in
the plant, the energy saved in the process is partly used to raise the leaf

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temperature. Many investigations have shown that the leaf canopy temperature is a
sensitive index in crops.
4. Soil water regime.
A commonly recommended method to decide on irrigation schedules is soil
moisture measurements in the field. When the soil moisture content has dropped to
a certain critical level, say about 50% of field capacity level in the crop root zone,
irrigation is applied. Normally irrigation is not delayed beyond 60% field capacity
moisture content.

3.6 General Guidelines on Planning Irrigation Schedules
The characteristics of different soil groups and soil depth influencing irrigation scheduling
are as follows:
(i) Shallow and I or sandy soil. In sandy soils in general and soils with limited depth (due to
the presence of hard pan close to the ground surface) the soil moisture holding capacity is
low. These soils require frequent irrigations and the amount of water applied in each
irrigation is kept low.
iii) Loamy soils. Loamy soils can store more water than sandy soils or shallow soils. Hence,
irrigation is applied less frequently with larger depth of water in an irrigation.
(iii) Clayey soils. The moisture holding capacity of clayey soils is more than loamy soils.
Hence, irrigations are less frequent and the quantity of water in an irrigation is higher.
Adjusting irrigation depth for non-peak periods. The most irrigation schedule is for
peak period moisture use situations. During the early growth stages of crops it is desirable
to keep the irrigation interval at shorter intervals and reduce the quantity of water used in
irrigation. If the irrigation interval is kept far apart due to the low value of ET0, the young
plants may suffer from water shortage as their roots are notable to take up water from the
lower layers of the root zone. On the other hand crops which are harvested dry like maize
need much less water at the late season stage. Hence frequency of irrigation during the late
seson is reduced.
Adjusting irrigation schedules to rainfall occurring during growing season –
the data presented mostly is on the assumption that no rainfall ocurrs during the crop
growing season. If contribution from rainfall is significant during the period of schedule is
to be suitably adjusted. This is usually done by making the irrigation interval longer or
reducing the net irrigation depth.
procedure to arrive at approximate solutions to irrigation schedule. A simple
procedure to make estimates leading to approximate values for scheduling irrigation
comprise of the following steps:
1. Estimate the net and gross irrigation depths (mm),
2. Compute the irrigation water need over the total growing season of the crop (mm),
3. Compute the total number of irrigations over the growing season,
4. Calculate irrigation interval (days).
The number of irrigation applications over the growing season of a crop is obtained by
dividing the irrigation water need during the growing season by the net irrigation depth
per application. The interval between two irrigations is obtained by dividing the number of

Page 23


days of the growing season by the number of irrigations. To be on the safe side the
irrigation interval is rounded of to the lower whole figure.
Exapie 10.13. Compute the gross depth of water application, the total number of irrigations
and the irrigation interval of tomatoes of 5 months growing period in loamy soil. The
irrigation water need of tomatoes for the whole growing period, estimated monthwise is 720
mm.
3.7 TIME OF IRRIGATION –
Time of irrigation is usually governed by two major conditions namely,
(1) water need of crops and (2) availability of irrigation water.
Water need of crops is, however the prime consideration to decide the time of irrigation.
(1) Water Needs of Crop
 Crop plants require water to meet the transpiration loss, build up body tissues and
to carry on biochemical and physiological activities within the body. Transpiration,
which is considered as a vital physiological activity of plants, occurs continuously as
long as the water supply is maintained.
 Also, a continuous evaporation occurs from the moist soil surface in crop field. After
irrigation, the evapotranspiration begins at a peak rate drawing water from the
moist soil below and continues till there is available water in soil. This causes a
continuous decline in soil water content.
 Rate of evapotranspiration decreases continuously sometime after completion of
irrigation with reduction in avalable soil water below the field capacity.
 A stage is reached within a few days after irrigation when the rate at which soil
water is available for extraction by crop plants becomes equal to the normal
consumptive use rate. This stage of soil water is considered as the lowest point of
the optimum soil water regime.
 The optimum soil water regime means the" range of available soil water in which
plants do not suffer from water stress and all the plant activities occur at an optimal
rate. Field capacity is the uppermost limit of optimum soil water regime for crops
other than rice.
 A soil water deficit below optimum soil water regime causes water stress in plants
causing decline in growth and yield, as the rate of availability of soil water falls short
of the normal consumptive use rate.
 Irrigation is, therefore, needed when this lowest limit optimum water regime is
reached and it is considered as the most opportune time for irrigation.
 The stage of available soil water below which water stress begins to cause a serious
fall in crop growth and subsequently the yield is termed as the critical level of soil
water for crop plants. This level of available soil water coincides with the lowest
level of optimum available soil water regime. The optimum available soil water
regime and the critical level of soil water may be diagrammatically represented as in
Fig. 10.1.
 Every crop has a characteristic optimum water regime. The regime varies with the
crop abilities to extract water from different soil layers.
(2) Availability of water

Page 24


 Irrigation water is often in short supply in most locations and therefore demands a
careful and economic use. Economy of water helps to bring more areas under
protective irrigation and leads to a greater crop production in areas of limited water
supply.
 In areas where water is scarce, farmers are not able to apply normal irrigation to
crops and are forced to skip some irrigations. It is therefore necessary that one
decides a priority of stages of crops when irrigations are to be applied and the
stages when one can afford to miss irrigation. The critical stages of water need of
crops receive the foremost attention. It is necessary to simultaneously consider and
weigh the relative the importance of the various stages for irrigation and the
availability of water.

 A preferential status of crop stages according to their relative importance to yield
should be considered for irrigation in areas of water scarcity.

3.8 CRITICAL STAGES OF WATER NEED OF CROPS
 During the life cycle of a crop plant, there are some crucial stages in the life cycle of a
crop plant when the plant is badly in need of water. Denial of water during these
stages causes a definite set back to growth processes and the yield is adversely
affected. These stages are referred to as the critical stages of water requirement.


Page 25


 These stages do not usually coincide with the periods of peak consumptive use by
crops. It will not be correct to consider that crops at these critical stages require
more water as their water needs are utmost.
 Critical stages of water requirement are usually the turning points in plant life
cycle. This can be represented by a sigmoid or S-shaped growth curve (Fig. 10.2).
 The curve shows two most important points of change in the growth rate, viz. the
point of inflection and the point of deflection.
 The point of inflection indicates a sudden increase in vegetative growth and the
point of deflection represents the slowing down of vegetative growth and
initiation of the reproductive phase.

 Crops demand for adequate water at these stages and cannot afford to stand water
stress without serious reduction in growth and yield. These two stages of crop life
are, therefore, considered as the most critical stages of water requirement.
 When crop plants are young and delicate, and start of grand growth period,
biochemical activities in plants occur at a higher rate. This leads to tremendous
increase in water need of the crop and the supply of water should be adequate to
maintain the normal rate of active growth and evapotranspiration.
 Water stress at the sensitive stages causes a serious retardation in growth process
that ultimately decresses the yield. The sensitive stages differ from one crop to the
others.
 Water stress at these stages causes lower tillering, branching, pegging, tuber
bulking, inadequate flowering and in extreme case, flower drops, poor setting of

Page 26








grains or fruits, bad filling of grains or serious fruit drops depending on the type of
crops.
In dwarf wheats crown root initiation stage is the most important critical stage of
water need as crown roots fail to develop in dry soils. Lack of adequate water at this
stage reduces tillering and affects the yield adversely.
It is true that crops require adequate water supply throughout their life cycle for
best growth and yield. Only in the later stages of crop maturity, water supply is
reduced or cut-off to obtain uniform and quicker crop maturity.
Crops may be allowed to stand water stress to some extent during certain periods of
life excepting at the critical stages to save some water under situations of water
scarcity. The critical stages of water need of crops that do not have distinguishable
stages are decided experimentally.
Determination of Critical Periods of Water Need To make a judicious use of
irrigation water particularly when the water supply is limited, it is essential to
determine the critical periods of water need of crops. For this purpose, a crop is
subjected to predetermined water stress at different stages of growth and then the
corresponding yield reductions are considered. It is then related to the yield of crop
that has not been subjected to any water stress and irrigated according to the
normal schedule. Another way to decide the critical periods is to miss irrigations at
different stages of the crop and then relating the corresponding yield reductions
with the yield from control plot which is irrigated normally. Periods at which yield
reductions are significant are considered as the critical periods of water need in the
life of the crop.

3.9 CRITERIA/APPROACHES FOR SCHEDULING IRRIGATION (ANSWER
THE QUESTION AS PER REQUIREMENTS.)
The criteria for scheduling irrigation as attempted from time to time may be grouped into
three categories, namely,
(1) plant criteria, (2) criteria based on soil water status and (3) meteorological criteria.
(1) Plant Criteria
Plants show up certain characteristic changes. These changes in plants are often valuable
pointers to the time of irrigation. Different plant criteria considered to schedule irrigation
are the general appearance of crop plants, plant water potential and water content of plant
tissues, growth, critical periods of water need, indicator plant, stomatal opening, leaf
diffusion resistance and plant temperature.
A. Plant appearance
 With water stress, some characteristic changes usually occur in the general
appearance of plants.
 There may be changes in the normal colour of plant or distortions of plants
such as wilting or drooping of plants and curling or rolling of leaves. Some
crops are very sensitive to soil-water changes and develop scarcity
symptoms easily, while others do not.
 Some plants develop symptoms easily, while others do not.

Page 27


 Changes in colour appear first in the lower leaves. On the other hand, deep
green and light green colours in alfalfa are indicative of water stress and
adequate supply of water respectively.
 Water stress is also shown by rolling of leaves during hottest part of day.
While some plants not easily shows water stress until decrease in growth
takes place.
 This technique is however quite simple and rapid, but suffers from many
deficiencies. Changes in colour may be misleading since nutritional disorder,
insect damage, disease attack and varietal character cause variable changes
in foliage colour.
 However, distinct changes in colour have been used to schedule irrigation to
beans.
B. Plant water potential and water content
 The relative leaf water content (RLWC) and leaf water potential change with
variations in soil water availability or make lag between water absorption by
plants and evaporative demand of the atmosphere.
 It has been noted adverse physiological and growth phenomena specific to
plant species with fall in the RLWC and water potential below certain critical
limits.
 However, sophisticated equipment, intricate measuring devices, high cost
and lack of proper standardization of instruments deter the use of this
technique on a large scale.
C. Plant growth
 Cell elongation, increase in height and radial changes in stem is considered as
the growth process that suffers first with water stress in plant.
 Timing of irrigation can be set as and when the normal growth rate is
observed to decline. This is, however, possible in places where a continuous
measurement of plant growth is maintained.
 This technique offers difficulties owing to unavailability and high costs of
equipment, inadequate standardization of the method, difficulties in
selection of proper growth parameter and precise growth measurement and
so on.
 The serious objection to this approach of scheduling irrigation is that the
plants suffer before they show any retardation in growth processes.
D. Critical crop stages of water need
 Irrigation scheduling may be decided based on stages of growth more
conveniently in crops in which the physiological stages are distinct to locate
the critical periods of water need.
 However, it may be a little difficult in crops where stages are not so well
defined.
E. Indicator plant


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 There are some plants sensitive to soil-water variations. They may be used
for detecting the water stress in crops that do not show symptoms of water
stress easily or exhibit the same when they have already suffered seriously.
 Sunflower plants are often used as in indicator plants in onion crop. An
indicator plant for irrigation should be such that it shows the water stress
before the crop has suffered from it.
 When an indicator plant is grown in a crop field, care should be taken not to
shade the plant by crop plants.
F. Stomatal aperture
 Opening of stomata in plants is regulated by soil-water availability. Stomata
remain fully open when the supply of water is adequate, whereas they start
closing with scarcity of water in soils to restrict the transpiration.
 Water deficit in plants is directly related to availability of soil water and that
may be used for scheduling irrigation in crops.
G. Leaf diffusion resistance
 Leaf resistance to vapour diffusion is regulated mainly by the leaf water
deficit.
 Several scientists reported a close relationship between leaf diffusion
resistance (LDR) and plant water stress. It is observed that on cloudy days
LDR is a sensitive index of internal water balance in the mild to moderate
stress range and holds a promise for scheduling irrigation.
 LDR, may be a useful tool in irrigation scheduling. LDR is a sensitive index of
plant water stress in sugarcane and can be used as a basis for scheduling
irrigation.
H. Plant temperature
 Solar radiation received on earth heats up leaf tissues besides causing
evapotranspiration and heating up the ambient air. With water deficit in
plant the temperature of leaf tissues rises. Many investigations have shown
that leaf or canopy temperature is a sensitive index of plant water status in
crops.
 The difference between the stressed and unstressed canopy temperatures
was a better index of water deficit than the difference between plant canopy
and air temperatures.
(2)Criteria Based on Soil Water Status
Scheduling irrigation based on soil water content is the most accurate and dependable
method. Determination of available soil water is rather more important than estimating the
total water content of soils. For the purpose, an information on the optimum water regime
of crops and the available water holding capacity of soils is essential.
A. Soil water content
 Early attempts were made to schedule irrigation when the soil water content
reached a certain value. The idea did not succeed since there existed a wide
variation in the water content retained by the different classes of soils.
 Later advanced a new concept of scheduling irrigation based on the lower limit
of soil water content for potential evapotranspiration of a crop.

Page 29


B.

C.

D.

E.




 They assumed that the growth of crop was likely to suffer below this level of soil
water. The threshold limit could be decided for various crops, soil types and
atmospheric evaporativity.
Depth-interval of irrigation
 Attempts to schedule irrigation based on depth of water applied or interval
between irrigations was not found very effective because of the water retentive
capacity of soils varies widely with soil types and soil physical conditions,
 Root zones of crops vary with types of crops and their rooting characteristics at
different growth stages, the depth and interval of irrigation require
modifications in different soils and at various crop growing periods.
 An arbitrarily fixed depth or interval of irrigation has misleading effects on crop
growth and yield.
Critical level of soil water
 This level once established experimentally for various crops in different soil
types and soil conditions can be profitably used for scheduling irrigation. This
approach has been widely suggested for adoption.
 A periodical determination of soil water content is made to know the time when
the soil water is likely to reach the critical level.
 This criterion is synonymous with the concept of available soil water depletion
for deciding the time of irrigation. The depth of irrigation however needs
revision upwards every time with increasing vegetative growth and rooting
depth of an actively growing crop.
Soil water tension
 Many scientific workers have used this criterion for scheduling irrigation to
crops in various parts of the world. In many countries, the tensiometers has been
considered as a useful device for scheduling irrigation.
 The use of tensiometers for controlling irrigation did not find much favour with
common farmers since the device presents certain difficulties in its use. The
tensiometers can be used only in the lower tensions up to 0.85 bars.
 It does not show the actual soil water content for direct calculation of the depth
of irrigation to be applied. The water content is calibrated from the soil-tension
curve.
 Sometimes the tensiometer showing the energy status of soil water earlier to the
existence of the actual soil water content.
Electrical resistance
The concept of electrical resistance that varies inversely with the water content in
soils was also tried to schedule irrigation. For this purpose, resistance blocks made
of gypsum, nylon, nylon-resin etc. were used.
Crops were irrigated when the electrical resistance reached a certain value. The
value could be decided experimentally for various crops by using the resistance
blocks.
This method has however many limitations and did not become popular. The
limitations are: (i) resistance blocks cannot be used at low tension at which most of
the available water is held by soils, (ii) difficulty of deciding the depth of irrigation
as resistance blocks do not directly show the prevailing soil water content and (iii)


Page 30


the existence of a time-lag in tension-equilibrium between the porous block and the
surrounding soil which causes showing up the energy status of soil water earlier.
(3) Climatological Approach
A. Empirical formulae
 For this purpose, empirical formulae using different meteorological parameters
have been developed. Penman (1948) and Thornthwaite (1948), Blaney-Criddle
(1950) and Christiansen (1968) developed formulae for estimating potential
evapotranspiration.
 This used the estimated evapotranspiration for scheduling irrigation by water
budget method.
 The adoption of empirical formulae for irrigation control demands the
knowledge of water holding capacity of soil and a continuous record of rainfall
and other meteorological parameters. This approach of scheduling irrigation to
crops is complicated for an ordinary farmer.
B. Evaporimeter
 Irrigation is applied when crops consume the available soil water to a certain
limit, calculated on the basis of consumptive use rate as determined by
evaporimeters. Sunken screen evaporimeter value can be used from the period
of 25 per cent ground coverage by crops till their maturity.
 The irrigation is applied when a certain amount of water gets evaporated from
the pan. The values of pan evaporation for this purpose are found for various
crops at their different growth stages under different soil and climatic
conditions.
C. Irrigation water/Cumulative pan evaporation ratio (IW/CPE ratio)
 The ratio of the amount of irrigation applied to cumulative pan evaporation
values has been used for scheduling irrigation. The pan evaporation values are
added up every day till it is equal to a certain ratio of the amount of water
applied as irrigation. The ratio for various crops is determined experimentally
by estimating the evapotranspiration by lysimeter studies.

3.10 NECESSITY OF IRRIGATION SCHEDULING
 Since irrigation water is of limited supply in most of the places, an emphasis should
be laid on making the most efficient and economic use of water for crop production.
 In a situation where adequate water is available on demand, farmers often irrigate
their crops earlier to the time of actual need in their eagerness to obtain good
growth and high yield of crops.
 Their attention is to produce more yields per unit area of land without much
consideration of the amount of water used. This leads to waste of valuable water
and it may sometimes cause damage to crops and lands owing to over-irrigation.
 On the other hand, a delay in irrigation for lack of proper knowledge may force
water stress to crop causing decline in yield.
 The optimum scheduling of irrigation under this situation should be based on crop
needs to avoid both over- and under-irrigation and to ensure a high water use
efficiency.

Page 31


 Since adequate water is not available in most places, attentions must be given to
produce the maximum yield per unit of water used by rational distribution of water
among crops over the growing seasons.
 A thorough understanding of the soil-water-plant-atmosphere relationships is
essential for proper scheduling of irrigation since irrigation needs of crops are
decided by the evaporative demand of the ambient atmosphere, soil water status
and plant characteristics.

3.11 FREQUENCY AND INTERVAL OF IRRIGATION
3.11.1 FREQUENCY OF IRRIGATION
 Irrigation frequency refers to the number of days between irrigations during
periods without rainfall. It depends on the consumptive use rate of a crop and on
the amount of available moisture in the crop root zone.
 It is a function of crop, soil and climate; sandy soils must be irrigated more often
than fine textured deep soils.
 Moisture-use rate varies with the kind of crop and climatic conditions and increases
as the crop grows larger and the days become longer and hotter.
 With higher frequency of irrigation, surface soils remain moist for longer periods
leading to higher evapotranspiration losses. Thus, frequency of irrigations should be
as low as possible to avoid waste of water. Of course, frequent irrigations with
smaller depths of water each time are often more conducive to higher yields than
heavier irrigations at long intervals, the delta of water remaining the same within a
certain limit. This has been represented in Fig.


Page 32


 In general, irrigation should start when about 50 per cent and not over 60 per cent
of the available moisture has been used from the zone in which most of the roots are
concentrated.
 The stage of growth of the crop with reference to the critical periods of growth is
also kept in view while designing irrigation frequency.
 In designing irrigation systems, the irrigation frequency to be used is the time (in
days) between two irrigations in the period of highest consumptive use of the crops
grown.
 Irrigation frequency depends on how fast soil moisture is extracted when a crop is
transpiring at its maximum rate. The average moisture-use rate during this period is
used to plan irrigation systems. For an irrigation system to be adequate, it must
have sufficient capacity to supply the water required during this period. The design
irrigation frequency may be computed as folows:

3.11.2 IRRIGATION INTERVAL
 The term, interval of irrigation indicates the time gap, usually expressed in days,
between two subsequent irrigations.
 The total amount of water required by a crop for producing an optimum yield is
termed as delta of water and it is synonymous with water requirement of crop.
 Fewer irrigations at longer intervals encourage saving of water. Losses of water
occur in the irrigation practice and the losses may take place in conveyance
channels and in fields every time irrigation is applied.
 The loss may get increase due to inadequate knowledge and experience of the
farmers in water application.
 Immediately after irrigation when the soil is wet, evapotranspiration occurs at a
potential rate. It starts declining some days after irrigation as the surface soil dries
up. Dry and loose soil surface helps to reduce evaporation.
 Since soil water declines progressively owing to continuous evapotranspiration, the
rate of evapotranspiration also declines progressively with the advance of time after
irrigation. Therefore, the longer is the interval between irrigations, the greater is the
saving of water.
 Besides, a longer interval between two irrigations cuts down the number of
irrigations during the growing season. Care should, however, be taken not to cause
any water stress beyond a certain limit by making the irrigation interval unduly long
unless compelled to do so for reasons of water scarcity.
 Irrigation is usually advised at the lowest limit of the optimum water regime, as
already stated earlier. The interval between two irrigations should normally be the
time taken by crops to reduce the soil water from field capacity to the lowest level of
optimum soil water regime.

Page 33


3.11.3 Design Irrigation Frequency
 It is same as the irrigation period and refers to the time, usually expressed in days,
between two irrigations that is necessary to irrigate the design crop area during the
period of peak consumptive use of the crop to be irrigated.
 The design irrigation frequency is used to decide the capapcity of the irrigation
system to be able to supply the required water to crops in the area.
 Irrigation frequency depends on how fast soil water is extracted when crop is
transpiring at its maximum rate. The average consumptive use rate during this
period is used for planning the system.
 The design irrigation frequency may be decided by the following relationship,

where,

Fc = field capacity, per cent
Mb = soil water content just before irrigation, per cent
As = apparent specific gravity of soil or bulk density of soil (dimensionless)
D = depth of crop root zone, cm

3.12 Factors Affecting Frequency of Irrigation
 The two main considerations namely, water need of crops and the availability of
irrigation decide the irrigation frequency. Once these two are known, the frequency
of irrigation is influenced mainly by:
(a) Climate and season
(b) Soil characteristics
(c) Crop characteristics and
(d) Crop and water management practices.
A. Climate and season
 Climate is responsible for causing variations in consumptive use rate and frequency
of irrigation. High temperature, low humidity, high wind velocity, greater solar
radiation in a place emphasis the need to irrigate crops more frequently.
 This is particularly evident in arid regions and during summer season. Since air
remains dry, temperature is high during summer months and the rainfall is low in
the arid region, there is a necessity to irrigate crops frequently.
 With greater evapotranspiration, frequent replenishment of soil water becomes
necessary to maintain the optimum growth of crops.
 On the other hand, higher rainfall and greater relative humidity during the rainy
season reduce the irrigation requirement of crops and irrigations may be applied at
longer interval, if it at all becomes necessary.
 In humid areas where rainfall is higher, the irrigation requirements of crops are
lower and irrigations are applied less frequently.

Page 34


 During winter months the crop demand for water is much less due to the lower
temperature and evaporative demand of the atmosphere. The irrigation
requirement during winter months is, therefore, lower and irrigations may be
applied at longer interval.
B. Soil characteristics
 A soil with greater water retentive capacity serves as a bigger water reservoir for
crops and can supply water for longer duration. Consequently, frequency of
irrigation is lower and interval of irrigation is longer in heavier soils and in soils
with crumb structure, good organic matter content and low content of soluble salts.
 On the other hand, the frequency is higher in porous sandy soils with coarse texture,
poor structure and low organic matter content. Retention of greater amount of
available water is considered more important than total quantity of water retained
by a soil.
 Depth of soil is another factor that influences the frequency of irrigation. A shallow
soil cannot hold enough water to meet the crop demand for a longer period.
Necessarily, frequent irrigations are required with smaller depth of water each time.
Irrigations at longer interval is applied to deep soil that has a greater water storage
capacity.
 Such a soil can supply water for longer duration particularly when the root system
is quite deep and extensive.
C. Crop characteristics
 Crops vary in their consumptive use of water, sensitivity to water stress, water
extraction capacity and optimum water regime. Frequency of irrigation thus varies
with crops. A crop having higher consumptive use rate consumes the soil water
quickly and requires more frequent replenishment of soil water.
 Many crops have varieties that are either sensitive or tolerant to drought conditions.
Varieties sensitive to drought conditions require frequent irrigations compared to
tolerant varieties.
 Rooting characteristics of crops such, When the root system is shallow and
fibrous, crops are not able to utilize water from deeper soil layers and are frequently
irrigated with smaller depth of water to wet only the upper soil layers.
 Crops with deeper and extensive root system command a greater depth of soil and
water reserve and require irrigations at longer interval.
 Maximum quantity of water is extracted from the upper 25 per cent of the effective
rooting depth and the extraction is least from the last 25 per cent of the rooting
depth which is about one-fourth of that from the upper section. The water
extraction pattern shows that a higher frequency with smaller depth of irrigation
each time is preferable for crops with shallow root system that extract most of their
water need from the upper soil layers.


Page 35


 Irrigation frequency varies with stages of crop growth. A crop when young and
delicate needs frequent irrigations. Plants at this stage are very susceptible to water
stress, because their root system is not deep and extensive enough to draw water
from deeper soil layers.
 Subsequently, the consumptive use rate gradually increases and at the same time
the root system also develops. Irrigations can then be applied at longer interval, as
roots are able to draw water from greater volume of soils.
 When a crop approaches maturity, the demand for water greatly declines because of
steep fall in consumptive use rate. Besides, the well-developed root system can also
draw water from deeper layers. The irrigation requirement, therefore, declines with
approach of maturity and crops are irrigated at longer intervals.
D. Crop and water management practices
 Soil water conservation practices such as artificial or soil mulching and crop cultural
practices like weeding and hoeing help to reduce the evaporation loss and conserve
more soil water for crop use. Thus, there is a reduction in irrigation requirement of
crops.
 Method of irrigation, depth of water applied each time and the water distribution
efficiency influence the frequency of irrigation. Sprinkler irrigation adopted in
porous and lighter soils demands frequent irrigations which cause more
evaporation loss as the surface soil remains moist for a longer period. A smaller
depth of irrigation is, therefore, applied each time.
 On the other hand, irrigations are applied at longer interval when the surface
irrigation is adopted in soils that have higher water retentive capacity and are not
too porous.


Page 36


3.13 Irrigation Period
 Irrigation period is the time, usually expressed in days, that can be allowed for
applying one irrigation to a given design crop area during the peak consumptive use
period of the crop.
 It is a function of the peak-period consumptive use rate. It is considered for
designing the irrigation system capacity and equipment.
 The irrigation system must be so designed that the irrigation period is not greater
than the irrigation interval. It is estimated as follows,

3.14 DEPTH OF IRRIGATION
 Depth of irrigation is a function of the water retentive capacity of the rootzone soil
and the extent of soil water depletion at the time of irrigation. It refers to the depth
to which the applied water would cover an area. As for example, a 10-centimetre
depth of irrigation to a hectare of land represents the volume of water which when
allowed to stand without any loss and infiltration into the soil would stand over one
hectare area to a depth of 10 cm.
 The net depth of irrigation is decided by the amount of water required to bring the
soil water content just before an irrigation to field capacity in the root zone soil. The
water content of soil just before irrigation must be known to calculate the net depth
of water required to be applied. It is calculated by the following formula,

 Therefore, irrigation should consist of the quantity of water required to replenish
the soil water depleted in the crop root zone just before irrigation applied. It should
neither be lighter nor heavier than actual depth of water needed.
 A lighter irrigation leads to under-irrigation keeping the lower soil dry, while a
heavier one results io over-irrigation causing loss of water through deep

Page 37


percolation. Roots do not grow in dry soil. Therefore, the root development gets
restricted and shallow rooting takes place when the lower soils remain dry in case
of under-irrigation.
 Roots of an actively growing plant move deeper and deeper provided the soil below
the growing point of roots is moist. Therefore, the depth of irrigation is increased
with growth of crop to wet the extending root zone.
 Shallow irrigation causes the roots already grown in deeper layers to gradually die
for lack of water. On the other hand, when a heavy irrigation is made, some amount
of water percolates down beyond the root zone and gets wasted.

3.15 Factors Affecting Depth of Irrigation
Factors that govern the frequency of irrigation, influence also the depth of irrigation.
Factors mainly concerned in modifying the depth are as follows:
(a) Depth of effective root zone soil,
(b) Water retentive capacity of soil,
(c) Degree of soil water depletion, and
(d) Apparent specific gravity of soil.
 Depth of soil to be made wet regulates the depth of irrigation. When roots extend
deeper into soil, more water is necessary to wet the soil layers up to which roots
have developed since roots function as the principal water absorbing organ of
plants. A smaller irrigation keeps the lower soils dry and it would lead either to
shallower development of roots or the deeper roots that have grown earlier will die
down gradually, if the amount of water applied continued to be smaller. Therefore, a
greater depth of water is applied each time to an actively growing crop so that roots
grow deeper.
 A smaller depth of water is applied to crops that have shallower root system. The
depth of water is also smaller when the crop is young and it roots are shallower. If a
higher amount of water is the applied, some water will percolate down beyond the
root zone and get wasted. With growth of the crop and its root system, the depth of
irrigation is increased.
 Depth of irrigation is a function of the water retentive capacity of root zone soil.
Soils of heavier texture with a greater amount of capillary pores can retain more
water in their pore spaces than lighter soils with more of non capillary pores. Again,
soils with crumb structure, higher organic matter content and water-stable
aggregates retain more water. The depth of irrigation is necessarily more in soils
with a greater water retentive capacity compared to soils with a lower water
retentive capacity.
 The consumptive use of crop decides the depth of irrigation. Soil water gets
depleted continuously after an irrigation is applied till the next irrigation. The
amount of soil water depleted from the field capacity level in the effective root zone
is to be replenished to bring back the soil water content to the field capacity to
continue the normal crop growth.

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 The apparent specific gravity of soil is determined to estimate the net depth of
water to be applied to the crop. The value of bulk density is important since it gives
the mass of soil solids involved in water retention. The depth of irrigation increases
with increase in bulk density of soil. The apparent specific gravity is the bulk density
without considering the units of the bulk density.
 Tillage operation, soil texture, structure, organic matter content influence the bulk
density of soil.

3.16 IRRIGATION WATER QUALITY CRITERIA
The several factors influencing water quality, the generally accepted criteria for judging the
quality are:
1. Total salt concentration, as measured by electrical conductivity,
2. Relative proportion of cations as expressed by sodium adsorption ratio (SAR), and
3. Bicarbonate and boron contents.
The suitability of irrigation water (SIW) can be expressed as
SIW= f (QSPCD)
'where,
Q = quality of irrigation water
S = soil type
P = salt tolerance characteristics of the plant
C = climate
D = drainage characteristics of the soil
 Some other factors like the depth of water table, presence of a hard pan of lime or
clay, calcium carbonate content in the soil and potassium and nitrate ions in
irrigation water also indirectly affect the suitability of irrigation water.
 This is probably the main reason for the several classifications, varying in limits of
salinity and other chemical indices. The soil type, major crops of the area, climate
and drainage characteristics profoundly influence the suitability of particular water
for irrigation,
 Highly saline water may be suitable in a well-drained, high textured, fertile soil
while a much less saline water may be more harmful for the same crop grown on a
heavy textured soil with impeded drainage.
 It is the actual salt concentration near the root zone, which determines the
suitability of irrigation water rather than the chemical properties of irrigation water
alone.
 The quality of irrigation water is generally judged by its total salt concentration,
relative proportion of cations or sodium adsorption ratio and the contents of
bicarbonate and boron,
1. salinity.
 Irrespective of the ionic composition, the harmful effects of an irrigation
water increase with its total salt concentration as it increases the soil salinity
signifiicantly.

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2.

3.

4.

5.

6.

 Waters of low salinity (EC < 3 mmhos/cm) are generally composed of higher
proportions of calcium, magnesium and bicarbonate ions. Highly saline
waters (EC >10 mmhos/cm) consist mostly of sodium and chloride ions.
 Moderately saline (EC = 3 to 9 mmhos/cm) waters have Varying ionic
compositions. Waters containing high concentrations of sodium, bicarbonate
carbonate ions have high pH.
Sodium adsorption ratio (SAR).
 Any increase in the SAR of irrigation water increases the SAR of the soil
soltion. This ultimately increases the exchangeable sodium of the soil.
Generally, there is a linear relationship between SAR and exchangeable
sodium percentage ESP) of the soil up to moderate ESP levels, and at high
ESP levels, the relationship tends to be curvilinear.
 In judjing the suitability of irrigation water, both salinity and SAR should be
kept in view along with the salinity and sodicity developed during the
cropping period.
 Salinity increases the osmotic stress while adsorption of sodium is increased
both by salinity and SAR
Megnecium Calcium ratio.
 At the same level of salinity and SAR, but with varying proportions of calcium
and magnesium, adsorption of sodium by soils and clay minerals is more at
higher Mg : Ca ratios. This is because the bonding energy of magnesium is
generally less than that of calcium, allowing more sodium adsorption. It is
more important if the Mg : Ca ratio in irrigation waters happens to be more
than 4.
Bicarbonate.
 Irrigation water rich in bicarbonate content tend to precipitate soluble
calcium and magnesium in the soil as insoluble carbonates:
 This leaves a higher proportion of sodium to divalent cations in the soil
solution and increases the SAR. This bicarbonate-induced increase in the SAR
of the soil solution ultimately results in higher adsorption of sodium on the
soil exchange complex.
Boron.
 Though boron is an essential nutrient for plant growth, it becomes toxic
beyond 2 ppm in irrigation water for most of the field crops. It does not
affect the physical and chemical properties of the soil, but at high
concentrations it affects the metabolic activities of the plant.
Potassium and nitrate.
 Potassium and nitrate ions are often present in significant amounts in
irrigation waters. Being essential nutrients, they act favourably in
reducing the harmful effect of saline water on crop growth by way of
providing these nutrients regularly, rather than by reducing soil salinity.
 Among these, the effect of nitrate ion has been found more spectacular
than potassium because irrigated soils are themselves deficient in
nitrogen status and are generally well supplied with potassium.


Page 40


 Regular supply of nitrate form helps in mitigating the salt-induced
nitrogen deficiency and in increasing crop productivity.

3.17 Quality of Water Sources
The quality of irrigation water is influenced primarily by its source.
1. River waters. The water quality of Indian a few exceptions, and those too in the hot
generally good. An analysis of 13 important river water taken at several places
during 1955 to 1966 at monthly intervals has revealed that the water quality of
Gandak, Kosi, Brahmaputra, Kaveri, Beas nad Narmada is good.
As their EC values fall below 350 micromhos/cm while salinity of those of Chambal,
Yamuna, Tapti, Godavari and Krisha ranged from from 450 to 1400
micromhos/cm. These waters are mostly alkaline. The rivers having low EC values
show less monthly and yearly variations as compare to those of moderate salinity.
The waters of low EC are suitable for irrigation purposes throughout the while while
those of moderate salinity can be used cautiously.
In South India, waters of Hagari and Tungbhadra rivers are of moderate salinity. In
some arid and salt affected areas of Gujarat, waters of some seasonal rivers show a
salt concentration up to 7000 ppm.
2. Canal waters. The quality of canal water depends mainly on the river from which it
originates Consequently, the quality of all the canal waters in India is good. The
development of salt affected lands in the canal irrigated regions mainly due to the
rise of water table. Extensive salt affected areas in the canal irrigated regions of
Punjab, Rajasthan and UP have been developed because of the rise in the depth of
water table and accumulation of salts on the soil surface through capillary
conductivity and evaporation.
3. Quality of other surface water sources. Under the minor irrigation projects, some
areas are irrigated by the waters of lakes, tanks, reservoirs and drains. In most of
the cases, their quality is fairly good, except in case of the drains or the reservoirs,
which are fed by small streams flowing over salt-affected areas. Drainage waters of
Godavari and drainage channels in UP are of moderate salinity.
4. Quality of ground waters Well water forms an important source of Irrigation. Its
quality is highly variable due to climatological and hydro-geological conditions.
Paliwal (1972) classified the quality of groundwater resources of India into three
main groups: (i) water quality of arid and semi-arid regions having rainfall below
450 mm per annum. (ii) water quality as influenced by hydrological conditions as
high water table, consisting of some of the areas (iii)water quality of wells in some
areas of the coastal regions of the country as influenced by backward flow of sea
water and inundation.

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Of all the factors, aridity is the most important single factor responsible for a very
degree off salinity of well waters.
The problem of boron hazard is not serious in the irrigation water sources of India,
except in a few cases in the arid areas where its concentration ranges from 2 to 5
ppm. The toxic effect of boron in irrigation water is primarily dependent on its
concentration, soil texture, calcium carbonate content, pH and organic matter. It is
also influenced by the type and amount of clay in the soil, its adsorption and release
characteristics; depth to water table and the drainage condition of the soil. The
equilibrium between different forms of boron and their interactions with the above
factors and the boron tolerance limit of crops determine the toxic effects of boron on
a particular crop species
5. Seasonal variation in water quality. Salinity of ground water is considerably
influenced by the climatic conditions. It increases during the summer and is
significantly reduced during the monsoon due to dilution by rain water. The degree
of variation in salinity and its ionic composition depends upon the depth of water
table, infiltration capacity of the soil and the rainfall characteristics of the area
concerned. Besides these factors, some recharge of water takes place in low-lying
areas due to flooding by rain water.
6. Variation in water quality with depth. The quality of ground-water in many
regions shows wide variations with the depth of the aquifers. The depth at which
good quality water occurs varies from place to place and even within the same area,
at a distance of a few metres. At several locations in the Delhi territory, for instance,
the water quality has been found to deteriorate with depth. In such areas, the water
quality of dug wells is superior to tubewells tapping deeper aquifers.

3.18 irrigation with saline water
3.19 improving quality of water
3.19.1 IRRIGATION WITH POOR QUALITY WATER
 Where there is no alternate source of good quality irrigation water in an area, it is
inevitable to use the available water of poor quality. However yield potential of such
areas can be increased by adopting proper management practices such as:
 Improvement of sodium and bicarbonate rich waters by gypsum application,
 choice of salt tolerant crop and their varieties,
 optimum fertilization and manuring.
 proper irrigation management, and
 breaking of any impervious layers by deep ploughing.
3.19.2 improvement of Water Quality
The harmful effect of irrigation water can be minimized to some extent by modifying its
ionic composition by adding such chemicals, which tend to precipitate the harmful

Page 42


constituents such as bicarbonate and carbonates in the form of less soluble salts or tend to
create a favourable cationic (Ca : Mg : Na) ratio.
1. For all practical purposes, gypsum should be powdered up to 0.5 mm size or passed
through a 30 mesh sieve. The gypsum requirement of water should be calculated,
depending upon the relative concentration of sodium, magnesium and calcium ions
in irrigation water and the solubility of gypsum. Mixing of gypsum directly in
irrigation water poses some problems. If it is placed in bags or containers between
the entry and outlet of water, it does not dissolve uniformly with increase of time,
and most of it is deposited at the place of entry of water in the field and forms bigger
granules from powdered gypsum on coming in contact with water, however, some
of the above limitations can be overcome by applying its saturated solution by a
mechanical device in the flowing water at a constant rate.
2. Choice of salt-tolerant crops.
Some crop and their varieties are more salt-tolerant than others. Hence salt-tolerant
crops are to be grown in salt affected areas till the soils are improved by vegetation
or adopting the reclamation procedures.
3. Use of fertilizers.
Generally, saline and alkali soils or soils irrigated with poor quality waters are low
in their fertility status, especially with reference to nitrogen or sometimes to
phosphorus. Better crops can be grown by raising their fertility status. Nitrogen
response to crops is better when it is applied to soil along with manures. It has been
observed that for wheat, barley, bajra and maize, the usual does of fertilizers, as
applied on normal soils can be applied up to an EC value of 6.5 millimhos/cm and an
ESP of about 30. However, excessive fertilization on a highly saline alkali soil is of no
value.
4. Soil management practices. In order to adopt irrigation with poor quality water
on a long-term basis, it is important to have a detailed analysis of representative soil
profiles for their physical, chemical and morphological characteristics Soil analysis
should include its structure, texture, pK, lime content, location and amount of
gypsum (if present in the profile), total soluble salts, their ionic composition and
exchangeable cations. Information on water transmission properties of the soil and
depth of water table are to be obtained. Data on the climatic parameters as amount
of rainfall, its intensity, distribution, and evaporation are to be obtained, as they
control the periodic as well as annual salt build-up in an irrigated area.
Accumulation of excessive amount of salt and development of high sodicity are the
main limitations in the safe utilization of poor quality irrigation water. These
depend upon the nature and amount of clay minerals in the soil and the quality of
irrigation water. Success of irrigation with saline water lies in the degree of accuracy
in the predicted values of soil salinity and sodicity expected to be developed during
the cropping period. Saline waters can more safely be used in coarse than in fine
textured soils.
5. Lime requirement for the reclamation of acid soils.
 The beneficial effect lime application in soils when soil acidity is a constraint to
agricultural production has been discussed earlier. As gypsum requirement for the

Page 43





6.

7.

reclamation of alkali soil is determined on the basis of desired replacement of
sodium from the soil complex, the lime requirement is determined based on the
requirement of raising soil pH from less than 6 to 8 desirable higher value.
A field infested with the problem of salinity should be levelled carefully. This will
help in uniform spread of water and facilitate its downward movement, otherwise at
high spots (if salts are not leached) it will lead to salt accumulation and will cause
heterogeneity in salt concentration.
The success of gypsum or flushing of salts lies in the drainability of the soil. Hence, if
the improvement of internal drainage by tile drains is not economically feasible.
Irrigation management. While irrigating with poor quality waters, accumulation of
salts increases with fineness of soil texture. it is essential to adopt irrigation
practices such that the salinity at the root zone is kept to the minimum. For this
purpose the quantity of water and the frequency of irrigation are planned to meet
the leaching requirement of soil and the consumptive use of the crop grown.
In case of shallow salt concentration, A heavy pre-sowing irrigation to leach this
surface salts will improve germination and early growth and is sometimes an
essential practice. It is made far enough in advance of the desired planting date to
allow for cultivation to remove weeds and prepare the seedbed.
Seed placement. Obtaining a satisfactory stand of furrow-irrigated crops on saline
soils when using poor quality water is important. If salinity is a problem, planting
seeds in the centre of a single-row raised bed will place the seed exactly in the area
where salts concentrate. A double-row raised planting bed by comparison offers an
advantage.
The two rows are placed so that each is near a shoulder of the raised bed, thus
placing the seed away from the area of greatest salt accumulation. By this method
higher soil and water salinities can be tolerated than with the single-row plantings
because the water moves the salts through the seed area to the centre of the ridge.
Alternate furrow irrigation is often advantageous. If the beds are wetted from both
sides, the salts accumulate in the top and the centre of the bed, but if alternate
furrows are irrigated, the salt can be moved beyond the single seed row , thus
reducing the extent of salt accumulation. Off-centre, single-row planting on the
shoulder of the bed closest to the watered furrow aids germination under salty soil
conditions.

3.20 Leaching requirements
The depth of irrigation water per unit depth of soil, required to produce any specified increase in
soil salinity for any given electrical conductivity of irrigation water can be calculated from the
following equation:

= depth irrigation water
= depth oi soil

Page 44

= density of soil (bulk density)
= density of irrigation water
SP= saturation percentage of the soil
= increase in electrical conductivity of saturation extract of the soil
ECW = electrical conductivity of irrigation water.
Under high water table conditions, the increase in soil salinity by the evaporation of ground water
can be determined by the following formula:

The fraction of irrigation water that must be leached through the root zone to keep the salinity of
the soil below a specific limit is termed as leaching requirement (LR). Mathematically, it can be
expressed as
in which,
LR = leaching requirement, expressed as a ratio or as per cent
ECt = electrical conductivity of irrigation water, mmhos/cm
ECd = electrical conductivity of drainage water, mmhos/cm
Information on the consumptive use of water by the crop is necessary if the leaching requirement
concept is to be used for determining either the depth of irrigation water that must be applied or
the minimum depth of water to be drained, in order to keep the soil salinity from exceeding a
pecified value. The depth of irrigation water DI, is related to consumptive use Dc and the equivalent
depth of drainage water Dd by the equation
Dl = Dc + Dd .
Using above equation eliminate Dd from LR equation

Expressing the leaching requirement (LR) in above equation in terms of the EC ratio of irrigation
and drainage waters, as given in LR equation, provides the following relationship:
In above Equation gives the depth of irrigation water, which is required to satisfy both the evapotranspiration and leaching requirements of the soil. Of the above factors, ECj is known from the
chemical analysis of the irrigation water and ECd is taken on the basis of permissible salt tolerance
limits of the crop. Generally, the salt tolerance limits are expressed in terms of EC of the saturation


Page 45

extract of the soil solution (ECe). In most cases, ECd is assumed to be approximately twice as high as
the corresponding ECe.


Page 46



Page 47


3.21 Plant response to Saline and Alkali Soils(this is less important)
 Excessive salinity usually causes poor stands of crop, stunted growth and reduced
yield. It may also cause leaf burns in some crops and deep blue-green colour in
other.
 In some cases, symptoms of excessive salts are similar to those of drought or low
fertility status. Soil salinity causes a decrease in the size and vigour of the plant. The
germination of seeds is delayed and retarded but the cropping period is slightly
reduced with the increase in salinity.
 However, its effects on the yield and quality of the crop are variable. In general,
grain yield is affected more than the height of the plant.
1. Effect of Salts on Plant Growth
It is effected by:
 increasing the osmotic pressure in the soil solution,
 accumulating certain ions in toxic concentration in the plant tissue, and
 by altering the plants mineral nutritional characteristics.
a. Osmotic pressure and plant growth.
It has been observed that there is an inverse relationship between the
osmotic pressure of the soil solution and plant growth. The reduced crop
growth is due to reduced water availability to plants. Thus, as the osmotic
pressure increases with the concentration of salts, it is presumed that
salinity inhibits plant growth primarily by reducing the availability of
water.

Page 48


On the other hand, recent evidence indicates that the water absorption
capacity is relatively unaffected by salinity and plants develop their own
internal pressure against salt.
Hence, this theory suggests that salt tolerance of the plant may be defined
as the degree to which osmotic adjustment can be made without any
sacrifice in plant growth.
In addition to the osmotic effect, the force by which water is held by soil
particles also affects plant growth. This force, called soil moisture tension,
increases as the moisture content of the soil decreases.
The osmotic pressure of the soil solution and soil moisture tension are
additive in their effect on plant growth. Their combined effect is called
total soil moisture stress.
It has been observed that plant growth is closely related to the soil
moisture stress, regardless whether this stress arises primarily from
salinity or soil moisture tension or both.
Through controlled leaching, the osmotic pressure of the soil solution
should be maintained at the lowest feasible level. By careful management
of the irrigation system, the soil moisture tension in the root zone is
maintained in a range that will provide the greatest net return of the crop
grown.
b. Specific ion effect.
Certain ions exert specific effects which depress crop growth and yield
independent of the salt concentration. These specific ion effects may be
toxic or may cause nutritional disorder in the plant, influencing its
physiological and metabolic activities.
The toxic effect may be due to the presence of an ion in excessive amount
in the soil solution, the higher uptake of which damages the plant growth.
Nutritional disorders in plants occur when the accumulation of one or
more nutrients in excessive amounts inhibits the uptake of other
essential nutrients.
When the amount of these essential nutrients is below their critical limit
which is essential for normal physiological functions of the plant, the
growth of the plant is inhibited. Plants vary widely in their nutrient
requirement and their ability to absorb specific nutrients.
Hence, the effects of salts on nutrition differ markedly from species to
species and even between varieties of the same crop.
c. Salt-tolerance of crops. Salt-tolerance means the ability of a plant to tolerate
salt concentrations in the root zone. Knowledge of salt tolerance is important in
selecting a particular crop and determining leaching requirement.
d. Boron tolerance of crops. Boron, though essential plant growth, becomes toxic
beyond a concentration m few ppm. It does not seem to affect the germination
seeds up to 20 ppm or even more, but the growth rate height of seedlings are
significantly reduced beyond 1 to 2 ppm.


Page 49


Relatively, legumes are salt-sensitive but tolerate more boron than
cereals. Amongst field wheat and barley can tolerate from 1 to 2 ppm of
boron in irrigation water.
Boron is translocated in the leaves and accumulates in the tip and margin.
Toxicity symptoms such as yellowing and tip burn is usually visible. If
boron toxicity is indicated in the plant, its source should be found.
If irrigation water has excessive boron, it should be mixed with good
quality water and if soil is the cause, the boron should be reduced by
leaching, though its leaching is more difficult than of salts.

3.22 Leaching methods.
 Leaching can be done by ponding the required depth of water on the soil surface by
means dikes or ridges, thus establishing a downward water movement through the
soil. Leaching can be done at any time of the year, depending on the availability of
water.
 It is advantageous to practice leaching during the monsoon season. The quality of
water applied in leaching should be in conformity with the tolerance limit of the
crops to be grown. The cost of leaching will be high if salts are leached beyond a
threshold value, as no additional benefits will accrue, because yields stabilize at the
threshold value.
 Information on crop tolerance of salt will help in selecting crops. From the initial
salinity and the crop tolerance level, fractions of salts to be leached could be derived
by the relation:
 In case any crop is known to be sensitive at the germination stage, the threshold
value of this particular stage should be used in deciding the fraction of the salts to be
leached.
 A thumb rule commonly used is that 1 cm of water should be applied for each cm of
the soil to be reclaimed. However, the quantity of water required may vary with soil
texture and fraction of the salts to be leached.
 For medium textured soils (sandy loam), 0.5 cm of water for each cm of soil is
usually sufficient to remove 80 per cent of the salts present in the soil profile.
continuous and intermediate leaching.
 In continuous leaching, water is ponded continuously over the soil surfc; e to allow
water to pass through the root zone. In the case of intermittent leaching, water is
ponded at intervals.
 A net saving of 30-35 per cent can be made through. intermittent, compared to
continuous leaching, in the amount of water needed to leach the salts to the same
degree. However, intermittent leaching may not be advantageous for highly
degraded alkali soils with restricted infiltration rates.
 Rectangular check basins and level borders are used when the land is flat. Contour
checks can be used when the land slope is considerable.

Page 50


 The effectiveness of leaching by furrows is limited. Sprinkling is an effective method
for leaching, especially when soils are cracked or are highly permeable.
 The efficiency of leaching depends on:
the amount of water applied,
uniformity of water distribution, and
the adequacy of drainage.
 In case of salt tolerant crops, it is sufficient to leach once or twice in a growing
season. But in less tolerant crops, it is advisable to apply irrigation water several
times for leaching during the cropping period. Generally, less tolerant crops show
higher leaching requirement.
 In practice, for better crop growth, it is desirable to apply slightly more water than
the calculated values. The best practice would be to follow the salinity trends by
making regular checks in the cultivated fields irrigated with saline water.
 Since nutrients are also moved down with water and salts, it is desirable to apply
fertilizers after leaching and make up the losses of the nutrients, if any, in leaching.
This is especially true in case of nitrogenous fertilizer.

3.23 Land reclamation methods
1. Drainage and lowering of the water table
 The source of water which is causing the waterlogging should be located, and
suitable measures should be adopted to check the flow from that source.
 If the waterlogging is caused by surface water, suitable surface drainage
systems should be adopted. If the waterlogging is caused by underground
water, the source should be cutoff by constructing intercepting drains.
Measures recommended for prevention of waterlogging are also useful for
reclamation of saline land.
 A good drainage system helps in avoiding efflorescence if the water table is
kept below the land surface at a safe limit so that water does not rise upto
the land surface by capillary action. The safe limits are different for different
soils.
2. Leaching
 Leaching is the process in which the land is flooded with an abundant
quantity of water to a depth of 15 to 25 cm over the surface. The excess salts
are washed down from the land surface to the ground water, provided the
water table had already been lowered to a safe limit.
 The process is continued till the salts in the surface layer are reduced to a
safe limit. The flooding operation is then repeated. The process is continued
till most of the salts have been washed to the depth below the root zone of
plants.
 Drying of the land surface between the flooding operations permits the
formation of surface cracks in the soil. These cracks increase the infiltration
during subsequent floodings.

Page 51


 Leaching operation can be repeated after 4 to 5 years, if necessary. However,
excessive leaching is not desirable, as it may remove the salts from the soil
which are essential for the plant growth.
 Moreover, over-use of flooding water creates the drainage problems. During
the reclamation process, weeds and grass are allowed to grow to consume
some of the salts. On completion of the leaching process, salt-resistant crops,
such as coarse rice and berseem, are grown for a year or two to reduce the
salinity further.
3. Use of chemicals
 Chemicals are sometimes added for the reclamation of land. Gypsum is the
most commonly used chemical for alkali soils. Sodium carbonate in the soil is
easily removed by spreading gypsum on the land at the rate of 2 tonne/ha
before leaching. Calcium chloride is also sometimes used. However, it is less
effective than gypsum.
 The low solubility of gypsum results in better activity than that in calcium
chloride. But addition of gypsum is less effective if the soil has excess sodium
salts ( as in Thur soils).
 For such soils, gypsum should be added after the excess salts have been
reduced by leaching. Moreover, gypsum should not be applied every year.
 Sometimes sulphuric acid is also applied to the land. The top 20 cm soil layer
is usually treated with 1 to 5 percent solution of sulphuric acid to neutralise
alkalies in the soil. It improves the growth of plants. Acid-forming fertilisers
are also effective.
4. Adopting rice cultivation
 Salinity of the land is reduced by adopting rice cultivation. In rice cultivation,
the large depth of water over the land leaches the salts and keeps them at a
safe depth below the surface.
 Rice cultivation also causes a reduction in the alkalinity of the soil. The roots
of the rice plants produce carbon dioxide which lowers the pH value,
increases the percolation and brings the exchangeable sodium ions of the soil
into solution.
 However, during rice cultivation, nitrogen in the soil, which is essential for
the growth of plants, is reduced due to excess water and absence of adequate
air. To compensate the nitrogen in the soil, a leguminous crop, such as gram,
is grown during the next Rabi season. Guara and San also improve the
nitrogen deficiency.
5. Crop rotation
 Salinity of a soil can be further reduced after leaching by adopting crop
rotation in which rice or maize is introduced. The following two crop
rotations are commonly adopted
 (i) Rice in rotation
(a) Rice-wheat-moong-rice (b) Rice-senji-sugarcane-rice
(c) Rice-berseem-rice
(ii) Maize in rotation
(a) Wheat-maize-berseem-cotton-wheat

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(b) Wheat-maize-senji-wheat
(c) Wheat-maize-rice-wheat
6. Green manuring.
 A crop called Jantar is generally used as a green manuring crop. It nas a rapid
crowth in saline and poorly drained soil. Other green manuring crops arc San.
Senji. Guara and Berscem, clc. Green manuring also improves the structure of the
soil. It releases the organic acids which lower pH value and add nitrogen to the
soil
7. Addition of agricultural waste products
 The salinity of the soil can be reduced by adding agricultural waste products
such as groundnut hull, saw dust, molasses with lime sludge, distillery waste,
sunflower hull, tamarind seed powder, etc. Molasses alone are not effective for
the reclamation of alkali soils,
 However, when combined with lime sludge, they are quite effective. Distillery
wastes are acidic and quite effective in reducing alkalinity and in replacing
sodium with calcium.
8. Use of argemona plants
Argemona is a plant which grows on the waste land. The plant is highly acidic and
can be effectively used to reduce the alkalinity of the soil. Other similar plants are
also sometimes used.
9. Use of processed coal
The alkanity of a soil can be reduced by the addition of a small quantity of processed
coal to the soil.
10. Electro-dialysis
Electro-dialysis can be used to reduce the alkalinity of the soil. When an electric
current is passed through the soil, it renders it porous and permeable. Hence the
infiltration rate is increased and soluble salts are washed out. However, the method
is quite expensive. It can be adopted where cheap power is available.


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Irrigation water management

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