Water requirement of a crop is the quantity of water needed for normal
growth,development and yield and may be supplied by precipitation or
by irrigation or by both. Water is needed mainly to meet the demands
of evaporation (E), transpiration (T) and metabolic needs of the plants.
The water requirement of any crop is dependent upon,
Crop factors like variety, growth stage, duration, plant population
and growing season.
Soil factors like texture, structure, depth,and topography.
Climatic factors like temperature, relative humidity and wind velocity.
Crop management practices like tillage, fertilization, weeding etc.,
Quantum of Water Requirement (mm) of Different Crops
Ikisan - Stages of Crop When Irrigation is Required
Stages of Crop When Irrigation is Required
During the growth span, the plant passes through various phases and
the stages of growth. The growth rhythm of plant is slow during some
stages and fast during some other stages. Accordingly plant demands
variable supply of water.
The growth period of irrigated dry (ID) crops can generally be divided
into 3 phases namely
vegetative,
reproductive and
ripening phases.
Each of these phases has different stages.
Vegetative phase: The early vegetative phase consists of crop establishment
or initial stage during the first 2 - 3 weeks after sowing. This is
followed by crop development stage which last for 2 - 6 weeks in different
crops.
Reproductive or flowering phase:The reproductive or flowering phase
comprises the period from initiation of buds to 75 % flowering. This
period in most of the seasonal ID crops last for 2 - 3 weeks and in
two seasonal crops and perennial crops for 4 - 6 weeks or more.
In yield formation stage otherwise known as ripening phase the end
product is formed. The flowering and yield formation period together
is known as mid-season stage. During the last part of the ripening phase
the crops undergo yellowing and drying to mature. This period is called
maturity stage or late season stage and it last for 2 - 4 weeks in most
crops. The entire reproductive phase is highly sensitive growth period
when the growth rhythm is fast. Therefore the soil water stress should
be avoided during this period. Active vegetative phase and yield formation
stage are moderate in sensitivity while initial establishment and maturity
stages are least sensitive to water stress.
Some crops like Cotton, Groundnut and pulses even prefer stress during
early vegetative growth to suppress excessive vegetative growth. In
many crops the initial establishment and flowering stages are highly
sensitive to excess water conditions resulting in poor performance of
the root system and also shedding of flowers, in addition to lodging
at maturity in some crops.
The water balance in ID crops is refered to the soil water storage
in the root zone and not to the level of standing water in the field,
as in case of paddy.
The critical stages or otherwise known as sensitive stages of different
crops for irrigation water requirement are as follows,
Rice
Panicle initiation, flowering.
Wheat
Crown root initiation, shooting, earing.
Sorghum
Booting, Blooming, milky and dough stage.
Maize
Tasseling, silking stages to early grain formation.
Pearlmillet
Heading and flowering.
Finger millet
Panicle initiation, flowering.
Groundnut
Flowering, Peg penetration, Seed development.
Sunflower
Two weeks before flowering to two weeks after flowering.
Cotton
Flowering and boll development.
Chillies
Flowering.
Sugarcane
Formative stage
Pulses
Flowering and pod formation.
Soybean
Blooming and seed formation
Tobacco
Immediately after transplanting and knee stage.
Citrus
Fruit setting and enlargement stage.
Banana
Early vegetative period, flowering and yield formation.
The practice of equating a hectare of canal irrigation area with a
hectare of area served by ground water is not appropriate. A striking
analysis carried out in four states, Punjab, Andhra Pradesh, Haryana
and Tamil Nadu has shown that the yield of food grains under well irrigation
is very much higher compared to the yield under canal irrigation. The
reason for this difference in yield between sources of water supply
is not so far to seek. The farmer who depends on canal irrigation is
at the mercy of a system over which he has no control.
The utility of irrigation is judged by the cropping intensity. In
most parts of the country, the cropping intensity is 200 % in the tube
well (or) dug well irrigated land as against 100 % or less in canal
irrigated land. About 18 % of the tail end area in canal commands of
South India are particularly vulnerable for erratic and insufficient
supply of water, not only because of losses to the extent of 50 % due
to seepage, percolation and evaporation in transit from the storage
reservoir to the farmers field but also because farmers in the upper
reaches of the systems often succeed in cornering more than their due
entitlement of water.
Wells
Awell is a hydraulic hole to the water strata. Water in the well
stands at a height equal to the static water level. There are different
types of wells namely open well, tube well, artesian well, and bore
well.
Open Wells
The dug out wells upto water bearing strata of the aquifer are open
wells. They derive water from the formation hole to the ground surface.
The large diameter of the open wells permits the storage of water.
Tube Wells
These are sunk by inserting pipes below ground surface and passing
through different geological formations of water bearing and non-water
bearing strata.
Artesian Wells
Due to pressure, water from well comes to the ground without pumping
are generally known as artesian wells.
Bore Wells
When ground water availability is at deeper layers exceeding 16 to
20 m with hard strata, bore wells are suggested.
Tanks
Large tanks irrigating more than 2000 ha are classified under medium
irrigation source. Small water reservoirs behind earthen dams are tanks.
Though the primary purpose of tank is for irrigating crops, it also
provides drinking water for humans and cattle in the villages. Monsoon
rains fall erratically and confined only to a few months in the year.
Irrigation tanks serve to store and regulate water for crop production.
In drought prone areas, tanks are considered to be a useful life saving
sources. But day by day the area irrigated by tanks decreases due to
neglect of maintenance of tanks, environmental degradation, cultivation
of foreshore areas and cultivation of tank beds.
Filter Points
These are shallow tube wells consisting of a well and a short length
of casing pipe. Filter points are generally bored in deltaic regions
where aquifer formation are of coarse sand and gravel and are very near
to the surface. In coastal sands open dug wells are to be lined with
concrete rings which is costly and also the availability of water is
dependent on seepage water and season.
To tap this water filter pipes (slotted filter pipes or PVC pipes
with a conical bottom point) is driven inside the soil to a depth of
about 9 to 15 m and water is lifted by means of ordinary pumpset from
this filter point.
Rainfall
Rainfall is dependent in different degrees, on the South-West monsoon,
North-East monsoon, on shallow cyclonic depressions and disturbances
and on violent local storms. India receives most of its rainfall from
the South - West monsoon originating in the Indian ocean. About 75 %
of the rainfall is received in four months i,e., June to September.
Unequal geographical distribution, unequal seasonal distribution and
frequent departures from the normal rainfall characterize the rainfall
of this country.
South - West Monsoon Rainfall received during the months of June
- July is critical and the fate of the Kharif crop depends very largely
on distribution and amount of rain during these two months. South-West
monsoon is responsible for 75-80% or more of the total annual rainfall
in the country.
North - East Monsoon During October - November cyclonic storms form
in the Bay of Bengal and when they strike coastal Andhra Pradesh or
coromandel coast they bring heavy rain to these areas. About 11 % of
the total rainfall in the country is received during this season.
Flooding method of irrigation is exclusive for lowland rice though
it is used for some other crops also. Water is allowed from the channel
into the field without much control on either side of the flow. It covers
the entire field and moves almost unguided.
The ideal size of each plot or basin is 0.1 to 0.2 ha for economising
water.Uneven distribution and low water application efficiency are the
common drawbacks of this method.
Basin Irrigation
Basin method is almost similar to check - basin method except that
in the check-basin method entire field is irrigated while in basin method
only the basin around the trees are irrigated.
This method is suitable for fruit crops. Basins are generally round
in shape, occasionally square in shape. The basins are small when the
trees are young and their size is increased with age of the trees. Basins
are connected by an irrigation channel.
Check-Basin Method
Check-basin method of irrigation is the most common method among surface
methods of irrigation. In this method the field is divided into small
plots surrounded by small bunds on all the four sides.
Water from head channel is supplied to the filed channels one after
the other. Each field channel supplies water to two rows of check basins
and water is applied to one basin after another. This method is adopted
when the field is quite large and is not easy to level the entire field.
In such situations, the field is divided into small strips and each
strip into several plots by putting bunds and these plots are called
check basins.
The advantage of this method is that the water can be applied uniformly
and effectively. It is suitable for close growing crops like groundnut,
wheat, fingermillet, pearlmillet, paragrass etc.,. The disadvantages
are more labour is required, more land is wasted under channels and
bunds. Intercultivation is not possible due to bunds.
Border Strip Method
The field is divided into number of stripes by forming bunds of around
15 cm height. These parallel earth ridges are called borders, and are
formed to guide a sheet of flowing water across a field.
The area between two borders is the border strip. Length of the strip
ranges from 30 to 300 m and width from 3 to 15 m. However, the most
common sizes are 60 to 90 m in length and 6 to 12 m in width.
The size of border strips depend on stream size, soil structure and
slope of the land. The borders are laid out along the general slope
or on the contour. Water from the channel is allowed into each strip
at a time. This method is suitable for close growing crops and medium
to heavy textured soils, but not suitable for sandy soils.
Drip Irrigation
It is defined as the precise, slow application of water in the form
of discrete or continuous or tiny streams of miniature sprays through
mechanical devices called emitters or applicators located at selected
points along water delivery lines.
It is also called trickle irrigation. Drip irrigation is adopted
extensively in areas of acute water scarcity and especially for crops
such as Coconut, Grape, Banana, Ber, Citrus, Sugarcane, Cotton, Maize,
Tomato, Brinjal and plantation crops. The advantages of drip irrigation
are,
No fertilizer nutrient loss due to localized application.
High water distribution efficiency.
Levelling of the field not necessary.
Only root zone is saturated.
Moisture always at field capacity in the root zone.
Soil factor plays less important role in frequency of irrigation.
No soil erosion.
Highly uniform distribution of water i.e., controlled by each nozzle.
Low labour cost.
Variation in supply can be regulated by regulating the valves and
drippers.
Fertigation can be adopted with drip irrigation.
The disadvantages of drip irrigation is expensive i,e., initial cost
is more in installing drip method.
Sprinkler Irrigation
Sprinkler irrigation system conveys water from the source through
pipes under pressure to the field and distributes over the field in
the form of spray of 'rain like' droplets. It is also known as over
head irrigation.
Different types of sprinkler systems namely portable, semi-portable,
semi-permanent and permanent are in vogue. But due to increased labour
costs and energy costs, different types of sprinklers are developed.
Centre-pivot system is largest sprinkler system with a single machine
can irrigate upto 100 ha. A centre - pivot sprinkler consists of a series
of sprinklers mounted on a lateral pipe, 50 - 800 m long, mounted or
carried by a row of five or more mobile towers.
One end of the lateral is fixed on a pivot pad. The unit rotates
around a centre pivot where water is pumped into the pipe, and water
is distributed through sprinkler fitted on lateral. The limitations
of this system are,
10 - 20 % of area is not irrigated at the corners of square or rectangular
plot.
High energy requirement and Huge cost of the equipment.
Now lateral - move systems are developed to overcome the draw backs
in centre-pivot system for irrigating square or rectangular plots. This
irrigation system consists of lateral - move systems which move up and
down the field.
Sprinkler irrigation can be advantageously chosen in the following
situations
When the soil is too shallow eliminating the possibility of levelling
of lands.
When the land is too steep ( > 1% slope).
When light (< 5 cm) and frequent irrigations are to be given.
When soils are very sandy (rapidly permeable coarse textured soils)
and
When supplemental irrigation is to be given to dryland crops during
prolonged dry spells, without any land preparation.
Disadvantages
High winds ( > 12 km/hr) cause improper distribution of water.
Evaporation losses are high from sprinkler irrigation especially
under high temperature and low relative humidity conditions.
The initial cost is high,
Some sort of knowledge is needed for successful operation of sprinkler
system.
Under irrigation causes reduction in photosynthesis due to reduction
in photosynthetic rate, chlorophyll content and leaf area.
Due to under irrigation, water deficit occurs, as a result stomata
are closed, so that reduction in transpiration takes place.
Translocation of assimilates is also affected by water stress.
Respiration rate decreases with increased moisture stress.
Due to under irrigation enzymatic activity decreases. So that accumulation
of sugars and aminoacids takes place due to breakdown of carbohydrates
and proteins.
Due to under irrigation hormonal balance is altered.
Due to under irrigation reduction in fixation, uptake and assimilation
of nitrogen takes place.
Excess irrigation causes several changes in the soil and plant resulting
in reduced growth and in some cases death of plants.
Germinating seeds are sensitive to waterlogging since they are totally
dependent on the surrounding soil space for oxygen supply.
Yield of cereals depressed if the excess irrigation given at panicle
development stage. iv. Excess water causes injury to the plant due to
low oxygen supply to the root system and accumulation of toxic substances
in soil and plant.
Wilting of tobacco takes place when bright sunshine occurs after a
prolonged wet spell.
Leaching of nitrates and denitrification occurs resulting in nitrogen
deficiency.
. Shoot elongation, senescence, abscission and production of adventitious
roots takes place as a result of continuous excess irrigation.
Respiration in the roots change from aerobic to anaerobic with the
result, toxic substances accumulates in roots and damage the root system.
Permeability of roots decreased due to shortage of O2. It results
in decreases water and nutrient uptake.
Generally water is last through leaching, drainage, evapotranspiration
and runoff.
The following disadvantages will be observed due to water loss,
Soil becomes very hard.
The germination percentage will be decreased.
· The nutrients in the soil leaches or evaporates.
The root growth retards, so that plant becomes stunted as a result
yields become reduced.
Stomata becomes closed, so that the transpiration process caused
as a result accumulation of gases or metabolic wastes increases, leads
to death of the plant.
Water use efficiency is the yield of marketable crop produced per
unit of water used in evapotranspiration.
WUE = Y/ET
Water use efficiency is also known as crop water use efficiency or
consumptive water use efficiency (Ecm) if the water used for metabolic
purpose of the crop (G) and is included with ET.
ECU = Y/G+ET
If yield is proportional to ET, water-use efficiency has to be a constant
but it is not so. Actually, Y and ET are influenced independently or
differently by crop management practices, while ET is mainly dependent
on climate and soil moisture. Fertilization and other cultural practices
for high crop yields usually increases WUE. The factors affecting WUE
are nature of the plant, agronomic practices, climate, ET, irrigation,
fertilization and plant population.
There are considerable differences between plant species to produce
a unit dry matter per unit amount of water used resulting in widely
varying values of water use efficiency. The water use efficiency for
few crops is listed below.
pH is the negative logarithm of hydrogen ion concentration. If pH
is 7.0, it is considered as neutral. If the pH is less than 7.0 and
H+ concentration exceeds OH- it is referred as acidic and if pH ranges
7 - 14 it is considered as alkaline. The pH is a sort of voltage measurement
to cover the entire range of 0-14. The pH is one of the parameters to
assess the water whether it is suitable for irrigation or not based
on pH values.
Main cations present in irrigation water are calcium, magnesium, sodium
and potassium. In effluents and sewage waste waters ammonium and heavy
metal cations are also found. The important anions like chlorides, carbonates
and bicarbonates, sulphates and nitrates are also present in irrigation
water.
For appraisal of irrigation water quality the water samples are mainly
analyzed for total salts (EC) relative proportion of cations, anions
and toxic substances such as excess boron and fluorine. For example,
the pH of bicarbonate (HCo3) waters is usually more than 7.5 and its
determination may reflect the degree of sodicity in the sample.
Sulphate content will be more in saline water having higher E.C.
If boron content is more than 2.0 mg/1(ppm) in irrigation water, it
is harmful to most of the crops. Fluorine content beyond 10 ppm in irrigation
water is harmful indirectly to animals who feed on plants irrigated
with high fluoride waters. Sodium at higher levels in irrigation water
exerts a toxic effect on crop growth.
Good irrigation water should not have excessive amounts of any salt
or toxic substances.
Water EC
Natural water has E.C value of much less than one unit. These values
are reported as milli mhos (EC x 10-3) or micro mhos (EC x 10-6) at
250C. Electrical conductivity serves as a guide to know the extent of
soluble salts present in irrigation water. The criteria for judging
the quality of irrigation water is the total salt concentration as measured
by electrical conductivity. The harmful effects increases with increase
in total salt concentration.
Irrigation water may be classified based on EC are,
C1 - Low Salinity Water
If electrical conductivity is less than 0.25 ds/m, the irrigation
water is classified as low salinity water. It can be used for irrigation
on all soils and on most crops but leaching is required in case of extremely
low permeable soil.
C2 - Medium Salinity Water
It has EC between 0.25 to 0.75 ds/m. This water can be safely used
for crops with moderate salt tolerance. The soil should have moderate
level of permeability and leaching to avoid accumulation of salts.
C3 - High Salinity Water
Water with EC ranges of 0.75 to 2.25 ds/m is called high salinity
water. This water can not be used on soils with poor drainage. This
water can be used for salt tolerant crops by providing good drainage
and also by practicing management practices for salinity control.
C4 - Very High Salinity Water
If EC is more than 2.25 ds/m the water is classified as very high
salinity water. It is not suitable for irrigation under ordinary conditions
but may be used occasionally if the soil is permeable by providing adequate
drainage.
Central Soil Salinity Research Institute (CSSRI) suggested
another classification of Irrigation water based on EC as follows,
Class
EC (ds/m)
Quality of water
Soils and crops suitable
A1
< 1.5
Normal waters
Most soils, most crops
A2
1.5 - 3.0
Low salinity waters
Most crops on light and medium textured soils. Semi-tolerant
crops on heavy textured soils.
A3
3 - 5
Medium salinity waters
Semi-tolerant crops on light and medium textured soils
and only tolerant crops on heavy texture soils not suitable for deep
black soils. The soils should have a fairly good drainage.
A4
5 - 10
Saline waters
Tolerant crops on light and medium texture soils. Soils
have excellent drainage
A5
> 10
High saline waters
Not suitable for irrigation under ordinary conditions.
Average yields of irrigated crops are below the economic optima because
data on the best combination of fertilizer, plant population and irrigation
regime are meagre to recommend to the farmers. Maximum WUE can not alone
be the goal always. The economics of obtaining high yields dominate
the scene. Yield increases from fertilizers, plant population, irrigation,
etc., follows some kind of decreasing increment function after a stage,
such that each successive unit of input produces less profit than its
predecessor.
The general tendency is to over-irrigate, especially if water is not
brought on the basis of quantity used. This tendency can be avoided
only if information is available on the most efficient way to use water,
and if field service is organized to advice the farmer on when to irrigate
and how much water to apply at each irrigation for a certain level of
fertilization and plant population. The three important approaches listed
below are to be taken into account for irrigation.
Soil Based Criteria
Depletion of available soil moisture i.e., feel and appearance method.
Plant Based Criteria
Critical stages approach, visual symptoms of the plant, water content,
leaf temperature.
Climatological Criteria Iw/Cpe Ratio
Surface irrigation methods are commonly used for various crops. But
rice is irrigated by flooding. Crops like Potato, Maize, Sugarcane,
Cotton are commonly irrigated with furrow method. Basin method of irrigation
is adopted for fruit trees.
The amount of water to be applied at each irrigation depends on the
amount of moisture depleted in the effective root zone depth.
The moisture extraction pattern from different depths of the soil
within the crops root zone depth in deep uniform soils is about 40 %
of the total moisture from first quarter of the root zone, 30 % from
the second, 20 % from the third and 10 % from the last quarter. At early
stages of crop growth, the depth of water applied should be less since
the root system is shallow.
Generally the amount of water applied at each irrigation is about
50 mm in red soils and 60 mm in black soils.
