SIGNIFICANCE SIGNIFICANCE OF IRRIGATION OF IRRIGATION SHEDULING SHEDULING AND AND TECHNIQUESTECHNIQUES

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ABSTRACTABSTRACT Scheduling of irrigation to crops is essential for

efficient utilization of available water, saving of input and enhancing yield.

It is prime process decides two important questions in irrigation, ‘when to irrigate?’, ‘how much to irrigate?’.

Soil indicators such as gravimetric method, feel and appearance method, tensiometer method, electrical resistance method and water budget technique; plant indicators like appearance and growth, leaf water potential and stomatal resistance techniques; meteorological indicators viz., evapotranspiration of the crop and IW/CPE approach, besides combination approach decides when to irrigate?.

The quantity of irrigation water to be applied (how much to irrigate?) at each irrigation depends upon the amount of available moisture in the soil (at effective root depth).

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INTRODUCTIONINTRODUCTION Scheduling of irrigation is a process to decide ‘when to

irrigate’ and ‘how much to irrigate’ to the crops. Proper scheduling is essential for efficient use of irrigation

water, inputs such as seeds, fertilizers, labour etc. Appropriate scheduling of irrigation not only saves water,

but also, saves energy besides, higher crop yield. Farmers are generally irrigating their crops on either time

interval basis (say weekly interval, ten days interval) or based on the appearance of the crops (based on wilting symptoms).

There are several soil, plant and atmospheric (meteorological) indicators in addition to combination approach, critical stage approach etc. to decide when to irrigate? the crop.

Similarly, based on the moisture content in the effective root zone quantity of irrigation water (how much to irrigate?) to crops is decided.

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LEARNING OBJECTIVESLEARNING OBJECTIVES

To study the importance of scheduling of irrigation to crops.

To learn the detailed methods of scheduling of irrigation along with their merits and limitations.

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MAIN BODYMAIN BODY Most of the farmers follow irrigation practices which are resulting

in either under-irrigation or over-irrigation of crops, resulting in low production per unit of water (water use efficiency).

There are two situations farmers are frequently faced: Where adequate water is available, farmer aims is to produce

maximum yield per unit of land and unit of water. Here, he has to provide optimum irrigation schedules, with time-

sequence for number of irrigations and quantity of each irrigation, for ensuring optimum crop yield with high water-use efficiency.

Where a limited quantity of water is available, he aims to produce maximum yield per unit of water.

In this case, information is to be provided for rationalizing the limited water distribution over the available land, applying water at moisture sensitive stage of crop growth and withholding irrigation at other stages.

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I. WHEN TO IRRIGATE I. WHEN TO IRRIGATE Crops vary with their soil moisture requirement for

maximum yields and quality of produce. Most plants are efficient in absorbing water from soil, if the

soil moisture level is nearing at field capacity (-0.33 bar). As the soil moisture level drops from field capacity due to

evapotranspiration and other losses, soil moisture tension naturally increase and eventually crops can’t extract needed moisture from soil for their optimum growth.

Crops start to wilt and growth is first retarded and then completely stops.

When the moisture level is restored again by addition of irrigation water or rain, some crops regain their growth and show little or no permanent damage.

Other crops, however, are permanently damaged.

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These crops are generally drought tolerant. Ex. Sorghum, pearl millet, finger millet, cotton.

For certain crops, providing irrigation at 25% depletion of available moisture enhance yield levels.

Ex. Maize, wheat. Crops should not experience moisture stress in the period between two irrigations, which naturally happens under field condition especially under light textured (sandy, sandy loamy) soils.

Irrigation has to be given when there is adequate moisture in the soil to meet transpiration demand of the crop and evaporation need of atmosphere.

By knowing the amount of moisture available in the root zone of the crop and the evapotranspiration demands of the crop and atmosphere, it is easy to determine when irrigation is needed.

There are several approaches to decide ‘when to irrigate’ based on soil, plant and atmospheric parameters, combination of soil and atmospheric parameters and critical crop stage approaches. Next Previous End

1. Soil Indicators1. Soil Indicators

These methods involve in determining moisture content of the soil and finding the deficit level in available moisture.

Based on pre-determined minimum water content, irrigation is given to bring the soil to field capacity.

The soil water content is determined either by direct measurement or inference from measurements of other soil parameters such as soil water potential or electrical conductivity.

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Gravimetric methodGravimetric method

It is the direct method of measuring the moisture content of soil.

Samples taken from the field, weighted, dried at 105°C for about 24 hours till constant weight is obtained and again weighed after drying.

The difference in weight between the wet (WS1) and oven dry (WS2) samples gives the moisture content (Pw) in percentage.

WS1-WS2

Pw (%) = WS2

The method is simple and reliable, but, time consuming and sampling is destructive.

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Feel and appearance methodFeel and appearance method

With experience, farmer can judge soil water content by the feel and also appearance of the soil.

Soil samples are taken with a probe or soil auger from each quarter of the root zone depth, formed into a ball, tossed into air and caught in one hand.

From the description given in Table 1, available moisture percentage is estimated for different textures of soils.

Considerable experience and judgment are necessary to estimate available soil moisture content in the sample within reasonable accuracy.

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Guide for judging the amount of available moisture in Guide for judging the amount of available moisture in soilsoil..Available soil

moisture rangeCoarse texture (loamy sand)

Moderately coarse texture (sandy loamy)

Medium texture (loamy and silt loamy)

Fine texture (clay loamy and silty clay loamy)

Field capacity (100%)

On squeezing, no free water appears on soil, but wet outline is left on hand

Similar symptoms

75 to 100% Tends to stick together slightly, sometimes forms a very weak ball under pressure

Forms weak ball, breaks easily, don’t slick

Forms a ball, very pliable, slicks readily Easily ribbons out between fingers, has slick feeling

50 to 75% Appears to be dry don’t form a ball with pressure

Tends to form a ball under pressure but seldom holds together

Forms a ball somewhat plastic, some-times slick slightly with pressure

Forms a ball, ribbons out between thumb and fore-finger

25 to 50% As above, but ball is formed by squeezing very firmly

Appears to be dry, don’t form a ball unless squeezed very firmly

Some what crumbly but holds together with pressure Somewhat pliable, forms a ball under pressure

0 to 25% Dry, loose, single grained flows through fingers

Dry, loose, flows through fingers

Powdery dry, sometimes slightly crusted but easily broken down into powdery conditions.

Hard, baked, cracked, sometimes has loose crumbs on surface.Next Previous End

Tensiometer methodTensiometer method Irrigation can be scheduled

based on soil moisture tension. Tensiometers (Irrometers) are

installed at specified depth in the root zone.

When the soil moisture tension reaches to a specified values (0.5, 0.75 or 1.0 bars etc.) irrigation is scheduled.

Tensiometers are generally used to schedule of irrigation in orchards, especially in coarse textured soils.

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This method however, fails to provide the quantity of water to be irrigated.

Electrical resistance Electrical resistance methodmethod

The principle involved in electrical resistance method is that a change in moisture content of the soil gives change in electrical conductivity in a porus block placed in a soil.

Gypsum, nylon, nylon and fibre, fibre glass blocks are generally used to measure a tension of different levels.

Use of tensiometers and electrical resistance (gypsum blocks) methods are not popular, because, tensiometer have working range of 0 to 0.8 bars, whereas, gypsum blocks don’t work at low level tensions.

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Also, both of these methods don’t provide moisture status.

Water budget techniqueWater budget technique

It is computed by posting everyday ET, effective precipitation, soil water content etc.

This method is cumbersome and lot of data is required.

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Determining the balance of moisture in the soil.

2. Plant indicators2. Plant indicators The primary objective

of irrigation is to supply of water to meet the plant needs.

Monitoring plants is the most direct method of determining irrigation scheduling.

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Plant parameters have to be related to soil water content to determine the irrigation scheduling.

Appearance and growthAppearance and growth Deliberate visual indicators to asses the water need in

plant are leaf and shoot wilting, leaf colour, drooping of leaves, rolling of leaves etc.

But, appearance and growth are not often effective parameters for deciding irrigation scheduling, as plants exhibit visible symptoms of deficiency long after they experience moisture stress.

When partial or full stomatal closure occurs due to reduction of transpiration (because of reduced availability of water to the plant), there is a rise in leaf temperature.

A hand-held infrared thermometer measures the difference between plant canopy temperature (Tc) and air temperature (Ta) and displays Tc-Ta values.

This Tc-Ta value is much useful for scheduling of irrigation. Positive values in Tc-Ta values are an indication of more temperature in the canopy than atmosphere (stress in plant canopy) and irrigation is to be given.Next Previous End

Leaf water potentialLeaf water potential Leaf water potential indicates the water needs to

plants. Leaf water potential is measured by removing a

leaf and placing it in a pressure chamber /apparatus.

The pressure in the chamber is slowly increased until fluid is forced from the leaf.

The pressure used is a measure of the leaf’s moisture potential.

The leaf age, leaf exposure to solar radiation and time of day when leaf is sampled all significantly influence the results.

Lower potentials indicate a greater need for water. This method is not extensively used since considerable time, care and training are required to obtain reliable results.

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Stomatal resistanceStomatal resistance Leaf resistance to vapour diffusion into the

atmosphere is primarily governed by the degrees of stomatal closure which under sufficient day light is mainly regulated by leaf water deficits.

Stomatal resistance is, therefore, an index to the need for water, since it is related to the degree of stomatal opening and the rate of transpiration.

High resistance generally indicates significant stomatal closure, reduced transpiration rates and the need for water.

Leaf diffusion parameters are used to measure stomatal resistance.

The skill required to take measurements and time involved to interpret limit the use of this method for research purposes.

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3. Meteorological indicators 3. Meteorological indicators When supply of soil moisture is adequate for

the plant, evapotranspiration is primarily controlled by the evaporative demand of the air atmosphere.

Meteorological concepts and approaches have been used as indicators to determine ‘when to irrigate?’.

Irrigation can be conveniently scheduled to a crop, if allowable water depletion in the root zone and evapotranspiration of the crop for short periods during the crop period is known.

At the end of each such period, the crop sown after the soil is brought to field capacity would require irrigation with the depth of water sufficient to meet the total cumulative evapotranspiration less effective rainfall during the period since previous irrigation.

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IW/CPE approachIW/CPE approach In this approach, a known quantity of irrigation water (IW) is applied when cumulative pan evaporation (CPE) reaches a predetermined level.

The amount of water given in each irrigation ranges from 4 to 6 cm, the most common being 5 cm of irrigation.

Scheduling irrigation at an IW/CPE ratio of 1.0 with 5 cm of irrigation water is applied when the CPE reaches 5 cm.

Generally, irrigation is scheduled at 0.75 to 0.8 ratio with 5 cm of irrigation water.

In IW/CPE ratio approach, irrigation can also be scheduled at fixed level of CPE by varying amount of irrigation water.

However, the equipment to measure CPE and IW are not easily available with the farmers.

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4. Combination approach 4. Combination approach Doorenbos and Kassam (1979) combined the soil moisture depletion approach and climatological approach for sufficient and deficit irrigated condition.

Similar to soil moisture depletion approach, total soil available water (Sa) is defined as the depth of water in mm/m between field capacity and permanent wilting point.

The proportion of Sa that can be depleted without casing actual ET (ETa) is known as fraction (P) of the Sa.

The P values become less than potential ET (ETm). The value of P depends on the crop, magnitude of ETm

and the soil. Some crops such as vegetables, continuously needs

relatively moist soil to maintain ETa = ETm.; other crops such as cotton and sorghum, ETa falls below ETm.

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Crops can be grouped according to the P to which the Sa can be depleted while maintaining ETa equal to ETm (Table 2).

The P value varies with growth period, due to greater P values during ripening phase.

For conditions where ETm is high, P is smaller; the soil is comparatively wet when ETa becomes less than ETm under high ETm conditions than when ETm is low.

Consequently, the fraction P varies with the level of ETm (Table 3).

Table 2. Crop groups according to fraction of soil water depletion (P)

Group No. Crops

1 Onion, potato

2 Cabbage, grapes, pea, tomato

3 Groundnut, sunflower, wheat

4 Cotton, maize, safflower,

sorghum, soybean, sugarcane,

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Table 3. Soil water depletion fraction (P) for different crop groups and maximum evapotranspitation rates (ETm)

Crop

group

ETm (mm/day)

2 3 4 5 6 7 8 9 10

1 0.50 0.43 0.35 0.30 0.25 0.23 0.20 0.20 0.18

2 0.60 0.58 0.48 0.40 0.35 0.32 0.28 0.25 0.23

3 0.80 0.70 0.60 0.50 0.45 0.43 0.38 0.35 0.30

4 0.88 0.80 0.70 0.60 0.55 0.50 0.45 0.43 0.40

FAO, Irrigation and Drainage paper No. 33

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5. Rough methods for farmers 5. Rough methods for farmers

Simple methods are suggested to the farmers to find out when to start irrigation and how much water to apply.

They use only the feel and appearance method described earlier as a rough guide to know when to irrigate and the probe is used to determine when to stop irrigation.

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Can evoporimetry methodCan evoporimetry method Small can of one litre capacity (14.3 cm height and

10 cm diameter) are used to indicate evaporation from the cropped field.

The cans are white painted and covered with 6/20 size mesh.

A pointed indicator is fixed at 1.5 cm below the brim of can.

When irrigation is given (bringing the soil to field capacity), the can is filled up with water to pointer level and kept to the crop height. Evaporation from the can is directly related to the crop evaporation.

Irrigation is scheduled when water level in the can falls to a predetermined level (equal to the amount of water to be applied at each irrigation) and can is filled again to pointer level.

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Soil cum mini-plot techniqueSoil cum mini-plot technique

In this method, 1x1x1 m size of pit is dug in the middle of the field.

About 5% of sand (by volume) is added to the pit, mixed well with soil and the pit is filled up in natural order.

Crops are grown normally in all areas including pit area.

The plants in the pit show wilting symptoms earlier than the other areas.

Irrigation is scheduled as soon as wilting symptoms appear on the plants in the pit.

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Sowing high seed rateSowing high seed rate

In an elevated area, one square metre plot is selected and crop is grown with four times thicker than the normal seed rate.

Because of high plant density, plants show wilting symptoms earlier than in the rest of the crop area indicating the need of scheduling of irrigation.

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5. Critical stage approach5. Critical stage approach

In each crop, there are certain growth stages at which moisture stress leads to irrevocable yield losses.

These stages are called as critical period or moisture sensitive period.

Hence, irrigation must be given to these stages to avoid yield losses.

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Table 4. Moisture sensitive stages of important cropsCrop Important moisture sensitive stages

Rice, pearl millet, finger millet Panicle initiation, flowering

Wheat Crown root initiation, jointing, milking

Sorghum Seedling, flowering

Maize Silking, tasselling

Groundnut Rapid flowering, pegging, early pod formation

Redgram, greengram, blackgram, peas Flowering, pod formation

Sugarcane Formative stage

Sesame Blooming to maturity stage

Sunflower Two weeks before and after flowering

Safflower Rosette to flowering

Soybean Blooming, seed formation

Cotton Flowering, boll development

Tobacco Transplanting to full blooming

Chilli Flowering

Potato Tuber formation to tuber maturity

Onion Bulb formation to maturity

Tomato From commencement of fruit setting

Cabbage Head formation to firming stage of head

Carrot Root enlargement

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II. HOW MUCH TO IRRIGATEII. HOW MUCH TO IRRIGATE

The quantity of irrigation water to be applied to the soil at each irrigation depends upon the amount of available moisture in the soil (specifically at effective root depth i.e. moisture extraction depth of the roots), at the time of starting irrigation (or the level of available moisture depletion from filed capacity) at which irrigation is proposed.

The effective rainfall expected in the period between this irrigation and the next one and the additional quantity of irrigation water required if salts are to be leached beyond root zone and the application losses.

The basic principle is mainly to give irrigation to bring the soil (at effective root zone depth of crops) to field capacity.

More often, allowance is given for expected effective precipitation to be stored in the soil.

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It may be more economical to irrigate to achieve maximum production per unit of water than the current practice of irrigating for maximum production per unit of land.

Farmers use a probe which is a steel rod of about 2.5 cm diameter and 60 cm long with a cross bar welded on the top to facilitate insertion into the soil and removal, to decide when to stop giving irrigation.

The probe is pushed into the soil with minimum effort and the depth that it can be pushed into the soil gives a good indication of the depth of wetting.

When the moisture extraction depth is wetted, irrigation is stopped.

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SummarySummary Scheduling of irrigation is a process decides ‘when

to irrigate’ and ‘how much to irrigate’ to the crops. Most plants are efficient in absorbing water from

soil, if the soil moisture level is nearing at field capacity (-0.33 bar).

Soil indicators, plant indicators, meteorological indicators, combination approach (of soil and meteorological), rough methods for farmers and critical stage approach are some of the means to scheduling irrigation.

Soil indicators involve in determining moisture content of the soil and finding the deficit level.

Gravimetric method, feel and appearance method, tensiometer method, electrical resistance method and water budget technique are used as soil indicators. Next Previous End

Plant parameters have to be related to soil water content to determine the irrigation scheduling.

Appearance and growth, leaf water potential and stomatal resistance techniques are used as plant indicators.

Meteorological indicators such as evapotranspiration of the crop are important to identify the irrigation need and IW/CPE approach is mainly followed here.

Simple methods such as can evoporimetry method, soil cum mini-plot technique and sowing high seed rate are used by farmers to decide irrigation scheduling.

Critical stage of crop for irrigation is identified to crops and irrigation is given during the stage to avoid yield losses is called critical stage approach.

The quantity of irrigation water to be applied (how much to irrigate) to the soil at each irrigation depends upon the amount of available moisture in the soil (at effective root depth).

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AssessmentAssessment Tensiometer works well in a range of 0 to

0.8 bars pressure (True/False) Hand held infrared thermometer is used to

measure the leaf temperature and in turn irrigation scheduling (True/False)

In IW/CPE ratio, most commonly used depth of water is 10 cm (True/False)

Critical stage for irrigation in wheat crop is Crown root initiation (True/False)

Plant absorbs water from soil efficiently when the soil moisture level is at Permanent wilting point (-15 bar) (True/False)

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ReferencesReferencesCambell, S.G. and D.M. Cambell. 1982. Irrigation scheduling

using soil moisture measurements. Theory and Practice. Advances in Irrigation. 1: 43-85.

Doorenbos, J. and A.H. Kassam. 1979. Yield response to water. FAO Irrigation and Drainage Paper No. 33. Food and Agriculture Organization of United nations, Rome.

Hagan, R.M. and J.F. Laborde. 1964. Plants as indicators of need for irrigation. In: Proc. 8th Int. Cong. Soil Sci. Bucharest. Romania, 11: 394-422.

Mishra, R.D. and M. Ahemed. 1990. A practical manual on irrigation. Oxford and IBH Publishing Co. Pvt. Ltd., New Delhi.

Prihar, S.S. and B.S. Sandu. 1987. Irrigation of field crops – Principles and Practices. ICAR, New Delhi.

Sankara Reddi, G.H.. and T. Yellamanda Reddy, 2009. Efficient Use of Irrigation Water. Kalyani Publishers, Ludhiana.

Yellamanda Reddy, T. and Sankara Reddi, G.H. 1995. Principles of Agronomy. Kalyani Publisher, Ludhiana.

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