SIGNIFICANCE OF IRRIGATION SHEDULING AND TECHNIQUES

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SIGNIFICANCE OF IRRIGATION SHEDULING AND
TECHNIQUES
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ABSTRACT

• 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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INTRODUCTION

• 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 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 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

• 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 cant 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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(Cont)..
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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.

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1. 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 method

• It is the direct method of measuring the moisture
  content of soil.
• Samples taken from the field, weighted, dried at
  105C 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 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 soil.
Available soil moisture range Coarse texture (loamy sand) Moderately coarse texture (sandy loamy) Moderately coarse texture (sandy loamy) Medium texture (loamy and silt loamy) Fine texture (clay loamy and silty clay 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 Similar symptoms Similar symptoms Similar symptoms Similar symptoms
75 to 100 Tends to stick together slightly, sometimes forms a very weak ball under pressure Forms weak ball, breaks easily, dont slick Forms a ball, very pliable, slicks readily Forms a ball, very pliable, slicks readily Forms a ball, very pliable, slicks readily Easily ribbons out between fingers, has slick feeling
50 to 75 Appears to be dry dont 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 somewhat plastic, some-times slick slightly with pressure 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, dont form a ball unless squeezed very firmly Some what crumbly but holds together with pressure Some what crumbly but holds together with pressure 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. Powdery dry, sometimes slightly crusted but easily broken down into powdery conditions. Powdery dry, sometimes slightly crusted but easily broken down into powdery conditions. Hard, baked, cracked, sometimes has loose crumbs on surface.
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Tensiometer 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.

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

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Electrical resistance method

• 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 dont work at low level
  tensions.

• Also, both of these methods dont provide
  moisture status.

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Water 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.

• Determining the balance of moisture in the soil.

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2. 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.

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

(Cont)
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Appearance 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.

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Leaf 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 leafs
  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 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

• 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 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

• 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, tobacco
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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) ETm (mm/day) ETm (mm/day) ETm (mm/day) ETm (mm/day) ETm (mm/day) ETm (mm/day) ETm (mm/day) ETm (mm/day)
Crop group 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

• 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 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 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 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 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
crops
Crop 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 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.
  
• 
  (Cont)

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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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Summary

• 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.

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• 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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Assessment

• 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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References

• Cambell, 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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