irrigation scheduling methods

1. IRRIGATION
SHEDULING AND
METHODS

2. 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 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).

3. 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.

4. 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.

5. ADVANTAGE OF IRRIGATION SCHEDULING

6. FARMERS FOLLOW TODAY
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).

7. SITUATIONS FARMERS FREQUENTLY FACE
 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.

8. Factors in Determination of Irrigation Scheduling
 The scheduling tool must consider information about
the crop, soil, climate, irrigation system, water
deliveries and management objectives.
 An irrigation scheduling tool needs only be accurate
enough to determine how much water to apply and
when.
 A good rule of thumb to follow when developing an
irrigation scheduling tool is to keep it simple and easy
to understand.

9. 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.

10. WHEN TO IRRIGATE
 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.

11. IRRIGATION SCHEDULING CRITERIA

12. 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.

13. GRAVIMETRIC 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.
PW (%)=
 The method is simple and reliable, but, time consuming.
WS1-WS2
WS2

14. FEELAND 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.
 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.

15. JUDGING THE AMOUNTOF AVAILABLE
MOISTURE IN SOIL

16. JUDGING THE AMOUNTOF AVAILABLE MOISTURE
IN SOIL
Available
moisture
range
Coarse Texture (Loamy
Sand)
Moderately coarse
texture (Sandy
Loam)
Med. Texture (Loamy
and silt loamy)
Fine texture (Clay
loam and silty clay
loam)
Field capacity
(100%)
On squeezing, no free
water appears on soil, but
wet outline is left on hand
Similar Symptoms
75-100% 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
50-75% 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
25-50 % 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
0-25 % 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.

17. TENSIOMETER METHOD

 Irrigation can be scheduled based on
soil moisture tension.
 Tensiometers 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 wheat /
orchards.
 This method however, fails to provide
the quantity of water to be irrigated.

18. 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.
Also, Electrical
resistance methods
like Gypsum, nylon,
nylon and fibre, fibre
glass blocks are
generally used to
measure a tension of
different levels

19. 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

20. METROLOGICAL 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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

26. Moisture Sensitive Stages of Important Crops
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
Red gram, Pea, Moong, Urd Flowering, pod formation
Sugarcane Formative stage
Sunflower Two weeks before and after flowering
Soybean Blooming, seed formation
Cotton Flowering, boll development
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

27. 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 field 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.
(Contd)…

28. How Much (Depth) to Irrigate ?
 Bout 70 % root mass is found in
the upper half of the max. root
depth.
 About 70 % of the water used by
top 50 % root zone
 Normally, 01 cm water irrigated
to 4-5 cm soil depth in loam soil,
but in sandy soil it may be more
than 5 cm.
 In case 60 cm root depth,
effective root zone will be 30 cm
and irrigation depth will be 30/5
means 6 cm.

29. Factors Affecting Crop Water Need
 Climatic factors eg. temperature, humidity, wind velocity
and sun shine.
 Type of soil eg. Soil texture, soil structure, soil density,
water holding capacity , soil reaction, biological
properties, organic content in soil etc.
 Plant features eg. Vegetative growth, number of stomata,
leaf area, depth of root.
 Agriculture activities. weed control and inter-culture
operations

30. IRRIGATION METHODS

31. Improvement in Water Use Efficiency
 Water Management at field
Levelling of field: Less time in irrigation, uniform water
distribution, water saving, equal moisture environment, proper
seed germination and growth etc
Irrigation scheduling
Improvement in conveyance efficiency
Improved irrigation method e.g. bed planting
Micro irrigation
Water holding in field : bund making
Mulching

32. Improvement in Water Use Efficiency
 Selection of suitable crop varieties
 Timely sowing/panting
 Conjunctive use of surface and ground water
 Renovation of ponds in rural areas
 Agriculture diversification
 Artificial water recharge
 Mass awareness generation

33. 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.
Contd.-----

34. Summary
 Gravimetric method, feel and appearance method,
tensiometer method, electrical resistance method and
water budget technique are used as soil indicators.
 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.
Contd.-----

35. Summary
 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).

36. Summary
 Different methods of irrigation are followed by farmers to
irrigate field.
 Generally, flooding of field is most common method
used by farmers, specially in canal area, which is the
most unwise method.
 Farmers should select the irrigation method, which are
water saving without compromising the crop yield.
 Micro irrigation is most effective method for irrigating
the crops, which save water with improved yield and
quality of produce.
 Major objective should be to improve the crop water
productivity.

37. THANKS