Irrigation and Scheduling and Types
of Irrigation
Net Irrigation Req
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•
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NIR= ET - ERAIN
where
NIR= net irrigation requirement,
ET= evapotranspiration, and
ERAIN = effective rainfall.
ERAIN is that portion of rainfall which can be
effectively used by a crop, that is, rain which is
stored in the crop root zone. Therefore, ERAIN is
less than total rainfall due to interception, runoff
and deep percolation (or drainage) losses.
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Crop and Base period
Gross Irrigation req
• The gross irrigation requirement (GIR) is the
amount that must be pumped. GIR is greater than
NIR by a factor which depends on the irrigation
efficiency (EFF):
• GIR = NIR / EFF
• where
• GIR = gross irrigation requirement (inches),
• NIR = net irrigation requirement (inches), and
• EFF = irrigation efficiency (decimal fraction).
Crop period = crop period is the time in days that a
crop takes From the instant of its sowing to that of
its harvesting
Base period
Base period of a crop refers to the whole period of
cultivation from the time when irrigation water is
first issued for preparation of the ground for planting
the crop to its last watering
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Delta & Duty
GCA and CCA
Delta
Delta is the total depth of water required by a crop during
the entire period of the crop in the field
Example
If a crop requires about 12 waterings at an interval of 10
days and a water depth of 10cm in every watering, then the
delta is 12*10= 120 cm
• Gross command area(GCA)
– The GCA is the total area lying between drainage
boundaries which can be commanded or irrigated by
canals.
Duty
Duty is defined as the no of hectares/ acres that one cumec
Or cusec of water can irrigate during the base period
• Culturable Command area (CCA)
– The GCA also contains unfertile barren land alkaline
soil, local ponds, villages, etc . These are known as
uncluturable areas. The remaining area is called CCA
Relation between duty and delta
Delta= 8.64*(B/D) meters
D= duty in hectares/cumec
B= base period in days.
• GCA= CCA+Unculturable area
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Monthly Estimation of water requirement
Estimation of water requirement
IRRIGATION REQUIREMENT TABLE IR(Cm)/Rotation period(Days)
• Seasonal
• Q= 28 *A * I/(R*T*E)
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CROP
– Where
– A = Area in ha
– I= Depth of irrigation in cm
– R= Rotation period in Days
– E= Irrigation efficiency ( In fraction)
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JAN
-
HY
COTTO
N(IR)
RT
S.CANE
SOYAB
EAN
HY
MAIZE
7
-
HY
JOWAR
FEB
-
-
MAR
-
APR
-
-
-
MAY
JUNE
-
-
-
JULY
-
-
AUG
-
SEP
-
OCT
-
NOV
-
DEC
-
7.50
7.50
7.50
7.50
7.50
7.50
7.50
30.00
30.00
30.00
30.00
15.00
15.00
15.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
15.00
15.00
10.00
10.00
7.50
10.00
30.00
15.00
30.00
15.00
15.00
7.50
7.50
7.50
7.50
7.50
30.00
30.00
30.00
30.00
30.00
7.50
7.50
7.50
7.50
7.50
30.00
30.00
30.00
30.00
30.00
7.50
7.50
7.50
7.50
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• ETo = 7.5 mm/d;
– Hence the net irrigation requirement is:
– 7.5 - (0.7 x 0.2) = 7.36 mm/d;
• and the field irrigation requirement is:
– 7.36/0.60 = 12.3 mm/d
• Canal losses = 48*1.5/(60*60)= 0.02 l/s per metre length
• A = 10ha = 10 x 10 000 m2
• Q = 12.3 x (10 x 10 000) + 800 x 0.02
• = 28 + 16 = 44 l/s
• This design discharge of 44 l/s should be compared with the water
available from the source. If less is available, the area may need
to be reduced, or the irrigation time
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Example: What design discharge is required for a
canal to irrigate an area of 10 hectares in the semi-arid
subtropics, when the mean daily temperature is 30oC,
and the mean rainfall is 0.2 mm/d during the peak
period (midseason)?
The canal is 800m long and is to operate for 12
hours per day.
•Losses from a similar canal are measured as 48mm
per hour with a water-surface width of 1.5m/m length.
•ETo= 7.5 mm/day
•Field irrigation efficiency= 0.6
•Effective rain fall =0.7
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IRRIGATION SCHEDULING
• Irrigation scheduling is defined as the process of
determining when to irrigate and how much
water to apply.
• Through proper irrigation scheduling, it should
be possible to apply only the water which the
crop needs in addition to unavoidable seepage
and runoff losses and leaching requirements.
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Irrigation Scheduling- A decision
process
Irrigation Scheduling
• Irrigation scheduling concerns the farmers' decision process
concerning 'when' to irrigate and 'how much' water to apply in
order to maximize profit.
• This requires knowledge on crop water requirements and
yield responses to water,
• the constraints specific to each irrigation method and
irrigation equipment,
• the limitations relative to the water supply system and the
financial and economic implications of the irrigation practice.
• Thus, the consideration of all these aspects makes irrigation
scheduling a very complex decision making process, one
which only very few farmers can understand and therefore
adopt.
An efficient watering program must
include three basic steps:
1. Determining when water is needed.
2. Determining how much should be
applied.
3. Deciding how water is to be applied.
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Irrigation Scheduling
Common irrigation scheduling approaches include:
1. irrigating on fixed intervals or following a simple
calendar, i.e., when a water turn occurs or according to a
predetermined schedule;
2. irrigating when one's neighbour irrigates;
3. observation of visual plant stress indicators;
4. measuring (or estimating) soil water by use of
instruments or sampling techniques such as feel,
gravimetric, electrical resistance (gypsum) blocks,
tensiometers or neutron probes;
5. by following a soil water budget based on weather data
and/or pan evaporation; and
6. some combination of the above.
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IRRIGATION SCHEDULE
CALENDAR DEVELOPMENT
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• Irrigation calendars for
each crop are normally
determined for two, or
in some cases, three
planting dates, for the
major soils (usually
two per scheme) and
perhaps for two
different initial soil
water contents at the
beginning of the
irrigation season.
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The Check Book Method: Crop
Evapotranspiration
Benefits of Irrigation Scheduling
Action _| August date _|ETc _ |Rainfall _|Accumulated ETc
• Scheduling maximizes irrigation efficiency
by minimizing runoff and percolation
losses.
• This often results in lower energy and water
use
• Optimum crop yields,
• - - - - - - - - - - - - - - inches - - - - - - - - - - - - - - - – Irrigate | 1 _ _ _ _ _ _|0.29 _ |_ _ _ _ _| 0.29
_ _ _ _ _| 2 _ _ _ _ _ _|0 37 _ |_ _ _ _ _| 0.66
_ _ _ _ _| 3 _ _ _ _ _ _|0.38 _ |0.08 _ _ | 0.96
_ _ _ _ _| 4 _ _ _ _ _ _|0.34 _ |1.45 _ _ | ----_ _ _ _ _| 5 _ _ _ _ _ _|0.37 _ | _ _ _ _ | 0.37
_ _ _ _ _| 6 _ _ _ _ _ _|0.26 _ | _ _ _ _ | 0.63
_ _ _ _ _| 7 _ _ _ _ _ _|0.31 _ | _ _ _ _ | 0.94
_ _ _ _ _| 8 _ _ _ _ _ _|0.28 _ | _ _ _ _ | 1.22
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When to Irrigate
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Efficiency of water application
• The most efficient way to water
is to apply water when it begins
to show signs of stress from lack
of water. The following signs are
indications of water need:
• Bluish-gray areas in the field
• Footprints or tire tracks that
remain in the grass long after
being made
• Many leaf blades folded in half
• Soil sample from the root zone
feels dry
• . An efficient watering does not saturate the soil, and
does not allow water to run off.
• . Typically, two to three waterings per week in the
summer and once every 10 to 14 days in the winter
are required. If rainfall occurs, irrigation should be
suspended according to the rainfall amount.
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Manner of applying water
• Water should never be applied at a rate faster than it can be
absorbed by the soil. If the sprinkler applies too much
water, it runs off, and is wasted.
• Avoid extremes in watering frequency and amount. Light,
frequent watering is inefficient and encourages shallow root
systems . Excessive irrigation, which keeps the root system
saturated with water, is harmful. Roots need a balance of
water and air to function and grow properly.
• The time of watering is important. The best time for
irrigation is in the early morning hours.
• Watering during the day can waste water by excessive
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evaporation.
Reading Tensiometers
The tensiometer gauge reads
the tension between soil
and water particles. Soil
moisture tension increases
when there is less water in the
soil. As a result the
tensiometer gauge, Figure 2,
reads high for dry soils
and low for wet soils.
A wet soil would be indicated
by a reading under 10
cbars and a reading above 50
cbars would indicate a
dry soil for most soil types.
•
Soil Moisture Measuring Techniques
Tensiometers
These sensors use a porous ceramic cup
attached to the bottom of a clear plastic
tube/water reservoir and calibrated
vacuum gage to measure soil moisture
tension in centibars.
• Tensiometers come in varying lengths,
from 1 foot to 4 feet in length,
• These devices are also soaked in water
for at least one day before installation.
Good contact between the ceramic cup
and the surrounding soil is also
essential for this device.
• As water flows out of the tensiometer
into the surrounding soil until moisture
equilibrates, it creates a partial vacuum
in the tensiometer body which is read
on the calibrated vacuum gage as
matric potential or soil moisture
tension.
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• There should be at least one, and preferably two,
tensiometer locations (two or more tensiometers
at one location being a station) for each area of
the field that differs in the soil type and depth
• A station located in each different soil type
enables you, through timing and duration of
irrigation to maintain the same amount of
available water in all areas.
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Placement of Tensiometers in the
Field
•For a sprinkler system the tensiometers should be placed in
Placement of Tensiometers in the
Field
the area irrigated by the first lateral within the root zone of the
crop.
•When operating a trickle system the soil should be
maintained at a constant soil moisture. Tensiometers
should be placed 12” to 18” from the emitter in an area
that is representative of where the plants are taking up
water
•With micro-sprinkler systems tensiometers are placed
along the crop row, in the root zone, at the midpoint
between two sprinklers. This should be in an area of the
field that represents typical soil and crop conditions.
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• For any system a second monitoring site should
be installed where a significant change in either
the crop, soil or irrigation system is evident.
• Deep rooted plants, such as fruit trees, should
have two tensiometers per site one at 12” and
one at 24”.
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Wild flooding
•Flood irrigation is the least expensive
irrigation method where water is
relatively cheap.
•It should only be used on very flat
fields, where ponding is not a problem.
•Flooding is a good way to flush salts out
of the soil.
It is highly inefficient
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Check flooding
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Basin flooding
Flooding- When and why
• Adopt
– An abundant supply of water and cheap
– Close growing crops
– Farms with low availability of labour and land is cheap
• Advantages
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–
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Any amount of water can be used
Installation and operation is low
System is not damaged by live stock
System does not interfere with use of farm implements
• Disadvantages
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Furrow Irrigation
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Excessive loss of water
Water distributed unevenly
Fertilisers are often eroded
Drainage must be provided
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Furrow
• Furrow irrigation is relatively
inexpensive where water costs are
low.
• Furrows must be carefully dug to
ensure an even distribution of
water.
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Furrow irrigation
The main objective is to direct
The water between the rows of a crop
and permit it to soak down to the roots
Adopt
•Variable water supply
•Slopes steep >6%
•Medium and fine texured soils
•Where skilled labour is available
Advantages
•Better water efficiency
•Can be used on any row
crops
•Relatively easy to install
Disadvantages
•High erosion
•High skilled labour
•Drainage to be provided
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Border strip irrigation
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Adopt
•Large and dependable supply water
•Soils at least 3 feet deep
•Farms with high land values
•Close growing crops
Advantages
•Efficient use of water
•Uniform application of water
•High water values
•Rapid method
Disadvantages
•Large supply of water is required
•Deep soils are required
•Land must be level
•Drainage must be provided
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• Sprinkler Irrigation: These
methods are more expensive
than flood or furrow irrigation,
but are more efficient at using
water.
• Still, much water is lost through
evaporation, and problems due
to foliar moisture-loving diseases
can arise if over watering occurs.41
Traditional systems
• Drip or Trickle Irrigation.
This is the most expensive,
but most water-efficient,
method.
• Low-quality water (high
in salts) should not be
used, unless filtered, due
to potentially devastating
effects of clogged emitters.
• Also, the use of water
high in soluble salts will
result in localized soil
salinity buildup around
plants, since drip
irrigation is an ineffective
leaching method.
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Picota
By Hand
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Bucket wheet
Mhot
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END
Leevy
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