Computation of irrigation efficiency

Useful of B. Tech Agril. Engg. Graduates

1. Computation of Irrigation Efficiencies
Md. I. A. Ansari
Department of Agricultural Engineering
(e-mail: [email protected])

4. Composition of Soil System
Solid: 50 % (Mineral:45%, Organic matter:
5%)
Water+ air:50%
Optimum growth: 25 % water +25% air

10. Soil Water
•When water is added to dry soil either by
rain or irrigation, it is distributed around
the soil particles, where it is held by
adhesion and cohesive forces.
•Water displaces air in the pore spaces and
eventually fills the pores.
•When all the pores, large and small are
filled, soil is said to be saturated and it is
at its maximum retentive capacity

13. Gravitational Water
•Gravitational water is free water moving
through soil by the force of gravity.
•It is largely found in the macropores of
soil.

14. Capillary Water
•Capillary water is water held in the
micropores of the soil, and is the water
that composes the soil solution.
•Capillary water is the main water that is
available to plants as it is trapped in the
soil solution.

15. Hygroscopic Water
•Hygroscopic water forms as a very thin film
surrounding soil particles and is generally
not available to the plant.
•This type of soil water is bound so tightly to
the soil by adhesion properties that very
little of it can be taken up by plant roots.
•The water uptake becomes zero when
wilting point is reached. Wilting point is the
water content at which plants will
permanently wilt.

16. Field Capacity
•Amount of water in soil after free drainage
has removed gravitational water (2 – 3
days)
•Field capacity is the amount of water that
a well-drained soil should hold against
gravitational forces.
•Soil is holding maximum amount of water
available to plants
•Optimal aeration (micropores filled with
water; macropores with air)

17. Measuring Soil Moisture
•Soil moisture can be indirectly
measured using devices and
instruments eg. tensiometers,
resistance blocks or neutron probes.
•Direct measurement of soil moisture
can be by weighing or the gravimetric
method.

23. Saturated Soil

24. Field Capacity

25. Permanent Wilting Point

30. •Depth of Irrigation:
•This is the depth of the readily available
moisture.
•This is the net depth of water normally
needed to be applied to the crops during
each irrigation

31. •The Moisture Content at Field Capacity of a Clay Loam
Soil is 28% by Weight While that at Permanent Wilting
Point is 14% by Weight. Root Zone Depth Is 1 m and the
Bulk Density Is 1.2 g/cm
3
. Calculate the Net and Gross
Depth of Irrigation Required If the Irrigation Efficiency Is
0.7.
Solution: Field Capacity = 28%; Permanent wilting point =
14%
•i.e. Available moisture = 28 - 14 = 14% by weight
•Bulk density (D
b
) = 1.2 g/cm
3

•Root Zone depth (D) = 1 m = 1000 mm
•Equivalent depth of available water (d)
• = 0.14 x 1.20 x 1000 mm = 168 mm
•This is the net depth of irrigation.

32. •Gross Water Application=
• Net Irrigation/Efficiency = 84/0.7 = 120 mm
Note: This is the actual water needed to be
pumped for irrigation.
It is equivalent to:
120 /1000 mm x 10,000 m
2
=
1200 m
3
per hectare.

33. Irrigation Interval
•This is the time between successive irrigations.
•Irrigation interval is equal to:
•Readily Available Moisture or Net Irrigation/
Evapotranspiration, ET
•The shortest irrigation interval is normally use in
design. The irrigation interval varies with ET.
•It is equivalent to Readily Available Water
divided by the Peak ET

34. • If Peak ET is 7.5 mm/day, Determine the
Shortest Irrigation Interval.
•Solution: Readily Available Moisture
(RAM) = 84 mm
• i.e. Shortest irrigation interval = RAM/
Peak ET = 84/7.5 = 11 days.

35. Transport of Irrigation Water
•Open Channel
•Pipe

38. Canal Lining

39. Canal Lining

41. Digging, Laying And Filling

42. •The lined channels require larger initial
investment.
•Reduce seepage loss by 90 – 95%
•Service life 20 to 50 years.

43. Irrigation Efficiencies
Irrigation efficiency is a critical measure of
irrigation performance in terms of the water
required to irrigate a field, farm, or an entire
watershed.
The objective of irrigation efficiency concept is to
determine whether improvements can be made
in both the irrigation system and the
management of the operation programmes
which will lead to an efficient irrigation water use.

44. Irrigation Efficiencies
Highly dependent on:
System Design
Management
Maintenance
Weather
Operating Conditions

45. •Several terms are used to evaluate
irrigation system performance.
Water conveyance efficiency
Water application efficiency
Water storage efficiency
Water distribution efficiency
Uniformity Coefficient
Overall Irrigation Efficiency

46. Water Conveyance Efficiency (Ec)
•It is used to measure the efficiency of water
conveyance system associated with canal, field
channel, pipelines, etc.
•Irrigation water can be diverted from a storage
reservoir and transported to the field or farm
through a system of canals or pipelines; it can
be pumped from a reservoir on the farm and
transported through a system of farm canals or
pipelines.
•Normally irrigation water pumped and carried in
closed conduits gives conveyance efficiency of
nearly 100 percent.

47. Water Conveyance Efficiency
)(,
)(
s
d
c
WwellorreservoirstreamafromdivertedWater
WFarmthetodeliveredWater
E=
Farm
Water lost by evap
and seepage
Ws
Wd
Stream

48. •Conveyance losses include any canal spills
(operational or accidental) and reservoir
seepage and evaporation that might result
from management as well as losses
resulting from condition of the irrigation
system. It also includes leakage from
pipes.
•Typically, conveyance losses are much
lower for closed conduits or pipelines
compared with unlined or lined canals due
to reduced evaporation and seepage
losses.

49. •45 m
3
of water was pumped into a farm
distribution system. 38 m
3
of water is
delivered to a field which is 2 km from the
well. Compute the Conveyance Efficiency.
Solution:
E
WaterdeliveredtotheFarmW
WaterofwaterdivertedfromastreamreservoirorwellW
c
d
s
=
()
, ()
= 38/45 = 84%

50. •Water Application Efficiency (Ea ):
Application efficiency relates to the actual
storage of water in the root zone to meet
the crop water needs in relation to the water
applied to the field.
•Application efficiency includes any losses
from o evaporation or seepage from
surface water channels or furrows, any
leaks from sprinkler or drip pipelines,
percolation beneath the root zone ,
evaporation of droplets in the air, or runoff
from the field.

51. •Water application efficiency below 100 %
is due to seepage losses from field
distribution channel, runoff and deep
percolation below crop root zone.
•Water losses due to inefficient application
of water in the field vary from 28 to 50 %.

52. •Proper irrigation management can
increase the application efficiency and
poor irrigation management can result in
inefficient use of water and reduce
application efficiency.
•Over irrigation may result in leaching
chemicals below crop root zone cause
yield reduction and result in wasting water
resources.

53. Application Efficiency
E
Waterinrootzoneafterirrigation
Totalvolumeofwaterapplied
a=
TotalvolofwaterappliedVolofTailwaterVolofdeeppercolation
Totalwaterapplied
. (. . )- +

54. Application Efficiency
Ea=(Vs/Vf) x 100
Vs=volume of irrigation water
stored in the root zone
Vf= volume of irrigation water
delivered to the field

55. •Delivery of 10 m
3
/s to a 32 ha farm is continued for
4 hours. Soil probing after irrigation indicates that
30 cm of water has been stored in the root zone.
Compute the Application Efficiency.
•
•Solution: Total volume of water applied
•= 10 m
3
/s x 4 hrs x 3600s/hr = 144,000 m
3
•Total water stored in root zone = 30 cm = 0.3 m
x 32 ha x 10,000 m
2
/ha = 96,000 m
3

56. = 96,000/144,000 = 66.7%.
E
Waterinrootzoneafterirrigation
Totalvolumeofwaterapplied
a=

57. •A 12-hectare field is to be irrigated with a
sprinkler system. The root zone depth is 0.9 m
and the field capacity of the soil is 28% while the
permanent wilting point is 17% by weight. The
soil bulk density is 1.36 g/cm3 and the water
application efficiency is 70%. The soil is to be
irrigated when 50% of the available water has
depleted. The peak evapotranspiration is 5.0
mm/day and the system is to be run for 10 hours
in a day.
•Determine: (i) The net irrigation depth
•(ii) Gross irrigation ie. the depth of water to be
pumped
•(iii) Irrigation period

58. •Solution: Field Capacity = 28%; Permanent
Wilting Point = 17%
• ie. Available Moisture = 28 - 17 = 11% ,
which is Pm
•Root zone depth = 0.9 m;
•Bulk density = 1.36 g/cm
3
•Depth of Available Moisture
• = 0.11 x 1.36 x 900 = 135 mm
• Allowing for 50 % depletion of Available
Moisture before Irrigation, Depth of Readily
Available Moisture = 0.5 x 135
mm = 67.5 mm

59. •i) Net irrigation depth = Depth of the Readily
Available Moisture = 67.5 mm

•ii) Gross Irrigation = Net irrigation
Application efficiency

• = 67.5/0.7 = 96.4 mm
•iii) Irrigation interval = Net irrigation or RAM

Peak ET
• = 67.5/5 = 13.5 days
• = 13.5 days = 13 days

60. Water Storage Efficiency (E
s
)
•The main goal in most irrigation
applications is to maximize water
storage in the soil root zone to satisfy
crop ET while minimizing deep
percolation and surface runoff.
•It is defined as the ratio of volume of
water stored in the root zone to the
volume of water required to fill the root
zone to field capacity.
•

61. Christiansen Uniformity Coefficient (C
u
)

C
X
mn
u
= -
å
10010(.
//
)
This measures the uniformity of irrigation
used for both sprinkler and drip system.

W here: is the summation of deviations from the mean depth
infiltered
m is the mean depth infiltered and
n is the number of observations.
The water distribution efficiency indicates the degree of uniformity in the
amount of the water infiltrated into the soil.
å//X

65. Distribution Uniformity is the measure of how uniformly the water is
applied
* DU is a measure of the irrigation system
Distribution Uniformity or DU
Example of a good DU Example of a poor DU

66. Example of a good DU Example of a poor DU
•The problem with a poor DU
–If enough water is applied to ensure every plant is given adequate
water, we overwater other plants.

67. •Poor water distribution causes water
stress in area receiving relatively low
amounts of water and oxygen stress in
areas that are waterlogged for several
days.
•Both application efficiency and water
distribution uniformity provide a better
indication of overall irrigation system
performance.

68. •A Uniformity Check is taken by probing many
stations down the border. The depths of
penetration (cm) recorded were: 6.4, 6.5, 6.5,
6.3, 6.2, 6.0, 6.4, 6.0, 5.8, 5.7, 5.5, 4.5, 4.9.
Compute the Uniformity Coefficient.
•
•Solution: Total depth of water infiltered =
76.7 cm
• Mean depth = 76.7/13 = 5.9 cm

69. Locations Depths (cm)
Deviations from Mean
1 6.4 0.5
2 6.5 0.6
3 6.5 0.6
4 6.3 0.4
5 6.2 0.3
6 6.0 0.1
7 6.4 0.5
8 6.0 0.1
9 5.8 0.1
10 5.7 0.2
11 5.5 0.4
12 4.5 1.4
13 4.9 1.0

70. •This is a good Efficiency. 80% Efficiency is
acceptable.

//Xå
C
X
mn
u
= -
å
10010(.
//
)
C
u
= -
´
10010
62
5913
(.
.
.
)
= 6.2
m = 5.9 cm; n = 13
= 92%

71. Overall Irrigation Efficiency (Eo)
•The overall irrigation efficiency represents the
efficiency of entire physical system and
operating decisions in delivering irrigation water
from a water supply source to crop. It is
calculated by multiplying the efficiencies of water
conveyance and water application:
•Eo=(Ec x Ea) x100, %
•Ec= water conveyance efficiency (decimal)
•Ea= water application efficiency (decimal)

72. Crop Water Use Efficiency
•Crop water use efficiency is mostly used
to describe irrigation effectiveness in
terms of crop yield.
•It is defined as the ratio of mass of crop
yield per unit of irrigation water used in
evapotranspiration (ET).

73. •High irrigation efficiency offers
lower operating cost,
improved production per unit of water
delivered
and improved environments benefit and
management.
•Several adjustments can be made to the
volume of water delivered to the field to
increase irrigation efficiency or uniformity.

74. •A stream of 135 l/s was diverted from a canal and 100
l/s was delivered to a field. An area of 1.6 ha was
irrigated in 8 h. The effective depth of root zone was
1.8 m. The run off in the field was 432 m3. The depth
of water penetrated varied linearly from 1.8 m at the
head end of the field to 1.2 m at the tail end. Available
moisture holding capacity of the soil is 20 cm/m depth
of soil. Determine water conveyance efficiency, water
application efficiency, water storage efficiency and
water distribution efficiency. Irrigation was started at a
moisture extraction level of 50% of available moisture.

75. Solution: Ec=(100/135) x 100=74 %
Water delivered to plot=(100 x 3600x
8)/1000=2880 m3
Water stored in root zone=2880-432=2448 m3
Ea=(2448/2880) x 100=85%
Water holding capacity of root zone=20 x
1.8=36 cm
Moisture required in root zone=36-36 x0.5=18
cm
=(18 x 1.6 x 1000)/100=2880 m3
Es=(2448/2880) x 100=85%
Mean depth=(1.8+1.2)/2=1.5 m
•

76. •Numerical deviation from mean depth of
penetration:
•At upper end=1.8-1.5=0.3 m
•At lower end=1.5-1.2=0.3 m
•Average numerical deviation=(0.3+0.3)/2=0.3
m
•Ed=100(1-0.3/1.5)=80 %

77. Ways of Improving Efficiencies
–Water Transport through underground pipelines/ lining
of water courses
–Reduce water evaporation, seepage and leakage from
pipes
–Improve equipment, technology, engineering
–Optimize total crop management
•Get a good design
•Maintain irrigation system
–Replace worn nozzles
–Fix leaky pipes
•Improve management
–Irrigation Scheduling
–Operate at designed pressure and flow
–Irrigate on calm cool days