Irrigation Principles
What is Irrigation Efficiency?
Irrigating efficiently means:
• applying the right amount of
water,
• at the right time,
• evenly and reliably,
• to achieve maximum crop
production,
• and minimal detrimental
impacts on the environment
What is irrigation efficiency?
In developing best practice irrigation management, a
number of performance indicators are assessed and
compared, which include:
• irrigation volumes pumped
• irrigation volumes applied
• crop water use
• cost of water
• crop productivity
• economic return
Irrigation Best Management
Practices
• Rate irrigation highly within the management system
• Get to know the soils on the property
• Design and maintain irrigation systems correctly
• Monitor all aspects of each irrigation event
• Use objective monitoring tools to schedule irrigation
• Use more than one tool for scheduling irrigation
• Retain control of irrigation scheduling
• Remain open to new information
(PIRSA, 2007)
Terms
Soil Water Content: Measure of amount of water held in the soil,
measured in millimetres per metre of soil depth (mm/m), and sometimes
expressed in millimetres of water within the crop rootzone (mm). Related to
soil water tension according to soil texture and structure.
Soil Water Tension: Measure of the amount of force (suction)
required to draw water from the soil, measured in negative kilopascals, or
kilopascals of suction (-kPa). Related to soil water content according to soil
texture and structure.
Terms
Field Capacity: Level of soil water at which the soil is holding the
maximum amount of water, allowing for drainage of excess water by
gravity. Generally corresponds to a soil water tension of –8kPa.
Refill Point: Soil water tension or content at which the irrigator
chooses to apply irrigation to refill the rootzone water store. May
correspond to soil water tension of –60kPa for many permanent plantings,
but can be much wetter or dryer, depending on crop type, growth stage,
management preference etc. The main aim though, is to replace the
amount of water that has been used by the crop and to re-irrigate before
crop water use slows down, unless the crop is being stressed for a
particular purpose.
Terms
Permanent Wilting Point: Soil water content or tension at which
plants can no longer withdraw water from the soil, leading to plant death.
Generally corresponds to a soil water tension of -1500kPa.
Readily Available Water (RAW): The amount of water held in
the soil between Field Capacity and a Refill Point generally corresponding
to a soil water tension of –60kPa, measured in millimetres of water per
metre of soil depth (mm/m).
Crop Water Use
Affected by:
• Soil type
• Climate
• Plant growth and development
• Irrigation system type and management of that system
Crop Water Use
rainfall
irrigation
transpiration
evaporation
runoff
+excesss rain
+leaching salt
+excess irrigation
+watering uniformity
drainage
Water Usage: Irrigation
Scheduling
Why schedule?
• Scheduling involves deciding when to irrigate and how much
water to apply.
• The aim of irrigation scheduling is to keep soil water within an
acceptable range, avoiding plant stress by not allowing your soils to
remain too dry or too wet.
• Scheduling makes the best use of your irrigation water.
• Plants that are not water stressed have the potential to produce
optimum yields and remain healthy and vigorous.
Water Usage: Irrigation
Scheduling
The benefits of irrigation scheduling are:
• optimising plant health;
• using irrigation water efficiently and effectively;
• maximising productivity;
• minimising the leaching fraction which reduces drainage and
nutrient loss;
• using irrigation water in an environmentally responsible manner.
Water Usage: Irrigation
Scheduling
The dangers of not scheduling:
Over-irrigation, which:
• causes waterlogging and reduces plant health;
• wastes water with unnecessary pumping and water costs;
• leaches nutrients out of the plant root zone;
• produces excess drainage water;
Under-irrigation, which:
• stresses plants and reduces plant productivity;
• causes nutrient deficiency;
• may cause salt damage.
Water Usage: Irrigation
Scheduling
Types of scheduling:
•Soil – based scheduling
•Climate – based scheduling
•Plant – based scheduling
Soil Water Monitoring
Devices
Tensiometers:
• placed in rootzone to indicate
crop available water
• drying soil exerts suction on
tensiometer water which
registers on gauge
• needs good contact with soil
(not suitable for cracking soils
and coarse sandy soils)
• suitable for tensions up to -85
kPa
Soil Water Monitoring
Devices
Gypsum Blocks:
• readings represent the suction plants need
to exert through there roots to obtain water
• measurements read by portable handheld
meter/logger
• GB Heavy (finer textured soils, low
sensitivity 0 to -50 kPa, accurate in very dry
soils up to -1500 kPa)
• GB Light (suitable for lighter/sandier soils,
sensitive from -10 to -200 kPa)
Soil Water Monitoring
Devices
Other devices include:
• Neutron Probes
• Capacitance Probes
• Heat Probes
• Time Domain Reflectometers
Comparing Devices
• Range
of measurement required
• Number of monitoring sites required (soil variation,
crop, irrigation system)
• Level of detail required (frequency of readings)
• Level of input available (time, employees)
• Backup (hardware, software, installation, calibration,
interpretation)
• System output (ease of interpretation)
• Price (initial and recurrent)
Where not to place devices
Outside rows: Outside rows are exposed to wind, especially on the edges
of roads or adjoining broad acre properties.
Wheel tracks / Gateways / Stock traffic areas: Soil is compacted and
may lead to inaccurate readings.
Disturbed soil: Sites where the soil has been disturbed can give inaccurate
readings.
Stunted or sick plants: Sick plants usually use less water.
Areas with poor sprinkler distribution uniformity: Can give readings
not representative of the whole shift.
Areas with shallow watertables: Rising watertables may be detected
giving a false impression of water needs across the crop.
Pasture or broad acre areas sheltered by trees or shelter belts:
Shading and shelter effect transpiration, and tree roots may rob soil water.
Climate-based Scheduling
• Daily crop water requirement (CWR) = monthly CWR
÷ no. of days in the month
• Approx irrigation interval (days) = rootzone RAW ÷
daily CWR
Climate-based Scheduling
• Monthly crop water requirement (CWR) = monthly
evapotranspiration x crop factor
•Crop factor: percentage of water used by the crop when
compared to evapotranspiration readings from a theoretical crop.
Calculated for each crop by researchers comparing crop water
usage and evaporation. Available from PIRSA and SENRMB sites
•Access evapotranspiration figures for your area from the Bureau of
Meteorology or nrmWEATHER
Crop factors should only be regarded as a guide. Site specific
crop factors can be developed over time by the irrigator.
Climate-based Scheduling
Climate-based Scheduling
• Similarly ETo data can be used, in conjunction with a
reference crop factor, to calculate soil water used on a
daily basis
• Arrival at refill point can be determined by summing
calculated daily crop water use
• ETo calculated by Bureau of Meteorology, AWS
• Soil-based scheduling should compliment climatebased scheduling
Climate-based Scheduling
Plant-based Scheduling
• Plant appearance (need to be careful wilted
appearance isn’t due to factors other than lack
of water)
• Sap flow meters
Meters
• Do not under estimate the importance of the
pump meter as a vital part of irrigation
scheduling and management
• Meters allow the user to determine accurate
volumes of irrigation water pumped
Distribution Uniformity and
Application Efficiency
• Pivot should be checked on regular basis throughout its life (first
test should be after pivot construction) to ensure reasonable
distribution of water along the spans
• Application efficiency and distribution uniformity can be
determined with catch can test
• Distribution uniformity calculated, via software package, to show
variation in water output along pivot spans
• Compare water pumped (from meter reading) with catch can
measurements to determine application efficiency
CATCH CAN METHODOLOGY
Objectives
To calculate property level water
balances.
To determine application and
transmission losses and Application
efficiency.
Methodology
Measure pivot
Read meter
Place catch cans
Record weather conditions
Record volumes caught by cans
Reread meter
Perform calculations
Pivot
Pivot Size (m)
Number
Time of
Irrigation
Event
Application
Average
Estimated
Depth range (mm)
Application
per Pass mm
Application
Application
Loss (%)
Efficiency %
1
320
Day
17- 21
19.4
30
70
2
310
Night
17- 30
23.9
9
90.7
3
268
Night
1.4- 9.2
6
18
82.1
4
324
Night
5.5- 7.5
6.6
7
93.1
5
421
Night
10.5- 14
12.2
14
85.7
6
427
Day
9- 20.5
15.5
12
88
7
454
Day
10- 17
13.6
10
90
Future Water Usage
• Drive to increase efficient and effective use of
groundwater may mean we have to use pivots
differently
• There will be a bigger requirement to extract the
greatest dollar value return for every Megalitre pumped
• Choosing what to grow under the pivot will become
increasingly important
• Pasture vs Crops
Acknowledgements
The South East Natural Resources Management Board
gratefully acknowledges the following information
providers:
• Rural Solutions SA – Irrigated Crop Management
Service
• Department for Environment, Water and Natural
Resources