Methods of Irrigation Scheduling and Determination of ... · Irrigation Scheduling Methods...
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Methods of Irrigation Scheduling and
Determination of Irrigation threshold triggers
Introduction
Principle of irrigation Scheduling
Methods of irrigation scheduling
Determination of Irrigation Triggers
Worked example – Water Balance Method
Determination of Field Capacity
1 meter deep
A kilometer wide
www.maps-world.net/africa-asia.htm
7 Million Km Long 180 times around the world
Water Required
3000 Calories of food
Every Day for a year
6.7 Billion Inhabitants
Irrigation Scheduling Combined management and technical tool which dictates:
When to Irrigate How much water your crop requires How fast to apply water to your crop How often to irrigate
Irrigation scheduling
is the key to improve irrigation Efficiency
Principle Irrigation Scheduling
FC-Soil water content in the soil after a saturated soil has drained by gravity
Threshold
Upper Threshold -FC
Lower Threshold -%FC
Irrigation Scheduling Methods
Soil Water Monitoring
Plant Water Monitoring
Soil Water Balance Modelling Approach
Irrigation Scheduling Methods
Irrigation Scheduling Soil Water Monitoring
Soil Water Monitoring
Feel and Appearance
Gravimetric
Direct
Volumetric
Neutron Probe
Dielectric Sensor
Indirect
Tensiometers
Tensiometric
Volumetric Electrical Resistive Sensor
Others Others
Why Irrigation Scheduling by Standalone sensors?
Soil Moisture Sensors simplifies these complexities into one measurement
Trigger can be a function of FC or AWC
Crop Factor Soil Water Factor Climatic Factor
Complex and Variable Processes Trigger
20-27th August 2008
15cm sensor
45cm sensor Average trend at 25cm Commencement of irrigation
Days
Irrigation Scheduling Methods
Irrigation Scheduling Methods
Plant Water Monitoring
Stem/Leaf Water Potential
Canopy Temperature
Plant Water Monitoring Based on plant response Does not answer the question “HOW MUCH water is required but “WHEN TO IRRIGATE”.
Plant water monitoring Leaf Water Potential
Water - xylem vessels Water - under tension (negative pressure)
Plant water monitoring Leaf Water Potential
Xylem vessel extends to the leaves
Leaf Water Potential
Preparation Board
Specimen Holder
Pressure Gauge
Hoses
Hose Connection
Metering Valve
The Pressure bomb
3-way ball valve
Plant water monitoring Leaf Water Potential
Integrate Soil factors Environmental Plant factors
indirect
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.06 8 10 12 14 16 18 20
Mid
day
stem
WP
Time of Day (hrs)
Plot of Midday Stem Water Potential (MSWP) Vs. Time for walnut
Fully Watered Mild stress Moderate Stress
Mid day sampling time Period 1-3 pm
Threshold range
-3 to -5 bars
-5.5 to -7.5 bars
-8.0 to -10.0 bars
Plant water monitoring Canopy Temperature
Transpiration cools the leaves below ambient temperature. If Tcanopy > Tambium, this imples reduced evapotransipration and increased stress
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
(Tc-
Ta) °
C
Date 1983
Plot of Difference Between Canopy and Air Temperature
Stressed
Reduced Stressed
Irrigation Intervals
Difference in Canopy temperature used in conjunction with Soil water potential (0.4 MPa at 76mm depth) to irrigation Kentucky Blue grass turf
Irrigation Scheduling Methods Irrigation Scheduling
Methods
Soil Water Balance Modelling Approach
Combination of Plant, soil and
Climate
Water Balance Model A soil water accounting system
Daily withdrawals Daily inputs Change in storage
Accounting is done up to some predetermined threshold. Soil is irrigated back to field capacity
Water Balance Model
Inputs
Evaporation
Runoff
Rainfall
Irrigation Transpiration
Capillary Rise Deep
Percolation
Root Zone
= Outputs
S
± ∆S
Water Balance Model Outputs Inputs
ETat
Ground Level SMDt-1
Dt
Rnt
SMDt
Threshold
SMDt = SMDt-1- (ETat+Dt +Rnt) + (Rt + It+C)
Summary
Soil Water Monitoring
Plant Water Monitoring
Soil Water Balance Modelling Approach
Irrigation Scheduling Methods
Soil moisture approach is simple if the triggers can be accurately calculated. Plant approach measures the stress level of the plant,
destructive and answers the question when but not how much. Generally used in conjunction with other methods Water Balance approach is a more holistic methods
Determination of Irrigation Trigger Points
Three important concepts are necessary Field Capacity Permanent Wilting Point Available water capacity Readily Available Moisture Maximum Allowable Deficit
Field Capacity Water contained in a soil after a saturated soil has been drained for at least two day
usually occurs typically at pressure heads of -0.1 (10kPa) to -0.33 bars (33Kpa).
Permanent Wilting Point Permanent wilting point (PWP) represents to lower limit of water available to plants. At this stage crops tend to wilt and cannot recover if irrigated. Typically occurs at 15 bars (15MPa)
PWP
Plant Available Water
Water within the soil profile between FC and PWP
PWP
AWC = θfc - θwp
Objective of understanding Plant soil water relationship – Maximize crop yields managing soil and water and crop
Soil Texture Field capacity (by% volume)
Permanent Wilting Pt
(by %Volume)
Available water (by %volume)
Sandy 15
(10-20) 7
(3-10) 8
(6-10)
Sandy Loam 21
(15-27) 9
(6-12) 12
(9-15)
Loam 31
(25-36) 14
(11-17) 17
(14-20)
Clay Loam 36
(31-42) 18
(15-20) 18
(16-22)
Silty clay 40
(35-46) 20
(17-22) 20
(18-23)
Clay 44
(39-49) 21
(19-24) 23
(20-25)
Available water
Plant Available Water Content and Available Water
Available water (AW) = (water content - wilting point) ×
rooting depth
Available water capacity (AWC) = (field capacity - wilting point) ×
rooting depth
Sample Problem1
Field Capacity Soil Available Water
Sample Problem A soil sample taken after gravitational
drainage has a total volume of 50 cm3, of which 12 cm3 is water. Find the field Capacity?
a. Field capacity = ? 12cm3/50cm3=0.24 or 24%
Sample Problem b. If the Permanent wilting
point is 0.11 or 11% and the plant rooting depth is 60 cm. Find the available water capacity?
60cm
-0.24
-0.11 Available Water Capacity
Available water capacity (AWC) = (field capacity - wilting point) × rooting depth
(0.24-0.11)*60cm=7.8cm or 78mm Answer 78 mm or 7.8 cm
Sample Problem C. When this soil has a
water content of 0.18 what is the Available water?
Available water = ?
Available water (AW) = (Moisture Content- wilting point) × rooting depth
(0.18-0.11)*60cm=4.2cm or 42mm Answer 42 mm or 4.2 cm
60cm
-0.24 FC
-0.11 PWP Available Water -0.18
Readily Available Water
Readily available soil moisture
AWC = θfc - θwp
PWP
Readily Available Water water which can be removed from the soil
with minimal energy applied. It is common to consider about 50% of the
available water as readily available water.
RAW = ½* AWC
θfc
θPWP
θAWC θRAW
Readily Available Water
All of the available water can be used by the plant, The closer the soil is to the wilting point, the greater the stress is that the plant experiences when water is being removed from the soil. Plant stress and yield loss are possible after the readily available water has been depleted
Maximum Allowable Deficit
The maximum level of depletion to which the soil can dry without causing water deficit stress in a crop that has a fully expanded root zone
For most vegetable crops its 30-40% of AWC The MAD therefore
become the lower trigger and field FC the upper trigger
Relationship between FC and AWC
Sandy loam FC= 24% PWP=11% AWC=24-11=13% 50% AWC = 0.5*13 = 6.5% In terms of Moisture Level = 6.5 +11 =17.5% What does 17.5% AWC
represent in terms of a fraction of FC?
%FC = 17.5/24=73% What does 50% FC represent
in terms of AWC? 50%FC=24%*0.5 = 12%≈ 1%
-0.24
-0.11
Available Water Capacity 50% AWC
In terms of AWC 50%FC = 1/(24-11)=7% AWC
73% FC
Relationship between FC and AWC
What does 50% FC represent in terms of AWC?
50%FC=24%*0.5 = 12% ≈ 1% more than PWP AWC =13% In terms of AWC, 50%FC =
1/(24-11)=7% AWC
-0.24
-0.11 7% AWC 50% FC
Available water content
Example of water budget approach for scheduling Irrigation scheduling of tomatoes
Location: ◦ Castries, St. Lucia
Soil Type, FC =21%, PWP =11% ◦ Loamy Sand
Rooting Depth ◦ Before flowering (before June 15) – 30
cm ◦ After flowering (after June 16) – 60 cm
Maximum total available water ◦ Before flowing – 30 mm ◦ After flowing – 60 mm
Allowable soil water Depletion ◦ Before flowering - 15 mm ◦ After flowering - 30 mm
DateRain (mm)
Eto (mm) Kc
Etcrop (mm)
Total available water (mm)
New Soil Moisture Level
(mm)
Irrigation Amount
(mm)
16.56/1 0.8 4.5 0.4 1.8 15.56/2 44.4 4.0 0.4 1.6 30.06/3 0 3.7 0.4 1.5 28.56/4 0 6.0 0.4 2.4 26.16/5 0.4 7.8 0.4 3.1 23.46/6 1.6 6.4 0.4 2.6 22.46/7 0 4.4 0.4 1.8 20.76/8 1.6 2.7 0.4 1.1 21.26/9 0 7.0 0.4 2.8 18.46/10 0 3.0 0.4 1.2 17.26/11 8.4 2.5 0.4 1.0 24.66/12 0 7.8 0.4 3.1 21.56/13 0 4.8 0.7 3.4 18.16/14 0 7.6 0.7 5.4 12.7 30.0 17.36/15 0 7.4 0.7 5.2 24.86/16 0 6.5 0.7 4.6 20.2 60.0 39.86/17 0 5.0 0.7 3.5 56.56/18 0 4.4 0.7 3.1 53.46/19 9.4 3.4 0.7 2.4 60.56/20 0 5.8 0.7 4.0 56.4
Soil Type - loam FC = 31% PWP=11% AWC=20%
Rooting Depth =60cm Area of Frame =
1m*1m How much water to
Saturate the soil
AWC over rooting depth =
0.2*60cm=12cm Vol. of Water required
Aear * depth 1m2*0.12m=.12m3
=120 l =120/3.78=31 gallons
Data Required Gravimetric water content Bulk density Volumetric water content Sensor reading corresponding to FC