Using Leaf Temperature for
Irrigation Timing
Daniel L. Bockhold
Graduate Research Assistant
University of Missouri - Columbia
Department of Biological Engineering
Methods of Irrigation
Scheduling
• Checkbook Methods
• Soil Moisture Sensors
• Evaporation Pans
• Crop Symptoms
• Canopy Temperature
Methods of Irrigation
Scheduling
• Woodruff Charts
• Arkansas Scheduler
• Michiana Scheduler
• Gypsum Blocks
• Tensiometers
• Watermark Sensors
• Washtub
• Visual
• Infrared Sensors
Infrared Sensors
• Measure leaf canopy temperatures, Tc
• Warm leaf canopy temperatures indicate stress
• Plant water-stress due to lack of water
When to Irrigate?
• Stress Degree Day (SDD)
• Crop Water Stress Index (CWSI)
• Temperature-Time Thresholds
• Other?
Stress Degree Day
• Measured once a day (1-2 hrs after solar noon)
• Summed over many days
• Replaced by CWSI
• Problem – temperature difference varies with
other climatic factors
Tc-Ta (oC)
CWSI
4
3
2
1
0
-1
-2
-3
-4
-5
-6
0
1
2
3
4
5
6
Vapor Pressure Deficit (kPa)
Non-water-stressed Baseline
Non-transpiring Baseline
• Plotted as (Tc-Ta) vs. VPD
• Tmax is non-transpiring crop
• Tmin is non-water-stressed crop
• Problem – difference between Tmax
and Tmin is small at low VPD
Temperature-Time Thresholds
• Irrigate when Tc is above an ideal
canopy temperature for a given amount
of time
• Problem – in humid conditions, Tc will
rise higher than the ideal temperature
without being water-stressed
Methods We Used for
Scheduling
• Tc greater than a calculated temperature for
a given amount of time
• Tc greater than air temperature for a given
amount of time
Evaluation Methods
Five different treatments
• Well-watered treatment (100%)
replaced water used with biweekly irrigations
• Semi-stressed treatment (50%)
• 2002 – received half the amount of water
of the 100%
• 2003 – received half the calculated ET
minus rainfall
• Dryland treatment received only rainfall
Evaluation Methods
(Cont.)
• (IR1) irrigated 1 inch when Tc was greater than
calculated temperature for 3 hours on 2
consecutive days
• (IR2) irrigated 1 inch when Tc was greater than
Ta for 3 hours on 2 consecutive days (2003 only)
Calculation of Canopy
Temperature
• Assumes canopy temperature is equal to the
temperature of a wetted leaf
• Calculated from measured weather data
Discussion of Calculation
• Accurately predicts canopy
temperature on most days
• Under-predicts on days where humidity
is low
Diurnal Patterns Canopy
Temperatures
Surface Temperatures (Cotton-2002)
Temperature (oC)
35
Irrigation of
IR Treatment
Rainfall
Cloud
Cover
30
100%
IR
Dry
Ts-cal
25
20
15
224
225
226
DOY
227
Canopy vs. Air Temperature
• Comparing canopy temperature to air temperature
is simpler and requires less instrumentation
• Problem – Average canopy temperatures of the
well-watered crops were less than air temperature
• Result – Under-irrigation could occur
Cotton Canopy Temperatures
• Dryland was always in grouping with
highest average temperature
• IR treatments were in the same or
lower temperature grouping as the wellwatered treatment
2002 Cotton Results
Treatment
100%
50%
IR1
Dryland
Yield
(lb/ac)
842
Irrigation
(in)
7.2
1003
1092
833
3.6
3.0
0.0
IWUE
(lb/ac-in)
1.25
47.2
0.397
-----
2003 Cotton Results
Treatment
100%
50%
IR 1
IR 2
Dryland
Yield
(lb/ac)
1137
897
1063
1037
931
Irrigation
(in)
5.8
0.7
4.7
3.3
0.0
IWUE
(lb/ac-in)
35.5
-48.6
28.1
32.1
-----
Soybean Canopy
Temperatures
• Well-watered treatment was not always
the lowest temperature
• Dryland treatment was not always the
highest temperature
2002 Soybean Results
Treatment
100%
Yield
(bu/ac)
50
Irrigation
(in)
13.5
IWUE
(bu/ac-in)
-0.37
50%
IR1
54
52
6.8
4.9
-0.15
-0.61
Dryland
55
0.0
-----
2003 Soybean Results
Treatment
100%
50%
IR 1
IR 2
Dryland
Yield
(bu/ac)
41
45
41
Irrigation
(in)
5.1
0.8
3.0
46
40
2.7
0.0
IWUE
(bu/ac-in)
0.20
6.25
0.33
2.22
-----
Corn Canopy Temperatures
• Well-watered treatment had the lowest
average temperature
• IR treatments had the next lowest
temperatures and were in the same
grouping
2003 Corn Results
Treatment
100%
50%
IR 1
IR 2
Dryland
Yield
(bu/ac)
159
Irrigation
(in)
7.9
84
136
122
87
1.7
3.3
3.6
0.0
IWUE
(bu/ac-in)
9.11
-1.76
14.85
9.72
-----
Option 1
Tc greater than a calculated temperature for a given
amount of time
•
Advantage
Detects stress faster
•
Disadvantage
Cost
Costs – Option 1
Instrument
Datalogger setup
Cost
$2260
Air Temperature and RH
$545
Solar Radiation
$275
Wind Speed
$195
IRTs
$430
Total
$3705
Option 2
Tc greater than air temperature for a given amount
of time
•
Advantage
Less expensive
•
Disadvantage
Not as responsive to stress
Costs – Option 2
Instrument
Datalogger setup
Air Temperature
Cost
$2260
$70
IRTs
$430
Total
$2760
Option 3
Tc measured with handheld infrared thermometer
greater than air temperature
•
Advantages
Inexpensive
Simple
•
Disadvantage
Measurements are taken by hand
Costs – Option 3
Instrument
Cost
Handheld IRT
$100
Air Temperature
Total
$30
$130
Conclusions
• Irrigation scheduling based on canopy
temperature can be used in humid regions with
certain restrictions
• Calculated canopy temperature accurately
predicted the measured canopy temperature when
the humidity was high
• Comparing canopy to air temperature can be
useful, but may cause under-irrigation
Conclusions
(Cont.)
• Yield results showed no statistical difference in
treatments of cotton and soybean, but did in corn
• Different setups can be made using infrared
thermometers that vary in cost
Questions ?