Using Leaf Temperature for Irrigation Timing Daniel L. Bockhold Graduate Research Assistant University of Missouri - Columbia Department of Biological.

Using Leaf Temperature for Irrigation Timing

Daniel L. BockholdGraduate Research Assistant

University of Missouri - ColumbiaDepartment 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

CWSI

• 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

-6-5-4-3-2-101234

0 1 2 3 4 5 6

Vapor Pressure Deficit (kPa)

Tc-

Ta

(oC

)

Non-water-stressed Baseline Non-transpiring Baseline

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

• 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

Five different treatments

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 TemperaturesSurface Temperatures (Cotton-2002)

15

20

25

30

35

224 225 226 227

DOY

Te

mp

era

ture

(oC

)

100%IRDryTs-cal

CloudCover

Irrigation of IR Treatment

Rainfall

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 well-watered treatment

2002 Cotton Results

Treatment Yield (lb/ac)

Irrigation (in)

IWUE (lb/ac-in)

100% 842 7.2 1.25

50% 1003 3.6 47.2

IR1 1092 3.0 0.397

Dryland 833 0.0 -----

2003 Cotton Results

Treatment Yield (lb/ac)

Irrigation (in)

IWUE (lb/ac-in)

100% 1137 5.8 35.5  

50% 897 0.7 -48.6  

IR 1 1063 4.7 28.1  

IR 2 1037 3.3 32.1  

Dryland 931 0.0 -----  

Soybean Canopy Temperatures

• Well-watered treatment was not always the lowest temperature

• Dryland treatment was not always the highest temperature

2002 Soybean Results

Treatment Yield (bu/ac)

Irrigation (in)

IWUE (bu/ac-in)

100% 50 13.5 -0.37

50% 54 6.8 -0.15

IR1 52 4.9 -0.61

Dryland 55 0.0 -----

2003 Soybean Results

Treatment Yield (bu/ac)

Irrigation (in)

IWUE (bu/ac-in)

100% 41 5.1 0.20  

50% 45 0.8 6.25  

IR 1 41 3.0 0.33  

IR 2 46 2.7 2.22  

Dryland 40 0.0 -----  

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 Yield (bu/ac)

Irrigation (in)

IWUE (bu/ac-in)

100% 159 7.9 9.11  

50% 84 1.7 -1.76  

IR 1 136 3.3 14.85  

IR 2 122 3.6 9.72  

Dryland 87 0.0 -----  

Option 1

Tc greater than a calculated temperature for a given amount of time

• AdvantageDetects stress faster

• DisadvantageCost

Costs – Option 1

Instrument Cost

Datalogger setup $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

• AdvantageLess expensive

• DisadvantageNot as responsive to stress

Costs – Option 2

Instrument Cost

Datalogger setup $2260

Air Temperature $70

IRTs $430

Total $2760

Option 3

Tc measured with handheld infrared thermometer greater than air temperature

• AdvantagesInexpensiveSimple

• DisadvantageMeasurements are taken by hand

Costs – Option 3

Instrument Cost

Handheld IRT $100

Air Temperature $30

Total $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 ?