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

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 (T c -T a ) vs. VPD T max is non-transpiring crop T min is non-water-stressed crop Problem – difference between T max and T min 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 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 Temperatures

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 TreatmentYield (lb/ac) Irrigation (in) IWUE (lb/ac-in) 100% % IR Dryland

2003 Cotton Results TreatmentYield (lb/ac) Irrigation (in) IWUE (lb/ac-in) 100% % IR IR Dryland

Soybean Canopy Temperatures Well-watered treatment was not always the lowest temperature Dryland treatment was not always the highest temperature

2002 Soybean Results TreatmentYield (bu/ac) Irrigation (in) IWUE (bu/ac-in) 100% % IR Dryland

2003 Soybean Results TreatmentYield (bu/ac) Irrigation (in) IWUE (bu/ac-in) 100% % IR IR Dryland

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 TreatmentYield (bu/ac) Irrigation (in) IWUE (bu/ac-in) 100% % IR IR Dryland

Option 1 Tc greater than a calculated temperature for a given amount of time Advantage Detects stress faster Disadvantage Cost

Costs – Option 1 InstrumentCost 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 Advantage Less expensive Disadvantage Not as responsive to stress

Costs – Option 2 InstrumentCost Datalogger setup$2260 Air Temperature$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 InstrumentCost 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 ?