Irrigation Scheduling Based on Crop Canopy Temperature for Humid Environments

The use of infrared thermometers (IR) to measure canopy temperatures for irrigation scheduling has been successfully applied in arid environments. Functionality of this technique in humid areas has been limited due to the presence of low vapor pressure deficits (VPD) and intermittent cloud cover. This study evaluated an alternate scheduling method for humid environments based on comparing measured canopy temperature with calculated canopy temperature of a well-watered crop. Irrigation was applied when the measured canopy temperature was greater than the predicted canopy temperature for more than three consecutive hours on two consecutive days. This method was evaluated against well-watered, semi-stressed, and dryland treatments of corn, soybean, and cotton on the basis of yield, irrigation amount, and irrigation water use efficiency (IWUE). Canopy temperature was underpredicted when the VPD was greater than 2 kPa. Limiting data to conditions when the solar radiation was greater than 200 W m-2 and the Richardson number was less than 0.2 resulted in very good prediction of canopy temperatures for cotton and soybean, particularly in the later growing period, but corn temperatures were consistently underpredicted. Although soybean and cotton yields were not significantly different across treatments, IWUE was improved for corn and cotton by use of this technique. Corn yield was greater for the well-watered crop, but the IR method resulted in 85% of the maximum yield while requiring less than 50% of the irrigation water. Results from this study suggest that the threshold temperature may be up to 1°C greater for corn and soybean and up to 0.5°C greater for cotton for humid compared to arid environments. This method shows potential as a tool for irrigation scheduling in humid environments. Further work is suggested to determine if conditions of excessive cloud cover and high VPD can be better accommodated, and to refine the threshold temperatures for corn, soybean, and cotton for humid environments.