SCANNED AND SPOT MEASURED CANOPY TEMPERATURES OF COTTON AND CORN

Authors: Wanjura, Donald; MAAS, S - TTU; WINSLOW, J - TTU; Upchurch, Dan

Submitted to: Computers and Electronics in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2004
Publication Date: 2/22/2004
Citation: Wanjura, D.F., Maas, S.J., Winslow, J.C., Upchurch, D.R. 2004. Scanned and spot measured canopy temperatures of cotton and corn. Computers and Electronics in Agriculture. 44:33-48

Interpretive Summary: Canopy temperature is a sensitive indicator of water stress and accurate measurement is important for efficient crop water management. Canopy temperatures are measured by sensors, which remotely sense the energy radiated by crops in thermal wavelengths. Temperatures measured by infrared thermocouple and thermal scanner sensors, which viewed different sized areas of canopies, were compared. Canopy temperatures for well-watered (HW) cotton and corn and limited-watered (LW) cotton were measured in field plots. In the HW cotton and corn temperature differences between the two sensors were significantly different on 25 % of days. In LW cotton temperatures were significantly different on all days, probably due to the infrared thermocouples observing some soil surface between wilted leaves in the canopy. Differences in canopy temperature measured by the two sensors averaged 0.2 °C in HW cotton, 0.6 °C in HW corn, and 3.2 °C in LW cotton. These results indicate that canopy temperatures measured from a small area by infrared thermocouples are comparable to those from a larger area sensed by a thermal scanner when canopy size is sufficient to mask the soil background. This information is important because infrared thermocouples are less expensive and provide more rapid temperature measurements than thermal scanners.

Technical Abstract: Canopy temperature is an indicator of crop water stress and can be used for making timely irrigation scheduling decisions for center pivot and subsurface drip irrigation systems. A study was conducted in 2001 in which cotton (Gossypium hirsutum L.) and corn (Zea maize L.) canopy temperatures were measured with infrared thermocouples and a thermal scanner in field plots irrigated by surface drip irrigation. Two water levels included full evapotranspiration replacement (HW) in cotton and corn and another water level in cotton, which applied 50 % (LW) of the HW amount. Canopy temperature measured from a small canopy area using infrared thermocouples was compared with those obtained from a larger area by thermal scanning. Canopy temperatures in HW cotton and HW corn were measured on 8 days during a 20 day period that started at first bloom in cotton and the V 14 growth stage of corn, including 4 successive days during one irrigation cycle. There were significant canopy temperature differences in HW cotton and corn between the two sensors on 25 % of the days, but the differences were only slightly greater than the measurement accuracy of the two sensors. In LW cotton, canopy temperatures from the two sensors were significantly different on all days of one irrigation cycle, probably due to the infrared thermocouples observing some soil surface through the canopy. Differences in canopy temperature measured by the two sensors averaged 0.2 °C in HW cotton, 3.2 °C in LW cotton, and 0.6 °C in HW corn. The largest canopy temperature differences during the irrigation cycle measured between HW and LW cotton was 6.8 °C and 9.1 °C, respectively, for the thermal scanner and infrared thermocouple. Canopy temperatures measured from a small area by infrared thermocouples were comparable to those from a larger area sensed by a thermal scanner when canopies masked the soil background. Thermal mapping showed that HW cotton and corn canopy temperatures were more uniform than those in LW cotton