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Modeling Diurnal Canopy Temperature Dynamics Using One-Time-of-Day Measurements and a Reference Temperature Curve

Conserv. and Prod. Res. Lab., USDA-ARS, P.O. Drawer 10, Bushland, TX 79012

View larger version (27K): [in a new window]   Fig. 1. Canopy temperatures of three replicate plots of the 28°C, 240-min treatment on corn in 1999 compared with air temperature. Also shown are horizontal bars drawn at the threshold temperature of 28°C and over the length of the threshold time. Because the canopy was above the threshold temperature for more than the threshold time on Day 234, irrigation occurred in the evening of that day but not in the evening of Day 235.

View larger version (21K): [in a new window]   Fig. 2. Diagram of the terms used in the scaled method (Eq. [4]). Time t might be any daylight time at which a canopy temperature (Trmt,t) was measured at a remote location in the field. A contemporaneous temperature (Tref,t) from the reference temperature data is then used in Eq. [4] along with the common predawn minimum temperature (Te) and each value in the reference temperature data (Tref) to predict corresponding temperatures at the remote location throughout the daylight hours (Trmt).

View larger version (27K): [in a new window]   Fig. 3. Example differences between the reference canopy temperature and the predicted canopy temperatures for both the Gaussian difference method (Eq. [5]) and the scaled method (Eq. [4]). Data are from 1999 corn, Day of Year 205 using the mean temperature of the 30/240 treatment as the reference temperature, Tref. In (A), the temperature at 1230 h CST from the 28/160 treatment was used for the value of Trmt,t in Eq. [4]; and the mean temperature of the 30/240 treatment at the same time was used for the value of Tref,t. The value of Td in Eq. [5] at 1230 h CST was set equal to Trmt,t – Tref,t. In (B), the time for evaluation of Trmt,t and Tref,t was 1945 h CST. In both (A) and (B), the actual difference in canopy temperature between the 28/160 treatment and the mean temperature for the 30/240 treatment is shown for comparison.

View larger version (51K): [in a new window]   Fig. 4. The mean absolute error between predicted (Eq. [5]) and measured temperatures using the temperature measurement for all times of day for each day of the year that canopy temperatures were measured. Values are shown for the 28/240 treatment for corn in 1999. The mean temperature of the 28/160 treatment was used as the reference.

View larger version (26K): [in a new window]   Fig. 5. Comparison between the two methods of the overall mean error across treatments for corn in 1999 showing 95% confidence limits for each. Also shown is the probability that the differences between the two methods are due to variation, denoted P(T <= t), using the students t test at each point. The mean temperature of the 28/160 treatment was used as the reference.

View larger version (26K): [in a new window]   Fig. 6. Comparison between the two methods of the overall mean error across treatments for cotton in 2001, showing 95% confidence limits for each. Also shown is the probability that the differences between the two methods are due to variation, denoted P(T <= t), using the students t test at each point. The 30/452 treatment mean temperature was used as the reference.

View larger version (26K): [in a new window]   Fig. 7. Comparison between the two methods of the overall mean error across treatments for soybean in 2002, showing 95% confidence limits for each. Also shown is the probability that the differences between the two methods are due to variation, denoted P(T <= t), using the students t test at each point. The 27/171 treatment mean was used as the reference.

View larger version (21K): [in a new window]   Fig. 8. Comparison of the cumulative number of irrigations calculated using the temperature–time threshold (TTT) method and the field-measured canopy temperature data compared with the average of the cumulative irrigation signals calculated using temperatures predicted by the scaled method. The data shown for the scaled method are averages using one-time-of-day temperature measurements at all times from 0815 to 2200 h CST. The 95% confidence limits are drawn around the average predicted cumulative number of irrigations. Data are from the 2001 cotton crop and the 28/452 treatment.