Effects of changing solar radiation on canopy-air temperatures of cotton and soybean

Abstract

A method is described that corrects canopy-air temperature ( Td) for rapidly changing solar radiation ( SR). Canopy (by infrared thermometer) and air temperatures, relative humidity, total solar radiation and wind speed were measured every 10 s with automatic data logging equipment in cotton ( Gossypium hirsutum L. cv. DES 119) and soybean [ Glycine max (L.) Merr. cv. Centennial]. Plots were located at the Delta Branch Experiment Station, Stoneville, Mississippi, U.S.A. (latitude 33°25′ N, longitude 90°50′ W). Results showed that Td changed relatively slowly when SR changed with cloud cover. About 100 s was required for Td to adjust to a new equilibrium temperature for a change in SR. Thirty percent of the remaining potential change in Td occurred each 10 s. These characteristic changes in Td were the same for cotton and soybean, for increasing or decreasing SR, and for a range of plant-water conditions. Accounting for the slower changes in Td with rapid changes in SR substantially improved the linear relationship between SR and Td. After accounting for non-equilibrium Td and using the improved linear relationship between SR and Td, Td values measured over a wide range of SR were adjusted to a Td value for a single reference SR level. Adjusting measured Td values for non-equilibrium and to a reference SR reduced the standard deviation of Td values collected repeatedly, at 10-s intervals, at a single location to ∼ 1 4 of that associated with the measured data. This represents a substantial reduction in the uncertainty in Td measurement and should result in improved application of the technique.