Factors that Affect the Quality and Quantity of Sunlight Reflected from Alfalfa Canopies

Experiments were conducted from 1997 to 2000 in Ames and Nashua, IA, to quantify the effects of incident radiation, sun angle, leaf wetness, and sensor height on the quality and quantity of sunlight reflected from alfalfa canopies using a hand-held, multispectral radiometer. Incident radiation had negligible effects on the percentage of sunlight reflected from alfalfa canopies in the visible wavelength bands (460 to 710 nm) and significant negative effects on the percentage of sunlight reflected from alfalfa canopies in the near-infrared wavelength bands (760 and 810 nm). Percentage reflectance values (810 nm) decreased between 0.20 and 0.32% for each 100 watts m^-2 increase in incident radiation. Percentage reflectance values in all wavelength bands obtained prior to 1100 h and after 1500 h CST were significantly higher than reflectance measurements obtained between 1100 h and 1500 h. The presence of moisture on leaves (leaf wetness) significantly increased percentage reflectance for all wavelength bands. Sensor height had variable effects on percentage reflectance in all wavelength bands. This variability was most likely due to changes in the heterogeneity of alfalfa canopies with increasing sensor height. In experiments conducted with more homogenous canopies (oat and turfgrass), sensor height had no significant effect on the percentage of sunlight reflected from these canopies. To minimize the effects of all of the above nondisease factors on the measurement of percentage of sunlight reflected from crop canopies, we recommend that percentage reflectance measurements be obtained between 1100 and 1500 h when plant canopies are dry, with a constant sensor height (sampling unit area), and within a small range of incident radiation for all measurements.