Diurnal changes in reflectance factor due to Sun-row direction interactions

Over a two year period, data were collected regarding the canopies of soybeans grown in rows in planter boxes placed on a turntable in an effort to investigate changes in the spectral reflectance factor related to row direction, Sun direction, soil background, and crop development stage. Results demonstrate that the direction of rows in a soybean canopy can affect the reflectance factor of the canopy by as much as 230%. The results for the red spectral region tend to support the validity of canopy reflectance models; results for the infrared region do not

INFLUENCE OF SOLAR ILLUMINATION ANGLE AND CULTURAL PRACTICES ON THE REFLECTANCE PROPERTIES OF SOYBEAN CANOPIES

In recent years considerable progress has been made toward the use of remote sensing technology to inventory crop acreages, assess crop stresses, and predict yields. Since soybeans (Glycine max L. Merr.) are a major crop in the United States, with over 28,800,000 hectares planted in 1979, it is important to fully understand those factors that are sources of variation in spectral measurements collected over soybeans. Spectral and agronomic measurements were acquired for soybean plots at the Purdue Agronomy farm several times during the 1978 and 1979 growing seasons. A Landsat band radiometer which acquired data in four spectral bands (0.5-0.6, 0.6-0.7, 0.7-0.8, and 0.8-1.1 (mu)m) was used both years. In 1978, 81 plots were observed periodically throughout the season to study the effect of row width, population, and cultivar on spectral response (bidirectional reflectance factor) of soybeans. A similar study was conducted in 1979 to examine the effects of row width, cultivar, planting date, and soil type on the spectral response on soybeans. As the 1978 soybean cultural practices data were analyzed, it was apparent that the time of the day or sun angle was related to the bidirectional reflectance factor (BRF). Therefore, an experiment to study specific sun angle and row azimuth interactive effects was conducted in 1979 by observing nine row directions, one complete canopy cover, and one bare soil plot in three diurnal studies. Analyses included the use of a near infrared/red reflectance ratio and greenness transformation. Results have shown a strong correlation of the bidirectional reflectance factor (BRF) of all the bands and transformations with percent soil cover. The changes in BRF associated with row width, planting date, or cultivar differences were often associated with differences in soil cover, leaf area index, and biomass. Differences in the BRF value due to cultivar were often noted late in the season when cultivars senesced at different rates. Differences in soil background due to soil moisture or soil type had an effect on the reflectance factor measurements and thus the correlations to changes in the vegetative canopy. Factors that affect the soil background such as soil color, soil moisture, or shadow due to sun angle-row azimuth interactions significantly affect the spectral response of an incomplete canopy cover, especially in the visible region. Diurnal variations in the BRF due to the shadow component in the canopy were greater than 100 percent in the visible wavelength region, while only very minor changes were noted in the near infrared. A function that utilizes both the solar azimuth and zenith angle explained most of the variation in reflectance observed for a given plot during the day

Variability of Reflectance Measurements Due to the Interaction of Row Azimuth and Solar Illuminati on Angle

Pronounced effects on the reflective response of a crop canopy due to changes in solar azimuth and row azimuth angle have been noted on soybeans planted in wide rows. Understanding the interaction between row azimuth and illumination direction on the crop canopy spectral response is necessary in order to utilize effectively the spectral data collected on row crops. The objective of this experiment was to determine the effects of rows and row direction on the reflective response of a soybean canopy as a function of solar azimuth and zenith angles. Data were acquired over eleven plots in 1979. One plot was planted in east-west and north-south rows 25 cm wide to obtain, at later growth stages, a canopy with negligible row effects. A bare soil plot was included in order to monitor the sunlit soil background reflectance of the soybean plots. The remaining nine plots were planted in soybeans with 76 cm wide rows with the following azimuthal directions: 90-270, 105-285, 120-300, 135-315, 150-330, 165- 345, 180-360, 210-030, and 240-060 degrees. The row directions were selected to favor the data collections during the morning hours when cloud free conditions are more likely. Reflectance data were acquired with a Landsat band radiometer (Exotech 100) at 15 minute intervals throughout the day on three days, representing three canopy growth stages with 65, 75 and 95 percent soil cover for those plots with 76 cm wide rows. Analysis of the data has shown minor effects on reflectance due to solar zenith angle. (Significant effects may be observed at lower latitudes whose wider diurnal variations of solar zenith angle are observed.) The two visible bands (0.5-0.6 µm) and (0.6-0.7 µm) were significantly affected by the interaction of solar and row azimuth angles. Reflectances of canopies with rows parallel to the solar azimuth illumination angle were more than double the reflectance of canopies with rows perpendicular to the illumination angle

Canopy Reflectance as Influenced by Solar Illumination Angle

An experiment was conducted at West Lafayette, Indiana in 1979 to quantitatively describe the interaction of the solar illumination angle and row azimuth angle on the measured reflectance factor (RF) of soybean canopies consisting of 11 plots. Nine of the plots were planted in 71 cm wide rows; the other plots were of bare soil and soybeans with 100 percent soil cover. Reflectance factor data in four spectral bands, 0.5-0.6, 0.6-0.7, 0.7-0.8, and 0.8-1.1 pm, were taken at 15 minute intervals during three clear days, August 12 and 31 and September 19 over nine plots of differing azimuthal direction with a Landsat-band radiometer (Exotech Model 100) at 5.2 meters above the soil. Diurnal changes of nearly 140 percent were observed in the red wavelength region when canopies covered 64 percent of the soil. The amount of shadow observed was a function of the plant geometry and row width. As soil cover approached 100 percent, the diurnal changes diminished. A function that describes the solar illumination angle with respect to the row azimuth explained most of the diurnal variation in the measured RF. Variation in near infrared response was much less and did not appear to be as strongly related to sun-row angle interactions. The ratio, near infrared/red, was highly sensitive to sun angle-row direction interactions, whereas the greenness function, utilizing all four spectral bands, was not

Canopy Reflectance as Influenced by Solar Illumination Angle

An experiment was conducted at West Lafayette, Indiana in 1979 to quantitatively describe the interaction of the solar illumination angle and row azimuth angle on the measured reflectance factor (RF) of soybean canopies consisting of 11 plots. Nine of the plots were planted in 71 cm wide rows; the other plots were of bare soil and soybeans with 100 percent soil cover. Reflectance factor data in four spectral bands, 0.5-0.6, 0.6-0.7, 0.7-0.8, and 0.8-1.1 pm, were taken at 15 minute intervals during three clear days, August 12 and 31 and September 19 over nine plots of differing azimuthal direction with a Landsat-band radiometer (Exotech Model 100) at 5.2 meters above the soil. Diurnal changes of nearly 140 percent were observed in the red wavelength region when canopies covered 64 percent of the soil. The amount of shadow observed was a function of the plant geometry and row width. As soil cover approached 100 percent, the diurnal changes diminished. A function that describes the solar illumination angle with respect to the row azimuth explained most of the diurnal variation in the measured RF. Variation in near infrared response was much less and did not appear to be as strongly related to sun-row angle interactions. The ratio, near infrared/red, was highly sensitive to sun angle-row direction interactions, whereas the greenness function, utilizing all four spectral bands, was not