Directional reflectance factor distributions of a cotton row crop

Abstract

The directional reflectance factor distribution spanning the entire exitance hemisphere was measured for a cotton row crop ( 39 per cent ground cover) as a function of the solar azimuth and zenith angles. Two spectral bands NOAA 7 AVHRR bands 1 ( 0.57-0.69 μ ) and 2 (0.71-0.99 μ ) were measured. Ancillary data consisted of leaf orientation distribution measurements, structural and agronomic measurements, as well as optical measurements of leaves and soil. Polar co-ordinate system plots of directional reflectance factor distributions and three-dimensional computer graphic plots of scattered flux were created from the radiometric data. These plots were used to study the dynamics of the directional reflectance factor distribution as a function of spectral band, geometric structure of the scene, solar zenith and azimuth angles and optical properties of the leaves and soil The polymodal nature of the directional reflectance factor distribution for incomplete row crop canopies was evident from these data. Aside from enhanced reflectance for the antisolar point, a reflectance minimum was found towards the forwardscatter direction in the principle plane of the sun. This minimum was often shifted towards the azimuth direction normal to the row direction and the side of the rows opposite to the sun. As the solar azimuth came into coincidence with the row direction, this reflectance minimum became bifurcated. These data, the literature data and the results of modelling studies found in the literature, suggested several physical mechanisms responsible for the observed dynamics of the reflectance distributions. Detailed knowledge of the dynamics of directional reflectance behaviour is important in interpreting aircraft and satellite data where the solar angle varies widely and the viewing angle can have different orientations with respect to the sun. Finally, the measured data and knowledge of the mechanics of the observed dynamics of the data can provide rigorous validation and verification tests for two- or three-dimensional radiative transfer models