ABSTRACT

For investigation of the angular distribution of plant canopy reflectance with high angular resolution a special field goniometer was designed. The measured distributions for barley and red clover canopies in the red and near infrared spectral regions including hot-spot profiles during vegetative growth, are reported. The dependence of four vegetation indices on nadir angle significant in off-nadir radiance measurements is studied, too.

SUMMARY

Regular measurements of angular distributions of barley and clover canopy reflectances during vegetative growth revealed seasonal trends of those distributions in two spectral regions. High angular resolution of radiometer and the construction of the field goniometer allowed to study the profile of hot spot and the reflectance distribution profiles for the whole upper hemisphere. The above contradiction of resolution with averaging to some extent shaded the validity of our results. Despite some randomness of shown distributions, the reported conclusions as based on a whole set of experimental results (not included in this paper) may be considered valid. The results from simultaneous optical and phytometrical measurements are possible to use in testing theo- retical canopy reflectance models and inversion procedures.

It is essential to consider the angular dependence of various vegetation indices to avoid misinterpretation of off-nadir measurements. The Normalized Difference NDVI is the least dependent on view direction. The dependence of Simple Ratio NIR/RED on view direction is very complicated, the other two indices (Greenness and Soil Brightness) are sensitive, too. To compare vegetation indices for different directions one must first reduce the measured radiances to nadir. In doing so, canopy reflectance model which describes the reflectance angular distribution as adequately as possible must be used (eg. Nilson and Kuusk, 1989).