Using a field radiometer to estimate instantaneous sky clearness

Reflectance measurements of crop plants and canopies show promise for guiding within-season, variable-rate nitrogen (N) application. Most research results have been obtained around solar noon with clear skies. However, for practical application, the system must work under cloudy skies or away from solar noon. The objective of this work was to assess the effect of cloud conditions on reflectance measurements of a corn canopy. The approach was to estimate an instantaneous sky clearness index (ICI) which could be used to correct field radiometer data for variations in cloud cover, such that the same reflectance reading would be obtained (and the same N recommendation made) for the same plants regardless of cloud conditions. Readings were taken from morning until night over 11 days with a range of sky conditions (sunny, overcast, partly cloudy). Data from clear days were used to estimate the theoretical expected spectral global radiation incident on a horizontal surface. The ICI was calculated as the ratio between the actual spectral global radiation and the corresponding theoretical global radiation. Analysis of the ICI for each band showed that the influence of cloudiness was different for each band. Thus, the cloud effect could not be compensated by the use of a band ratio or vegetation index.