|SOUZA, E - UNIV OF MO
|SCHARF, P - UNIV OF MO
|Sudduth, Kenneth - Ken
|HIPPLE, J - UNIV OF MO
Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Other
Publication Acceptance Date: 4/13/2002
Publication Date: 4/13/2002
Citation: SOUZA, E.G., SCHARF, P., SUDDUTH, K.A., HIPPLE, J.D. USING A FIELD RADIOMETER TO ESTIMATE INSTANTANEOUS SKY CLEARNESS. AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS MEETINGS PAPERS. 2002. MID-CENTRAL CONFERENCE PAPER MC02-07.
Technical Abstract: 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. Our objective in 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. An upward-looking multispectral radiometer (460, 550, 680 and 800 nm bands with a10nm bandwidth) measured the actual spectral global irradiation incident on the horizontal plane. 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 of the multispectral radiometer bands showed that the influence of cloudiness was different for each band. Thus, the cloud effect could not be compensated for by use of a band ratio or vegetation index.