|Souza, E - UNIOSESTE BRAZIL|
|Scharf, Peter - UNIVERSITY OF MISSOURI|
|Hipple, J - UNIVERSITY OF MISSOURI|
Submitted to: Revista Brasileira de Engenharia Agricola e Ambiental
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 13, 2005
Publication Date: October 1, 2006
Citation: Souza, E.G., Scharf, P.C., Sudduth, K.A., Hipple, J.D. 2006. Using a field radiometer to estimate instantaneous sky clearness. Brazilian Journal of Agricultural and Environmental Engineering. 10(2):369-373 Interpretive Summary: For most grain crops, farmers apply some form of nitrogen (N) to help meet the crop’s nutrient needs. While current practices generally apply one rate of N over a whole field, or even an entire farm, research shows that crop N need often differs dramatically within fields. Consequently, in places where a single N rate exceeds crop need, the left-over N may be lost to the environment. In places where a single rate is less than crop N need, yield will be less than optimal. One promising approach for better matching N need and application is within-field sensing of the reflectance characteristics of the crop and subsequent N application based on this sensed information. However, because the intensity of sunlight varies with time of day and sky conditions, data from sunlight-based sensors also varies, even if the crop characteristics are constant. One way to overcome this obstacle is to use sensors that include their own light source. Another approach, described in this paper, is to compensate for differences in sunlight as a function of sky conditions. We developed an “instantaneous clearness index” to quantify differences in cloudiness and applied it to reflectance data obtained from corn plants. The analysis showed that commonly used reflectance variables – band ratios and vegetation indices – did not completely compensate for cloudiness, because the effect of clouds was different for different wavelengths of light. Thus, additional compensation, perhaps based on the instantaneous clearness index, should be applied to provide more accurate data across a range of cloud cover. The results of this study provide information useful in the development of corn N application algorithms based on sunlight reflectance sensing.
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. 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 for by use of a band ratio or vegetation index. Explicit inclusion of ICI in a compensation algorithm should be investigated to provide more accurate data across a range of cloud cover.