Agriculture Reference
In-Depth Information
zenith
sun
zenith-angle
(vertical)
elevation-
angle
(vertical)
point
in the field
N
nadir
Fig. 9.21
Geometry of solar radiation
In order to evaluate the influence of the solar zenith-angle, records were taken in
Northern Germany on a sunny day in mid-May with a field spectrometer vertically
fixed on one spot in winter-barley. During the day, the zenith angle varied from 40°
to 80°. Similarly, reflectances were recorded on a day with varying cumulus clouds.
Finally, the soil-colour was altered by watering a dry, bright soil and thus creating a
dark, wet soil. For further details see Heege et al.
2008
or Reusch
1997
.
The effects of the interfering factors zenith angle, cloud cover and soil brightness
were defined by coefficients as listed below:
Factor zenith-angle of the sun
se
indexstand deviationcaused by change of zenith angle
indexdifferencecaused by altering thenitrogendo
Coefficient Cz
=
.
Factor cloud-cover
se
indexstand deviationcaused by change in cloud cover
indexdifferencecaused by altering thenitrogendo
Coefficient Cc
=
.
Factor soil-brightness
indexdifferencecaused by altering thenitrogendo
se
Coefficient Cs
=
indexdifferencecaused by wettingdry soil
The numerators of the coefficients or ratios are given by the index difference due
to the change in nitrogen applied. The denominator is the index difference pertaining
