Íslenskar landbúnaðarrannsóknir - 01.03.1980, Page 21

TEST OF A HAND-HELD RADIOMETER 19
500
r2 = 0.84 e'
450 •
400
5 • •
w 350 •
LU •
2 300 • • •
< •
< 250 * • L
QC 200 •5 —
150 •
100 I I I 1 1 1 1 1 1
0 60 120 180 240 300 360 420 480 540 600
TOTAL DRY BIOMASS (g/m2)
TOTAL DRY BIOMASS (g/m2)
Figure 4. Plot of data values, regression determined equations, and associated r2 values for (A) red
radiance, (B) IR radiance, (C) radiance ratio, (D) radiance difference, (E) radiance sum, and (F)
vegetation index.
(the most significant results), and the fifth
trial (the least significant results) will be
presented. Selected graphs from the other
four measurement trials will be presented
where appropriate. It should be noted
again that the sixth trial was for the plots
after clipping and thus this trial should
not be compared to the other five.
The results for the second measurement
trial, having the higher coefficient of de-
termination or r2 values, were similar to
the results of the other four preclipping
trials (Figure 3). The experimental results
from the fifth trial were similar to the sec-
ond trial, and with the exception of the
radiance sum, had lower r2 values (Figure
4).
In general, the analysis results were
very similar for each of the fxve preclipping
trials. Slight differences did exist but there
were no systematic reasons for these dif-
ferences.
INTERPRETATION
AND DISCUSSION
RED and IR radiances
Both the red and IR radiances de-
monstrate the phenomena of asymptotic
spectral radiance (Figures 3a,b and 4a,b).
Asymptotic spectral radiance is defined as
the stable or unchanging spectral
radiance which is reached as the green
vegetation biomass increases until addi-
tional biomass causes no change in the
resulting canopy spectral reflectance or
radiance (Tucker 1977). The red
radiance decreases as the total dry
biomass increases until the red radiance
asymptote was reached (Figures 3a and
4a). The actual numerical value of the red
radiance asymptote was dependent upon
the solar illumination. Regardless of the
solar illumination, however, the red