Íslenskar landbúnaðarrannsóknir - 01.03.1980, Side 25

TEST OF A HAND-HELD RADIOMETER 23
vegetation. Dead vegetation is not sensi-
tive spectrally to this method.
(2) The method works better in
homigeneous vegetation or cultivated
grasslands. Heterogeneous vegetation
tends to degrade the relationship between
the red and IR radiances and the green
biomass present and necessitate using
larger area experimental plots to
adequately sample the spatial varibility.
(3) The presence of herbaceous flow-
ers, being quite reflective, an seriously de-
grade the utility of this method.
(4) The spectral radiances vary from
day to day, hour to hour, and therefore
any field application of this method will
need frequent recalibrations. Additional
work needs to be done to see if the calibra-
tion procedure(s) can be simplified.
(5) The asymptotic nature of spectral
radiance with respect to green biomass
effectively limits the use of the method
from low to medium-high amounts of
green biomass.
Associated with the method are several
advantages which include:
(1) The method is fast (the 29 plots
could be measured in 10 minutes by two
people) and reasonably accurate under
many conditions.
(2) The method is not seriously af-
fected by the presence of wet soil, water
droplets on the vegetation, or light mist.
This is in contrast to the adverse effects
these conditions have upon capacitance
meters, the other widely used nondes-
tructive biomass estimation method.
(3) The instrument is light (2 kg),
easy to carry, sturdy, and relatively cheap
($ 2,000 U. S.).
(4) The same instrument can be used
for laboratory live/dead determinations
without any modifications (Tucker
1980).
Additional testing of this method is
planned to further evaluate its utility in
Iceland.
TABLE 1.
Date, time, and description of conditions for the six series of spectral measurements.
Number Date Time Description
1 13/07/76 1300 Overcast, very cloudy, sun coming through patches of clouds
2 13/07/76 1400 Overcast, very cloudy
3 13/07/76 1500 Overcast, very cloudy, very windy
4 14/07/76 0815 A lot of dew on vegetation, intermitent clouds solar flux varied greatly
5 14/07/76 1000 Mild wind, cloudy with frequent sunny patches, drier than for number 4
6 14/07/76 1245 After clipping; several plots had very mossy soil surfaces and some low lying herbaceous plants