Dn. Mungai et al., MEASURING SOLAR-RADIATION TRANSMISSION IN TROPICAL AGRICULTURE USING TUBE SOLARIMETERS - A WARNING, Agricultural and forest meteorology, 86(3-4), 1997, pp. 235-243
In Kenya, the ratio of the output of north-south (N-S) mounted tube so
larimeters (TSLs) to that of dome solarimeters and an east-west (E-W)
mounted TSL confirmed large divergences of the N-S mounted instrument
around solar noon and early and late in the day, owing to direction-re
lated geometrical factors of radiation interaction with the glass enve
lope, which are not fully understood. A difference in solarimeter outp
ut of NE-SW mounting compared with E-W mounting remained within the ac
curacy limits. High ambient temperatures (near 40 degrees C) with low
wind speeds (less than 0.3 m s(-1)) appeared to increase tube output r
elative to that of the Kipp solarimeter by 10-20%. Condensation inside
the tube increased the output by less than 5%. Ancillary indoor data
in the Netherlands confirm these ambient effects. These errors may add
to the geometrical ones and together they lead to instrument-based di
stortions in the interpretation of derived parameters such as radiant
energy conversion to dry matter and radiation extinction in mulches an
d crops. An example of the latter, for grass mulches, is given that co
nfirms earlier results obtained in Dar es Salaam. Calibrations of the
tubes as a function of time and ambient conditions against a standard
instrument and, where possible, near E-W mounting are therefore necess
ary under tropical conditions. Under these conditions, filtered TSLs a
ppear to be unreliable for determination of photosynthetically active
radiation, as a comparison with a Line Quantum Sensor confirmed. (C) 1
997 Elsevier Science B.V.