Measuring solar radiation transmission in tropical agriculture using tube solarimeters; a warning

In Kenya, the ratio of the output of north-south (N-S) mounted tube solarimeters (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-related geometrical factors of radiation interaction with the glass envelope, which are not fully understood. A difference in solarimeter output of NE-SW mounting compared with E-W mounting remained within the accuracy limits. High ambient temperatures (near 40°C) with low wind speeds (less than 0.3 m s −1) appeared to increase tube output relative 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 distortions in the interpretation of derived parameters such as radiant energy conversion to dry matter and radiation extinction in mulches and crops. An example of the latter, for grass mulches, is given that confirms 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 necessary under tropical conditions. Under these conditions, filtered TSLs appear to be unreliable for determination of photosynthetically active radiation, as a comparison with a Line Quantum Sensor confirmed.