Abstract
Solar radiation is extremely useful in modelling many agricultural applications, but ishardly used due to the difficulty in obtaining data, and the time consuming process inestimating it by the Angstrom (1924) formula which uses world geographical relationships.To estimate solar radiation at the Coconut Research Institute, Lunuwila (7° 20' N; 71° 5 3 'E; 30.5 m ) an alternative model was developed from measured sunshine hours data only.The model had good fit (R2 = 0.90, P < 0.001) and was found to have agreement with theestimates obtained from the Angstrom model. The alternative model is more flexible anduseful in estimating crop evapotranspiration, and for crop-weather modelling. The mean dailysolar radiation at Lunuwila was estimated to be 18.3 MJ m"2d "' and .the total annual solarradiation receipts is 6680 MJ m'2 (66.8 TJ ha-1). The monthly solar radiation was highestin March (21.7 MJ m'2 d ' ) and the estimated 75% probability value was 22.5. June had thelowest (16.1 MJ m"2 d'1) value and the estimated 75% probability value was 17.8.
Highlights
The amount of radiation received at the higher level of the atmosphere is known as extra-terrestrial radiation
The results show that mean daily solar radiation decreases from February to June and remain virtually constant at 17 MJ m"2 d'1
The alternative model is extremely useful to agricultural scientists to estimate the amount of solar radiation received at the canopy of a crop
Summary
The amount of radiation received at the higher level of the atmosphere is known as extra-terrestrial radiation In the more advanced agro-meteorological stations Solar radiation is measured using various radiometers and pyranometers. Measured solar radiation data are not available front many agro-meteorological stations. Stanhill (1965) showed that the estimation of solar radiation, using the Angstrom formula is more accurate than the measurements taken from actinograph. The data on solar radiation are useful for determining the irrigation water requirements for crops. Foale (1993) estimated that if a coconut canopy intercepts all the incoming solar radiation and the canopy operates at maximum light use efficiency (about 20.5 MJ m'2 d"1), coconut palms would be able to produce 51 t/ha/year of dry matter. Detailed studies on the impact of solar radiation on coconut have not been carried out mainly due to non availability of such data (Peiris, etal, 1995).
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