AN 'ALTERNATIVE' MODEL TO ESTIMATE SOLAR RADIATION

Abstract

Solar radiation is extremely useful in modelling many agricultural applications, but is
hardly used due to the difficulty in obtaining data, and the time consuming process in
estimating 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 the
estimates obtained from the Angstrom model. The alternative model is more flexible and
useful in estimating crop evapotranspiration, and for crop-weather modelling. The mean daily
solar radiation at Lunuwila was estimated to be 18.3 MJ m"2d "' and .the total annual solar
radiation receipts is 6680 MJ m'2 (66.8 TJ ha-1). The monthly solar radiation was highest
in March (21.7 MJ m'2 d ' ) and the estimated 75% probability value was 22.5. June had the
lowest (16.1 MJ m"2 d'1) value and the estimated 75% probability value was 17.8.