A semi-empirical model for estimating diffuse solar near infrared radiation in Thailand using ground- and satellite-based data for mapping applications

This work has developed a semi-empirical model for estimating monthly average hourly diffuse solar near infrared (NIR) radiation in Thailand. Diffuse NIR radiation data were recorded at four stations in Thailand namely, Chiang Mai (18.78oN, 98.98oE), Ubon Ratchathani (15.25oN, 104.87oE), Nakhon Pathom (13.82oN, 100.04oE) and Songkhla (7.20oN, 100.60oE). Our analysis has shown that diffuse NIR radiation is related to cloud, precipitable water and cosine of solar zenith angle in terms of an exponential function. Therefore, the model was formulated with these parameters and validated with independent diffuse NIR measurements. The result showed that diffuse NIR radiation from the model and that from the measurements were in reasonable agreement with root mean square difference and mean bias difference of 16.7% and 1.5%, respectively. The semi-empirical model was used to generate diffuse maps of NIR for the country which displayed both diurnal and seasonal variations.