Modification of Angstrom Model for Estimation of Global Solar Radiation in an Intermountain Region of Southern Iran

Estimation of solar radiation is widely used for prediction of crop water requirements. Its value is not measured, at many areas; therefore, several empirical models have been developed to predict solar radiation. Its value is predicted using Angstrom model with measuring daily sunshine hours. In this study, daily sunshine hours and global solar radiation were measured in Agricultural Research Station at Shiraz University, located in southern Iran for the 2003 to 2006 period. Because of the site topographic conditions, the measured daily sunshine hours ( n ) in clear-sky days were less than the calculated maximum possible sunshine hours by astronomical equations ( N ). Therefore, Angstrom model was modified based on substitution of N-t by N, where t is the mean difference between the values of n during clear-sky days and daily values of N. The estimated value of t in the modified Angstrom model was 1.85 h. The modified Angstrom model was more appropriate for predicting global solar radiation in comparison with its original form and is recommended to be used in intermountain regions, where sunshine hours are measured only.