Empirical models of daily and monthly global solar irradiation using sunshine duration for Alagoas State, Northeastern Brazil

The Ångström-Prescott model called M1 together with ten modified versions, all based on the sunshine duration were adjusted to estimate the daily global and monthly averaged solar irradiation for some sites in the hinterland of Alagoas State in the eastern coast of the Northeastern Brazil. The models were adjusted with meteorological data from 2007 to 2010 and their skills were analyzed using: the Mean Bias Error, Root Mean Square Error and Willmott's Index of Agreement. The results indicate that the fitted coefficients depend on the geographical coordinates, altitude and local microclimate with 15% differences among the coefficients and estimates. The largest errors are observed in the regions with more cloudiness. Mean Bias Error and Root Mean Square Error for the daily evaluation of models M1, M9 and M11 were similar, with high values of Willmott's Index. The daily estimates obtained with models M1 and M11 did not differ more than 5%. Models M9 and M11 showed a better performance than that of M1 on a monthly basis. Finally, models M1 and M11 yielded the best results and due to their efficiency and simplicity are recommended to estimate the daily and monthly solar irradiation where sunshine duration data are available.