Estimation of Angstrom Coefficient and Developing a Regression Equation for Solar Radiation Estimation (Case study: Mashhad)

Solar radiation, nowadays has a lot of application in different fields of agriculture, irrigation, and hydrology engineering and due to these various applications, different models has been proposed for it’s estimation. Angstrom-Prescott equation is one of the most important well known models for solar radiation estimation. This equation has empirically coefficient that various for each location. In this paper, the data gathered in Mashhad Synoptic station during 1378 and 1380, Angstrom-Prescott coefficient has been identified according to the ratio of actual sunshine hours (n) to the maximum sunshine hours (N). Also a Regression local equation has been proposed considering several meteorology parameters including daily gathered data of saturation vapor pressure deficit, precipitation, air temperature mean, relative humidity percentage and n/N. Finally the proposed model has been evaluated according to the independent measured data during 1381 to 1382. The statistical analysis of the results not show a significant difference between multi coefficients-local equation with Angstrom-Prescott equation, and therefore without more accuracy and more additional meteorology data and only with the data including sunshine hours and calculating extraterrestrial solar radiation, global solar radiation can be used with a high precision. For instance our model for Mashhad can be used with a=0.23 and b=0.44 which are the coefficient of the Angstrom-Prescott equation. This coefficient should be calibrated and validated for each zone individually.