Solar Radiation Estimation from Rainfall and Temperature Data in Arid and Semi-arid Climates of Iran

Document Type: Research Paper

Authors

1 Asst. Prof., College of Soil and Water Engineering, University of Tehran, Karaj, Iran

2 Shahid Bahonar University of Kerman, Kerman, Iran

Abstract

Precipitation and air temperature data, only, are often recorded at meteorological stations, with radiation being
measured at very few weather stations, especially in developing countries. Therefore there arises a need for suitable
models to estimate solar radiation for a completion of data sets. This paper is about an evaluation of eight models for
an estimation of daily solar radiation (Q) from commonly measured variables in six synoptic stations of Iran, namely:
Mashhad, Kerman, Tabriz, Esfehan, Hamedan and Zanjan using daily rainfall and temperature data for a duration of
three years of 2000, 2001 and 2002. These stations represent several arid and semiarid sub-climates of Iran as based
on extended-De Martonne climatic classification (semiarid-cold: Mashhad and Tabriz, arid-cold: Esfehan, Kerman,
semiarid-extracold: Hamedan and Zanjan). The STATISTICA (ver. 6.0) software was employed for non-linear
multivariate regression. The results indicated that most of the models overestimated in lower values of solar radiation
while underestimating in the higher ranges, indicating a systematic error. Performance of the models was evaluated
based on the Root Mean Square Errors (RMSE) as well as R2. RMSE ranged from 1.14 to 7.76 Cal cm-2min-1 for the
whole data range and in all the six stations. Among the eight models, the Richardson model rendered the best
agreement with the measured data in Kerman and Zanjan stations. In case of Hamedan station, Bristow and Campbell
model was the most suitable. As for Tabriz station, De Jong and Stewart model using rainfall and range of daily
temperature data led to the best performance. In Mashhad station, McCaskill equation can be recommended. Analysis
of the data in Esfehan station showed no significant difference among the models. Due to variation in equations'
performances, to come to valid conclusions and to choose the most suitable radiation models, further study would be
required from other climatic regions the country.

Keywords