Validation in an arid area of an algorithm for the estimation of daily solar radiation

The Thornton- Running algorithm to estimate daily global radiation was tested at a site in a coastal desert of the eastern Mediterranean. In this algorithm three factors are multiplied in order to compute the daily global radiation: the total daily extraterrestrial radiation impinging on a horizontal plane, the maximum daily total atmospheric transmittance possible on days with clear skies (T-t,T-max), and the realized proportion of T-t,T-max for days during which the sky is not clear (T-f,T-max). Estimates of T-t,T-max compared very well with measured values while this was not the case for T-f,T-max. A good correlation was, however, obtained between predicted and measured values of daily global radiation with a noticeable underprediction of global radiation when measured values exceeded 20 MJ m(-2). A regression analysis showed that errors in the estimated global radiation were linearly related to errors in T-f,T-max. Analysis of the data used in this study indicated that for the conditions prevailing in the area it was not necessary to introduce a correction factor for rainy days. The possibility of using the saturated vapor pressure at minimum daily temperature in lieu of the actual measured daily averages of saturated vapor pressure is evaluated, and the results indicate that this approximation is reasonable and does not noticeably affect the estimation of global radiation. A systematic underestimation of daily Penman's potential evapotranspiration ( PET) during the dry summer period was observed when the computation of PET was carried out using the estimated values of global radiation instead of the measured ones. During this period a further underestimation of PET was observed when the latter was computed using the saturated vapor pressure at minimum daily temperature instead of measured daily values in all the equation terms. This underestimation was not noticeable during the winter period.