Regression equations for determining the daily diffuse radiation as a function of daily beam radiation on a horizontal surface in the semi-arid Negev region of Israel

Global and normal incidence radiation intensities measured at two sites located in the semi-arid Negev region of Israel have been utilized to develop a set of monthly empirical regression equations for determining the diffuse radiation that simulate the data shown graphically in the classic paper by Liu and Jordan. This equation expresses K_d, the ratio of the daily diffuse on a horizontal surface to the daily extraterrestrial radiation on a horizontal surface, as an exponential function of K_D, the ratio of the daily beam on a horizontal surface to the daily extraterrestrial on a horizontal surface. The databases utilized in developing the equations consisted of measurements made in Beer Sheva and Sde Boker during the years 1990-1998 and 1990-1999, respectively. The monthly empirical regression equations were validated by using an independent set of data measured in Beer Sheva during the years 1983-1989. It was observed that a single set of monthly equations was capable of characterizing the Negev region of Israel. The statistical analysis of the results is presented and discussed in detail.