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

Global and normal incidence radiation intensities measured at two sites loc ated 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 diffus e on a horizontal surface to the daily extraterrestrial radiation on a hori zontal surface, as an exponential function of K-D, the ratio of the daily b eam on a horizontal surface to the daily extraterrestrial on a horizontal s urface. The databases utilized in developing the equations consisted of mea surements made in Beer Sheva and Sde Boker during the years 1990-1998 and 1 990-1999, respectively. The monthly empirical regression equations were val idated by using an independent set of data measured in Beer Sheva during th e years 1983-1989. It was observed that a single set of monthly equations w as capable of characterizing the Negev region of Israel. The statistical an alysis of the results is presented and discussed in detail.