Spatial and temporal changes in rainfall frequency distribution patterns in Israel

Annual and monthly rainfall totals in Israel were analyzed to reveal any long term changes in their temporal and spatial distribution patterns, since the 1930s. The data consists of 60 rainfall stations, spread all over Israel from the far North to the Negev desert in the South, with long-term records of rainfall covering two normal periods. A gamma distribution function was fitted to the annual rainfall at each station for the two normal periods, and the shape and scale parameters of the distribution, as well as their percentage changes during the last normal period with respect to the first one, were analyzed. The analysis of the annual distribution function parameters reveals some appreciable changes, that are statistically significant, in the spatial rainfall distribution patterns in the southern, northern and central parts of the country. The most striking feature is revealed in the South, where a more than 60 percent increase in the shape parameter occurs, and a similar rate of re-scaling, i.e. a decrease of about 40 percent in the scale parameter. Analysis of the monthly distributions revealed considerable changes in October and November, at the beginning of the rainfall season, and an appreciable change in March, at the end of the rainfall season.