EVALUATION OF DROUGHT CHANGES OF ISFAHAN CITY BASED ON THE BEST FITTED PROBABILITY DISTRIBUTION FUNCTION

Generally, in arid and semi-arid region due to climatic conditions, a good match between conventional agriculture, soil and water conditions as well. Personal experiences of rural and old traditions teaches them how to avoid minor changes in environmental conditions of drought risks. In this study, the monthly rainfall statistical period 1951 to 2005 for Isfahan synoptic stations were used. Climate of the mentioned area is dry, according to climate Domarten Climagram. By choosing the best fitted distribution function basis of statistics Kolmogrov-Smirnov, the cumulative probability values were calculated and SPI values were corrected accordingly. The monthly scale results showed that in the months of June, July, August and September can be used Gamma function, but for the other months, the greatest difference between the results of the Gamma distribution with the best distribution in the frequency of droughts classes, can be seen in December. Also, the results showed that in addition to the frequency, the intensity values of drought is significant difference. It is often recommended that the default SPI index (Gamma) is not used. It should be noted that in the years studied, 1955 and 2000, respectively, the year was as wet and dry.