عنوان مقاله [English]
Drought is a phenomenon refers to precipitation less than normal value which is possible in any climate and geographic location. In this study, the Standardized Precipitation Index on a scale of 1, 3 and 12 months used to determine the intensity, duration and frequencies of Shiraz droughts and the graphical mann-kendall test used to analyze the shifts or possible trends in both annual and seasonal time scales. The data used was monthly precipitation data of Shiraz synoptic station ranged from 1984 to 2013. Results indicated the increase in the intensity, duration and the frequency of droughts and trend of normal years to drought. Also the drought of 1386-1387 was the biggest with the duration of 16 months and the maximum intensity of -2.57. Since the negative trend of annual and seasonal precipitation scales is not statistically significant at 5% level, the trend of decadal moving averages was applied to examine the whole trend. Due to the low variability of decadal moving average from fluctuations, the trends were determined in high confidence level. Results indicated that precipitation averages decreased by 100 mm in the last ten years and negative trends were identified with high confidence of 99.99% in annual scale and winter. Also, results showed that negative slope of winter precipitation trend line is steeper than reduction of annual precipitation.