عنوان مقاله [English]
Drought is one of the natural disasters and it is very much frequent in dry and semidry areas of Iran. Lack of rainfall has different effects on underground waters, soil moisture and river currents. Examining and analyzing of drought severity duration curves and evaluation curves are essential to water resources and agricultural management planning. So, the drought indices should be used. Standardized precipitation index (SPI) is one of the most important indices that aims to assign numeric values to the most important climate factor (rainfall). It is used to determine precipitation deficit in different time scales. Time scales show the drought effects on water resource abilities. In present study, the drought severities were determined in a 30 year statistical period for 20 stations across Fars province of Iran in three 6, 12 and 24 months scales. Then based on drought severities in different months, the change process curves of SPI index were provided at different scales. Considering the short-time, mid-time and long-time scales, and the drought was examined from different view points and its severest events and their alternation periods were compared and analyzed. The results showed that Fars province has been encountered with droughts for many years and it has deteriorated in recent years. Other results showed that the short-time droughts had very much fluctuation and were much sensitive to the moisture changes. But in long-term time scale, the sever droughts had long standing and reflected the drought in better fashion. Thus, since drought severity and its frequency are all directly or indirectly time scale depended functions, they may be regarded as the initial warning for drought and help to evaluate it. Result was also revealed the changes process from low severity to high one from northwest to the southeast of the province. The alternation period of drought occurrence with a mild severity varied 3-10 years in average at the studied stations. These quantities for moderate and severe droughts were obtained 5-15 and 10-15 years, respectively.