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Drought Monitoring Using SPI Index in Central Region of Iran

Authors:
[ Tayeb Raziee ] -
[ Alireza Shokoohi ] -
[ Bahram Saghafian ] -
[ Peyman Daneshkar Arasteh ] -

Abstract:

Climate variability is an aspect of natural climatic cycles, which causes anomalies and fluctuations in climatological variables such as precipitation and temperature. In some parts of the world, these anomalies are intense and induce vast turbulence in natural ecosystems. Droughts are clear signs of climate variability, which frequently hits arid and semi arid regions around the world, including Iran, with severe intensity. There are many indices used to investigate drought and wet events, including Percent ofNorma1 (PN), Deciles, Palmer Drought Severity Index (PDSI) and Thornwait Moisture Index (TMI). Each of these indices has advantages and disadvantages. For example the PN index is site-dependent and, due to variation of mean precipitation from site to site, it is unable to monitor spatial pattern of droughts. The PDSI index, which requires many variables and is complex in calculation, is not convenient for regions such as Iran due to lack of data. A few investigations on drought occurrence in Iran have been based on PN and Deciles indices, In this research drought phenomenon was analyzed using the well-known Sri index. The Sri index, solely based on precipitation is a convenient and applicable index for separation of drought and wet periods. Sri index allows an analyst to detect drought and wet spells in any desire timescales. Since Sri index is a non-dimensionalized index, it is more appropriate for development and analysis of drought spatial patterns. Using Sri index, one can detect most the severe and extreme wet and drought spells in a region and conduct a frequency analyses. In this research, monthly rainfall data of 22 rain-gauge stations in central part of Iran with 40 years of data records were used to compute Sri index. "'(Hen, probability distribution function was determined from the longterm record by fitting a suitable hnction. The cumulative distribution hnction was then transformed using equal probability to a normal distribution with a mean of zero and standard deviation of one, so the values of the Sri are really in standard deviations. Using this method, Sri values for 3, 6, 9, 12 and 24-month timescales were computed and dry and wet spells in each station were identified. In order to investigate spatial pattern of droughts in the study area, Sri values of December 1998 and 1999 for different timescales were mapped using geostatistical tools in GIs. The results showed that drought intensity increases from west to east of the study region and reaches its maximum value to the east of Yazd province.

Keywords:

Drought, Monitoring, SPI Index, Iran.