Evaluation of temporal trends of the drought indices SPI and SPEI in the Southern Coast of Iran

This research aims to compare the performances of the Standardized Precipitation Index (SPI) and Standardized Precipitation minus Evapotranspiration Index (SPEI) in terms of the monitoring and trend analysis of the climatic droughts at several synoptic stations located in the coastal areas of Persian Gulf and Oman Sea during 1992 - 2014. To this end, trend analysis of drought was carried out using the traditional Mann-Kendall and Trend Free Pre-Whitening (TFPW) Mann-Kendall tests. Results showed that both indices report the same behaviors, but the SPEI presents higher percentages in different drought classes and a rapid responses to humidity variations at all time scales when comparing to the SPI. There were significant positive correlations between the two indices at the corresponding timescales such that the highest correlations in most of the stations with the values 0.97 and 0.98 were related to the time scales of 12 and 24 months, respectively. The same results were obtained for the frequency percentages of different classes of the mentioned drought indices. Trend of droughts in the region is generally increasing and the slope of trend for the SPEI is stronger than SPI as the time scale increases. Finally, the use of SPI is not sufficient to monitor droughts in arid and hyper-arid climates of coastal regions, and due to the role of temperature as the limited factor of humidity, the study recommend the use of SPEI for effective and accurate drought monitoring in such regions.