Investigating the drought trend using the modified SPEI and MSPI indices in different time scales (case study: Fars province)

The effective and timely monitoring of drought can help to develop drought systems and optimal management of water resources. Therefore, in this research, the drought situation in the synoptic stations of Fars province (Abadeh, Shiraz, Fasa and Lar) was investigated in different time scales (3, 6, 9 and 12 months) using precipitation-evaporation and transpiration (SPEI) and Multivariate standardized precipitation (MSPI) indices. The Mann-Kendall test was used in order to check the trend of the investigated indicators. Based on the results, the frequency of dry and wet periods is high in short-term time scales. But with the increase of time scales, the frequency of dry and wet periods decreases and their duration increases. In terms of the SPEI index, in the 3-month time scale, the most severe drought is at Abadeh station (-5.20) and the most severe drought is at Shiraz station (5.21). In the time scale of 6 months, the most severe drought and drought were observed in Abadeh (-3.45) and Shiraz (3.10) stations, respectively. In the time scale of 9 months, the most severe drought and drought are in Lar station with values of -3.63 and 2.48, respectively. Also, on a 12-month scale, the most severe drought is in Shiraz station (-3.37) and the most severe drought is in Abadeh station (2.37). Principal component analysis of SPI time series showed that the first principal component (PC1) can account for a large percentage of changes in the original SPI time series in all regions studied in this research. Also, the results showed that MSPIs follow the fluctuations of the SPI time series, especially in long-term dry and wet periods. So, partial wet and dry periods in severe and prolonged dry or wet periods may be excluded by MSPI.