Evaluation of SPEI Drought Index and Trend Analysis Using Nonparametric Methods in the Selected Stations of Urmia lake Basin

   Identification of trend and drought characteristics play an important role in the development and management of water resources. In this study, the spatio-temporal characteristics of SPEI index in short, medium, long and annual scales (SPEI3, SPEI6 and SPEI9) in Urmia Lake basin were investigated. for this purpose, temperature and precipitation data from 4 synoptic stations of Tabriz, Urmia, Mahabad, Ahar during 26 years (1988-1983) were used. Mann-Kendall test and LOWESS graphical method on monthly, yearly scale were used to evaluate SPEI index trend. Drought trend increased in all synoptic stations studied and monthly SPEI index decreased in all three short, medium and long term time scales (drought increase).The annual drought trend at Mahabad Synoptic Station also increased drought, and at all three time scales the drought trend at Mahabad station was downward compared to other selected stations. Based on the annual results of the SPEI index, the longest humidity period from 1990 to 1995 is detectable. The most severe drought was in 2001 in Urmia and Ahar stations. Also, the longest drought period (moderate, severe, very severe) on the time scales is at Mahabad Station. and Mahabad station is the first priority in terms of increasing drought process and Urmia station is the last priority.