Study of the changes in precipitation regimes is very important in proper water resources management. In this research, in order to analyze the state of precipitation and annual dry spell periods, the trends of SPI, Annual Maximum Dry Spells length and Annual Number of Dry Spell Periods indices were studied. For this purpose, daily precipitation data of five synoptic stations in northwest of Iran (Urmia, Tabriz, Ardebil, Khoy and Maragheh) having a 34-year data record (1981-2014), were used. After examining the homogeneity and normality of the data, the significance of autocorrelation in each of the time series was tested. According to outcome of Mann-Kendall test, all three indices had a decreasing trend in the most of stations. Decreasing trend in the two AMDSL and ANDSPs indices reflects improvement of spell and number of dry periods, and decreasing trend of SPI index indicates reduction of precipitation. So, we can conclude that despite of the reduction in precipitation in the region, rainfall distribution has improved over the years. Decreasing trend of the two SPI, and ANDSP indices is significant in Maragheh and Mann-Kendall test criteria (Zm) for SPI, and ANDSP indices were -3.622 and -2.408 respectively (P<0.05). In order to identify the change points of the trends of series, sequential Mann-Kendall technique were used. The results indicated the occurrence of significant jump in Maragheh station.
AMDSL , ANDSPs , Drought , Mann-Kendall Test , SPI , Trend Analysis
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