Spatiotemporal trends and change point of precipitation in Iran

The analyses of the spatial and temporal trends of precipitation are pertinent for the future development and sustainable management of water resources of a given region. Annual and seasonal precipitation data from 28 synoptic stations of Iran (1967–2006) were analyzed to determine the spatial and temporal trends and approximate year of the beginning of the significant trends by using the Mann–Kendall and Mann–Kendall rank statistic tests, respectively. The trend free pre-whitening (TFPW) method was applied to eliminate the influence of serial correlation on the Mann–Kendall test, and the magnitude of the precipitation trends was obtained from the Theil–Sen's slope estimator. Over the 40-year period, negative trend in annual precipitation occurred at 22 sites (79%), while just three sites had statistically significant (α = 0.05) negative trend in precipitation. The magnitude of the significant negative trends of annual precipitation at the 95% confidence level varied from (−)2.53 ± 0.69 mm/year at Tabriz station to (−)3.43 ± 0.81 mm/year at Khoy station. The change points of the annual precipitation at Khoy, Oroomieh and Tabriz stations were 1982, 1994 and 1981, respectively. In the seasonal series, the negative trends in spring and winter precipitation were larger compared with those in the other seasonal series, so that, five significant negative trends were detected in the winter time series. A noticeable decrease in the winter precipitation series was observed mostly in northern Iran, as well as along the coasts of the Caspian Sea. In summer precipitation, two significant positive trends were found at Mashhad and Torbateheydarieh stations, whereas no significant positive or negative trends were detected by the trend tests in autumn precipitation.

► The significant trends were found in the annual precipitation time series.
► The significant annual trends started in 1982, 1994 and 1981.
► The significant trends were more obvious in winter than the other season.