Trend Analysis of Precipitation in Northern Half of Iran during the Recent Half of the Century

Precipitation is one of the important components of the hydrologic cycle that its amount and intensity change in different space and time scales. The researchers believe that changes in concentration of greenhouse gases due to consumption of fossil fuels, leads to strong changes in some of the components of the hydrologic cycle such as precipitation in different parts of the world. In this study, trend of precipitations in the northern half of Iran was invistigated in monthly, seasonal and annual time scales using the Mann-Kendall method. The precipitation data of 18 selected synoptic stations in the period of 1961-2010 were used for this analysis. The Sen's slope estimator was used for determining the slope of the precipitation trend line. Results showed that in the annual time scale, the precipitation at 44% of the stations (8 out of the 18 stations) had a significant downward trend, six stations had insignificant downward trend and only 4 stations (Babolsar, Qazvin, Sabzevar and Tehran) had insignificant upward trend in precipitation. Among the stations, the most strong negetive trend line slope in the case of annual precipitation belonged to the Gorgan station, which was equal to -4.192 mm/yr. In monthly time scale, the number of months with negative trends was 1.3 times greater than that with positive trends. In seasonal time scale, similar to monthly time scale, most of the sites had negative trends. In general, the results indicated that the downward trends of precipitation in northwest of Iran were stronger than those in the central and eastern parts of the country.