Trend Analysis of Precipitation of NW of Iran Over The Past Half of The Century

In this study, trends of precipitation of the NW of Iran were examined in monthly, seasonal and annual time scales using the Mann-Kendall method, which is developed in three different variations namely MK1, MK2 and MK3. Data used here were the information of precipitation for 16 selected stations in the period of 1955-2004. The Sen's slope estimator was used for detecting the slope of precipitation trend line. Results showed that there are downward trend in the most of stations for the NW of Iran's precipitation (except the Astara, Rasht and Mahabad stations) in annual time scale. The most strong positive trend slope in the case of annual precipitation belonged to the Astara station, which was equal to 6.42 mm/yr, however, the most strong negative trend slope belonged to the Anzali station, which was equal to -6.18 mm/yr. In monthly time scale, the numbers of months having negative trends were greater than that of the positive ones. All stations were experienced both negative and positive trends except the Jolfa, in which all months experienced negative trends for monthly precipitation data.