Trend Analysis of precipitation Regime in the Gorganroud Basin

Precipitation is an important climatic factor that reflects particular complexities. The statistical distribution of total precipitation of a location may vary with time. Due to the arid and semiarid climatic conditions of Iran, knowledge of the precipitation amount and variation is an important issue. Therefore in this study the characteristics and trend of precipitation in 7 rain gauging stations in the Golestan province is investigated during the 31-year period of 1974-2004. The linear regression and Mann Kendall trend analysis methods were used. The results indicated that at 5 % level, significant increasing trends have been observed in the annual precipitation of the Tamar, Tangrah, and Gonbad stations. Increasing trend has been found in the autumn precipitation in all stations, but it was significant only at the Tamar station at 5% level. Precipitation in winter and spring did not show a significant and homogenous trend across all stations. While the summer precipitation shows an increasing and homogenous trend across all stations. However it is significant at 5% level only for the Gonbad, Tamar, and Cheshmeh khan stations. The maximum daily precipitation of the Rabat Gharebil and Galikesh stations shows decreasing trend which is significant for the latter at 5% level. But it shows increasing trend in the other stations from which the Tamar and Tangrah stations have significant trends at 1% level. In case of number of days with rainfall events, all stations showed an increasing trend which is significant at 1% level except for the Tamar and Cheshmeh khan stations.