V. Sheikh - Assistant professor of Watershed Management Department, Gorgan University of Agricultural Sciences and Natural Resources, Shahid Beheshti St., Postcode: ۴۹۱۳۷-۱۵۷۳۹
A. Behremand - Assistant professor of Watershed Management Department, Gorgan University of Agricultural Sciences and Natural Resources, Shahid Beheshti St., Postcode: ۴۹۱۳۷-۱۵۷۳۹
Y. Mooshakhian - B.Sc Student of Range and Watershed Management Department, Gorgan University of Agricultural Sciences and Natural Resources

Changes in the hydrologic behavior of river basins give rise to concerns of water resource managers and policy makers. In this study trends in hydrological and precipitation variables are examined for the 35-year period using the data of 6 hydro-climatological stations in the Atrak River basin in north east of Iran. Two annual series of hydrologic variables were examined, namely mean discharge and peak discharge. Regarding the precipitation, three annual series of total precipitation, maximum daily precipitation, and number of rainy days were studied. Four main trend tests (linear regression, normal score regression, Mann-Kendall and Seasonal Kendall) were adopted. Initial data analysis indicated that the linear regression which is a widely used parametric technique is not applicable for the trend detection of the discharge variables. Because, the assumption of normality has not been met by discharge observations. The other three tests, which are robust non-parametric tests, provide similar conclusion about the trend existence. Results indicated that for more than half and one third of the stations the hydrologic variables show significant downward and upward trends for the mean and peak discharges, respectively. However, the results of analyses give no evidence of significant trend for the precipitation variables in majority of cases. Overall, although the precipitation pattern is considered as the major driving variable for the river discharge regime, but it is not the only possible cause. Amongst other causes are the landscape and land use changes which we assume as most likely causes of trends in the hydrological regime of the Atrak River basin.

trend, discharge, precipitation, Mann-Kendall, Atrak River basin