A Comparison of Trends in Hydrologic Variables in the Atrak River Basin Using Non-parametric Trend Analysis Tests

Identification of the existence or non-existence of trends in streamflow and floods by the statistical analysis for water resources management and infrastructure designing is an inevitable issue. Thus, in this study, three hydrologic variables of mean daily discharge, maximum daily discharge and peak discharge were analyzed for detection of any probable trend. Using three non-parametric tests including Mann- Kendall, Spearman’s Rho and Thiel-Sen estimators, the trend of hydrologic variables in the Atrak River basin in northeast of Iran was studied for 10 discharge gauging stations during the 1972-2003 period. The results indicated that the three tests proved the same conclusion about trend existence. Except one station, all other stations within the study area show increasing trend for the peak discharge but for the mean daily discharge show opposite trends. For 70 percents of stations, the observed downward trends in the mean daily discharge are significant at 0.05 or lower levels whereas 30 percents of stations exhibit significant upward trend in the annual peak discharge at 0.05 or lower significance levels. None of the stations present a significant downward or upward trend for the maximum daily discharge. Based on the results it can be concluded that the hydrological regime of the Atrak River basin has undergone considerable changes which is indicated by decrease in annual discharge and increase in magnitude of floods. Therefore, water resources management and planning authorities should consider these changes in the assessment, designing and planning processes. The changes in hydrologic regime of the Atrak River basin is either related to climate or land use changes.