Trend analysis of streamflow in Turkey

Abstract

Abstract This paper presents trends computed for the 31-year period of monthly streamflows obtained from 26 basins over Turkey. Four non-parametric trend tests (the Sen's T, the Spearman's Rho, the Mann-Kendall, and the Seasonal Kendall which are known as appropriate tools in detecting linear trends of a hydrological time series) are adapted in this study. Moreover, the Van Belle and Hughes’ basin wide trend test is included in the analysis for the same purpose. Homogeneity of trends in monthly streamflows is also tested using a procedure developed by Van Belle and Hughes. Thus, this study includes a complete application of both the Van Belle and Hughes’ tests for homogeneity of trends and basin wide trend (originally developed for trend detection in water quality data) on a hydroclimatic variable. As a result, basins located in western Turkey, in general, exhibit downward trend, significant at the 0.05 or lower level, whereas basins located in eastern Turkey show no trend. In most cases, the first four tests provide the same conclusion about trend existence. Use of the Seasonal Kendall, which involves a single overall statistic rather than one statistic for each season, is justified by the homogeneity of trend test. Moreover, some basins located in southern Turkey exhibit a global trend, implying the homogeneity of trends in seasons and stations together, based on the Van Belle and Hughes’ basin wide trend test.