Streamflow trends in the Tigris river basin using Mann−Kendall and innovative trend analysis methods

In this study, the trend of monthly mean and annual streamflow values of 16 streamflow gauge stations in the Tigris basin, which holds 13% water potential for Turkey, is determined. The monotonic trends are calculated using non-parametric Mann−Kendall (MK) test. To remove serial correlation from time series, a modified ‘pre-whitening’ method is used. The trend slopes are determined by Sen’s slope method. Moreover, innovative trend analysis (ITA) method is also used to determine trends of low, medium and high streamflow values. As a result of the study, trend indicators, namely, Z values of MK tests and D values of the ITA method are compared and it has been observed that the trend directions by MK and ITA are generally similar. Nevertheless, only the negative D values calculated by ITA are mostly higher than the negative Z values calculated by MK. The ITA results of the annual mean streamflow show 80% of the stations show a strong decrease in trend for high values. Most of the significant trends found by MK are obtained in a decreasing direction, and the linear slopes are mostly determined in the range of ±2% per year. According to the spatial analysis, the significant decreasing trends are generally located in the middle region of the basin.