River flow trends in the west of Iran during the past 40 years: Impact of precipitation variability

A better understanding of hydrological changes associated with climate change can be achieved by analyzing the temporal trends in river discharge time series. We used the non-parametric Spearmanʹs rho, Kendallʹs tau, Mann–Kendall and Theil–Sen approaches to evaluate annual and monthly trends in river flow data from 12 stations located in the mountainous western regions of Iran over a 40-year period spanning the hydrological years 1969–1970 to 2008–2009. The Pearson correlation analysis was also applied to explore the relationships between precipitation and river flow series. Using a 5% significance level, we found an evidence of downward trends in annual river discharge at Varayeneh, Omar-Abad, Saleh-Abad, Zehtaran and Bahadorbeig stations at the rates of 0.491, 1.00, 0.101, 0.044, and 0.020 m3/s per decade, respectively. These significant downward trends started in the hydrological years of 2005–2006, 2000–2001, 2006–2007, 1995–1996 and 1995–1996, respectively. The greatest numbers of stations with significant river discharge trends occur in October and November, accounting for seven and six stations respectively. Annual precipitation exhibited no strong directional trend, although the tendency was toward increasing annual precipitation. Monthly precipitation variability seems to be also increasing for most of the months but no overall trend pattern in monthly precipitation totals emerged. Correlation analyses indicated strong relationships between river discharge and precipitation at the annual scale and for most of the months.