Impact of climate change on streamflow timing (case study: Guilan Province)

The timing of streamflow characteristics is important for water resources management and agricultural irrigation. Using daily discharge data from 37 gaging stations, located in Guilan Province, Iran, for the period 2002-2016, this study investigated nine streamflow timing measures and tested their trends by the Mann-Kendall non-parametric test at the 90% significance level. The nine measures included Q5%, Q10%, Q50%, Q90%, and Q95%, date corresponding to the center of fall volume, and dates corresponding to centers of spring and winter volumes. Results revealed that the median values of low flow timing slopes (Q5% and Q10%) had significant negative trends. Q5% had the steepest downward trends, implying that low flows originating from groundwater occurred earlier than in the past. In contrast, high flow timings (Q90% and Q95%) showed upward trends. Also, Q50% exhibited upward trends for all 37 stations. The trends suggest that climate change has altered the river flow regime. Among the nine measures, the date corresponding to the center of fall volume (FCV) and the dates corresponding to the centers of winter/spring volumes (WSCV) had the most significant downward trends. Trends in the spring freshet were mainly significantly negative in the northwest. However, trends in the pulse date were negative mainly in the west. The occurrence of low flows became earlier in time, and the length of dry periods within a year became longer.