روند تغییرات ضریب سیلابی در ایستگاه‌های هیدرومتری استان اردبیل

      Flood as a rapid and destructive event causes great human and economic losses in Iran and around the world, annually. This study aimed to assess the temporal trend and spatial changes of flood coefficient of Ardabil Province rivers using daily discharge of 22 hydrometric stations over a 22-year (1991 to 2011) of recorded data. The flood coefficient amount was calculated after subtracting the defined base flow resulted from the local minima technique. The non-parametric Mann-Kendall test was used to detect the temporal tend in the time series values. The recorded data length were divided into 5-year periods and the variations of changes of flow rate in different periods were analyzed. The significant differences were determined by using the Kruskal-Wallis statistical test. The results showed that changes of flood coefficient was significant (p<0.05) between different periods. The spatial changes of flood coefficient variation coefficient showed a similar spatial pattern in different studied periods and the highest variability were defined at North-East and West parts of the study area. The highest rate of flood coefficient was observed in Akbardavood station with the value of 0.8 in the fourth sub-period. According to the results of trend analysis, there is an increasing significant temporal trend in 6 stations, and one station had decreasing trend, while other stations had increasing trend with no statistical significance. The increasing trend of flood coefficient can be related to rainfall variations and land use change along with intensified use of extensive rangelands areas which increases the potential runoff and surface flow resulting increase in the flood coefficient in the studied statistics.