Seasonality Analysis of Extreme Flood in Urmia Lake Basin Using Circular Statistics

One of the important consequences of the climate change on extreme floods is the change in their frequency, timing and magnitude. In this research, the seasonality of flood in the Urmia Lake basin was analyzed based on the extreme flood data from 14 stations. This work was done in the statistical period of 2003-2019 using the new method of circular statistics. The uniformity of occurrence time of extreme floods (OTEF) was examined by Rayleigh-test and Kuiper-test, in three significance levels of 0.1, 0.05 and 0.01. The slope of the trend lines for the OTEF were estimated using the modified Sen’s estimator. The results of the Kuiper test indicated that there is no uniformity in the OTEF. It was found that there are two distinct flood seasons in the region including a) late March to mid-May (S1) and b) mid-September to early December (S2). The mean of seasonal strength index of events was equal to r̅=0.695. By dividing the year into above mentioned seasons, this index was obtained as 0.9 for the first season and 0.78 for the second season. The two eastern and western parts of the Urmia Lake had same seasonality strength in S1. However, in S2 the seasonality of the western part of the lake was stronger than the eastern part. In S1, on average, the OTEF in 40% of the stations had a negative trend while for the rest it showed a positive trend. For S2 the stations with negative and positive trends were respectively 11% and 78% of the total.