Application of Circular Statistics in Seasonality Analysis of Extreme Precipitation Occurrence Time in Urmia Lake Basin

A hydrological parameter affecting the management of water resource systems is changes in the amount and occurrence time of extreme precipitation (OTEP). In this research, the seasonality of precipitation in the Lake Urmia (LU) basin was analyzed using the daily extreme precipitation data of 30 rain gauges in the statistical period of 1991-2018. The uniformity of OTEP was tested by Rayleigh and Kuiper’s tests at 0.1, 0.05, and 0.01 levels. The slope of the trend line for OTEP was estimated using the modified Sen slope estimator. The uniformity of OTEP was rejected at each level. The results revealed two strong seasons: late winter and early spring (S1) and autumn (S2) for OTEP. The results showed a general median seasonality index of 0.3, which changed to 0.82 and 0.9 for S1 and S2, respectively, after dividing the whole year into two seasons. The seasonal strength of S1 was similar in both the western and eastern parts of LU, but the west of the lake was stronger than the eastern part in S2. In S1, negative and positive trends in the OTEP were observed on average in 40% and 60% of the stations, respectively, with corresponding values of 77% and 27% for S2, respectively.