Application of circular statistics in seasonality analysis of extreme precipitation occurrence time in Urmia Lake basin

Urmia Lake (UL), one of the most important saline ecosystems in the world, has faced a severe water level drop in the last two decades. In this research, the seasonality of precipitation in the Urmia Lake basin (ULB) was analyzed using the daily precipitation data of 30 rain-gauge stations in the period 1991–2018. The occurrence time of extreme precipitation (OTEP) was determined by using circular statistics. The uniformity of OTEP was examined by Rayleigh test (RT) and Kuiper test (KT). The slope of the trend lines for the OTEP was estimated using the modified Sen’s estimator. Trends in the OTEP were analyzed by the non-parametric Mann–Kendall test. The results indicated no uniformity in the OTEP at 0.1, 0.05, and 0.01 levels in the basin. Seasonal events throughout the year were divided into two separate seasons denoted by S1 for late winter and early spring and S2 for autumn. The results showed that the mean seasonality increased from 0.3 to 0.82 (for S1) and 0.9 for S2. The comparison of seasonal strength in the west and east parts of ULB revealed that these two parts of ULB had the same seasonality strength (SS) in the S1. However, the seasonality of the western part of the lake was stronger than the eastern part in S2. Trends in OTEP showed that about 60% of the stations witnessed upward trends in S1. This was about 27% in S2. The findings of this analysis can provide useful information about the changes in the OTEP and its hydrological impact on the studied basin. This information is helpful in the scientific management of water resources in the Urmia Lake basin.