Determining the of Seasonal Distribution of Monthly Rainfall Occurrence in some Rain Gauge Stations of Ardabil Province using Circular Statistics

Intensification of natural disasters make it important to know the time variations of rainfall for wet and dry spell management and adaptation strategies. Measuring and predicting of precipitation anomalies using statistical methods is an important tool for climate studies. Seasonal changes determine the distribution of precipitation occurrence over a year period.

The purpose of the present study was to investigate the seasonality variations of monthly rainfall data using circular statistics method at 28 rain gauge stations in a 30-years period. According to the results, the seasonality index of monthly rainfall, standard deviations and standard deviations were calculated in the monthly time scale. In the circular statistics method, the time of occurrence of the event is determined based on the angle on the scale of the studied event interval.

According to the results, the value of the seasonality index (r) in the 8 rain gauge stations are greater than unity, which includes Pol Almas, Kousetopragi, Nir, Lay, Sarein, Siyahpoosh, Neur and Yamchi, meaning that the occurrence of precipitation in these stations occur completely with seasonal natureAccording to the results, there is no correlation between the mean amount of rainfall and the values of the seasonality index (r). The highest Seasonal index (r) is calculated for the Lay station with a value of 1.53 and the minimum value of the seasonality index (r) is assigned to the time series of Totoonsin station with zero SI value. At most of the studied stations, it is observed that the value of the SI value (r) is close to 1 which indicate the seasonal nature of the precipitation occurrence.

Based on the values of the seasonality index, wet periods and the time distribution of rainfall in different months of the year can be determined. Also, the seasonality index can be used to determine the duration of droughts. The numerical calculation of the index over a long period of time can provide a more comprehensive understanding of precipitation distribution. Most of the rain gauges with seasonal rainfall pattern located in the east and west part of the Ardebil province. In conclusion, determining the distribution of precipitation in the study area showed that the precipitation occurrence in the Ardabil province occurs seasonal with distribution of precipitation in two to three months of the year.