Trends analysis of annual and seasonal precipitation characteristics in the Hyrcanian region of northern Iran

We investigated the trend analysis of rainfall characteristics in the Hyrcanian region of northern Iran in a 33-years period (1987 to 2019). Annual, seasonal, and three 11- years periods (1987-1997; 1997-2008; 2008-2019) were analyzed using data recorded at eight synoptic weather stations of Astara, Rasht, Anzali, Ramsar, Noshar, Babolsar, Ghaemshahr, and Gorgan. The Mann–Kendall (MK) non-parametric test was used to determine the rainfall trends. The mean annual rainfall was 1143 mm with the highest and lowest amounts recorded in Anzali (1694 mm) and Gorgan (529 mm), respectively. The MK test demonstrated no significant trends for annual rainfall in each station and the whole region (MK value: 0.32). Rainfall of 11-year periods showed slight differences (1140, 1145, and 1146 mm for the three consecutive periods). Rainfall and the number of annual events per year illustrated positive significant correlations in all weather stations (mean r = 0.58). The mean annual number of rainfall events was 118 with a mean rainfall event of 9.5 mm increased by approximately 0.5 mm (5%) during the recent period. In the Hyrcanian zone, 21% and 31% of the annual rainfall were less than 10 mm and larger than 40 mm, respectively. 34% of yearly rainfall (383 mm) was recorded in the first half of the year and the largest amount of seasonal rainfall was in autumn (43% of total annual rainfall). Plant communities, forest ecosystems, and agricultural activities in the Hyrcanian region will definitely be affected by any change in precipitation.