Time series analysis of rainy season duration in Iran

Iran has an arid and semi-arid climate that 61% of the country receiving less than 250 mm of rainfall and only 4% of the country receiving more than 600 mm of rainfall. This fact indicates a very uneven distribution of precipitation in Iran. On the other hand, given the fact that Iran is located in a dry region, the time variability of precipitation is very high. 85% of Iran is in the arid territory. Drought and water scarcity its main characteristics of Iran's climate (Karimi et al. 2018). After the heat and humidity factors, the third factor is precipitation that one of the 6 most important factors in shaping Iran's climate which is associated with temporal and spatial fluctuations (Masoudian 2003). Increased greenhouse gas emissions, global warming and subsequent climate change, especially in recent years, have caused major changes in this important climate factor, with Iran increasing 0.5 mm in volume over the last half-century (Masoudian 2011). Increased greenhouse gas emissions, global warming and subsequent climate change, especially in recent years, have caused major changes in this important climate factor, with Iran increasing 0.5 mm in volume over the last half-century (Masoudian 2011). On the other hand, forecasts for an increase of 1.5 Celsius degrees in 2030 and 2050 will add to the probable severity of these changes in the future (IPCC 2018).

In this paper, for the first time, the temporal variations of the rainfall duration of the country during the wet season were considered. In order to achieve this goal, after extracting the beginning and ending days of three-day rainfall and more, the rainfall season duration at 108 stations studied was calculated and first, using hierarchical cluster analysis, zoning the duration precipitation regions of the country. Subsequently, the study of rainfall season fluctuations based on the 25-year time series (1991–2015) was carried out using the Yue & Wang modified Kendall method.

The results showed the duration of the rainy season in 5 regions (very long, long, short, short and very short). As it was noted, these areas, like the rainfall regions of the country, mostly followed latitude and had more orbital arrangement (Masoudian 2009). Investigating the trend of the duration of rainy season changes in the country showed that these changes decrease significantly in the southern latitudes relative to the northern latitudes as the dry and semi-arid regions of the country, which have the shortest rainfall season, experience the least significant changes. (Modares & Dasilva 2007). Except for the southwestern coastal area of the Caspian Sea and the northwest of the country, significant incremental changes occur during the rainy season, along the southern slopes of central Alborz and the eastern slopes of northern Zagros. It should be noted that the Caspian coastal region is dominated by precipitation and humidity and in the northwest of the country up to the southern slopes of central Alborz, the thunderstorms are dominated pattern. In addition to the eastern slopes of the northern Zagros are prevailing by the thermal radiation climate, and is one of the low rainfall areas of Iran (Masoudian 2009, 2005). Therefore, the duration of the rainy season is increasing in regions with different climates. In general, as we move from north to south and from highlands to plains, inland deserts and coastlines, the long-term changes in the rainy season also decrease significantly. The most significant changes in these changes are mainly seen in the western half of the country and the southern slopes of Alborz and scattered in the northeastern and southern heights of the country.

Regardless of the significant incremental changes mentioned above, in general, the trend of long-term changes in the country during the wet season indicates a significant decrease in most areas and regions of the country. In other words, the country's rainfall regime is more concentrated and the rainfall season is shorter. Considering the importance of the duration of the rainy season and its variations in the growing season, flowering, sprouting (Zhou 2019, Moor & Lauenroth 2017, etc.) and crop yields (Daewoo 2019, Karimi et al. 2018, Maddah et al. 2015, etc.). As well as in the storage and management of water resources, it is hoped that the results of the forthcoming research will be useful in taking steps to achieve water and food security in the coming years.