Measuring trends and regimes of rainfall with the use of seasonality patterns in Iran

Abstract

Abstract The trend and regimes of rainfall considerably are different effects on the bio-environmental process. Therefore; climatic elements changes and changed trends and regimes of rainfall are both makers, with severe changes to bio-environmental conditions. In a more detailed analysis, it is essential to detect both trends and regimes of rainfall, distribution of monthly and annual rainfall, as well as seasonality patterns. Therefore, this study used monthly and annual rainfall series records for 1975–2019 from 140 synoptic stations and satellite data such as geopotential height, Southern Oscillation Index (SOI), Northern Oscillation Index (NOI), North Atlantic Oscillation (NAO) to detect the trend and regimes of rainfall in Iran. Statistical analysis with parametric and non-parametric tests for monthly and annual rainfall series was used to detect the rainfall patterns based on selecting the Mann–Kendall test (MK), Sen.’s slope method (SSM), and the t-student test at a 5% significance level. Based on the seasonality index (SI) method to detect the seasonality patterns, we recognized spatial patterns of the regimes of rainfall and trends of rainfall in Iran. Finally, we evaluated the trends and the regimes' patterns of rainfall spatially, the type of trends with decreasing and increasing patterns, and rainfall changes range with spatial statistics models and the temporal distribution. Results show the different slopes from -0.409 to -0.156 mm based on spatial statistics models with the south-north oriented with a total decreasing pattern for rainfall changes. Rainfall patterns show the different patterns temporally (non-trend, decreasing, and increasing) based on stations with different periods in Iran, which may relate this difference to temporal distribution in recorded periods. Based on the seasonality, rainfall regime distribution shows diversity in seasonality in rainfall from an extreme seasonality pattern, mainly in most rainfall in < 3 months pattern, markedly seasonal with a long dry season pattern, mainly seasonal pattern, and extreme seasonality pattern in Iran. Therefore, the results of this study, effects of trends, and rainfall regimes were shown on environmental planning in Iran.