Risk Assessment of Extreme Precipitation in Northwest Iran in the Light of Changing Climate

Abstract

The objective of this study is to conduct a regional risk analysis based on extreme precipitation trends at annual and seasonal time scales. It evaluates the trends of precipitation extremes over the Northwest of Iran, covering 600,000 km². ECMWF-ERA5 reanalysis precipitation data with a relatively high spatial resolution of 0.25° × 0.25° and daily temporal resolution are used for the long-term historical period from 1941 to 2020. The provided risk assessments are based on the integration of the special distribution of trends for 10 selected extreme precipitation indices (EPIs), which were assessed with a non-parametric Mann–Kendall test at the significance level of α = 0.05. Additionally, the difference in precipitation for the recent climate reference period (1991–2020) was compared to a subperiod 1941–1970, pursuing to distinguish shifts in precipitation patterns. The findings have found a meaningful increase in the frequency of daily heavy precipitation events over the explored period, specifically in the few latter decades. The trends reveal primarily positive patterns, with many being statistically significant. Furthermore, in some regions, total rainfall has increased by nearly 25% over the past three decades. The highest number of events occurs during the winter (DJF) followed by the spring (MAM) season. Nevertheless, looking at the monthly timescale, March records the highest number of extreme precipitation events. The strongest positive trend in intensity and frequency is associated with the autumn season (SON), particularly October and November in this region. Moreover, the highest number of grid points with notable positive trends is observed during autumn on the seasonal timescale and in November on the monthly timescale. Whilst the results of the annual survey show that 56.5% of the region is at risk of extreme precipitation, this percentage grows to 80.3% on the seasonal scale (SON), which highlights the areas with a higher probability of the occurrence of extreme precipitation events. Overall, the combined results of the EPIs trends indicate a significant shift towards more intense and frequent precipitation on interannual scales. Notably, most of this region can be classified as a susceptible area to extreme precipitation events and therefore at a high probability of flash floods, especially during the autumn season.