Analysis of Long-Term Changes in Precipitation Pattern in the Khuzestan Province: Lessons Learned from April 2019 Floods

In this study, long-term changes in the precipitation pattern and how they affect the future flood events were explored by collecting, completing and validating precipitation data in the Karkheh, Dez, Karun, and Great Karun Basins. Also, the depth and frequency of precipitation in the water year 2018-2019 have been carefully investigated. Finally, the quality of precipitation forecast and awareness by some official organizations has been discussed. The study area has a sufficient number and density of reliable meteorological stations. However, this does not hold for the synoptic stations in mountainous areas, such that only 4% of the stations have recorded data in more than 10% of the entire 30-year statistical period. The results showed that the climatic trend of the region is on average in the direction of decreasing precipitation amount. The investigation of rainfall pattern has revealed the continuation of high soil moisture due to the increase of low rainfall events in recent years, which can lead to enhanced probability of flood occurrence. The analysis of April 2019 precipitations showed that in the entire study area, the amount of precipitation was 24% to 108% higher than the last 10-year average. The return period of total precipitation in Ahvaz station in the water year 2018-2019 was 297 years, which is unprecedented. In addition, the forecasts of the Iran Meteorological Organization and the Water Research Institute for the flood of April 2019 in the Khuzestan province were significantly erronous compared to the observations.