Paola et al. (2026) The May 2023 flood in Emilia-Romagna and 50-year trends in extreme precipitation based on ERA5-Land

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-01-07
• Authors: Francesco Di Paola, Domenico Cimini, Maria Pia De Natale, Donatello Gallucci, Sabrina Gentile, Edoardo Geraldi, Salvatore Larosa, Saverio Teodosio Nilo, Elisabetta Ricciardelli, Mariassunta Viggiano, Michele Volini, Filomena Romano
• DOI: 10.1016/j.ejrh.2025.103070

Research Groups

• Institute of Methodologies for Environmental Analysis - National Research Council (CNR-IMAA), Italy

Short Summary

This study analyzes the extreme precipitation events of May 2023 in Emilia-Romagna, Italy, and places them in a multi-decadal context by deriving long-term (1974–2023) precipitation trends from ERA5-Land reanalysis. It finds that while overall wet hours have decreased and precipitation has become more fragmented, the intensity and frequency of the most extreme precipitation events have increased, consistent with thermodynamic expectations.

Objective

• To characterize the extreme precipitation events of May 2023 in Emilia-Romagna, Italy, using rain gauge observations and reanalysis data.
• To analyze long-term (1974–2023) trends in sub-daily and multi-day precipitation characteristics, focusing on extremes (intensity, frequency, duration, spatial extent), to provide a multi-decadal context for the May 2023 events.

Study Configuration

• Spatial Scale: Emilia-Romagna region, northern Italy. Analysis performed at individual ERA5-Land grid points (~9 km resolution) and regionally.
• Temporal Scale: May 2023 for event characterization (specific events lasting 36-120 hours). Long-term trend analysis over 50 years (1974–2023) using hourly data.

Methodology and Data

• Models used: ERA5 and ERA5-Land reanalysis datasets (ECMWF).
• Data sources:
  • Rain gauge observations (284 gauges) from Agenzia regionale per la prevenzione, l′ambiente e l′energia dell’Emilia-Romagna (Arpae-SIMC).
  • ERA5-Land (316 grid points, 0.1° x 0.1° resolution) and ERA5 (0.25° x 0.25° resolution) reanalysis.
  • Copernicus Emergency Management Service (CEMS) for May 2023 flood extent and depth.
  • Shuttle Radar Topography Mission (SRTM) for terrain elevation.
  • EU-Hydro River Network Database for river channels.
• Methods:
  • Percentile threshold method (99th percentile) to define extreme precipitation and other metrics (persistence, areal extent, mean precipitation).
  • Theil–Sen (TS) slope estimator for trend magnitude.
  • Mann–Kendall (MK) trend test for statistical significance and direction of monotonic trend.
  • Inverse Distance Weighting (IDW) for interpolating rain gauge data onto the ERA5-Land grid.
  • Analysis of ETCCDI (Expert Team on Climate Change Detection and Indices) precipitation indices.
  • Clausius–Clapeyron relationship for temperature-precipitation scaling.

Main Results

• The May 2023 flood events in Emilia-Romagna resulted in approximately €8.8 billion in economic losses, 17 fatalities, and widespread damage, including a maximum flooded area of 1.25 x 10^9 m^2 with peak water depths reaching 5.5 m. High soil moisture content from prior rainfall exacerbated the flooding.
• Over the 50-year period (1974–2023), mean hourly precipitation for extreme accumulations increased by 10 % for 1-hour durations and 5 % for up to 2-hour durations.
• The increase in 1-hour extreme precipitation (7.4 % per degree Celsius) is consistent with the Clausius–Clapeyron thermodynamic rate, given a concurrent 2-meter air-temperature increase of 1.3 °C during extreme precipitation.
• The frequency of 1-hour extreme accumulations decreased by 12 %, while extreme accumulations with durations up to 3 hours increased in frequency by an average of 26 %.
• Overall, there is statistical evidence for a reduction in wet hours and a more fragmented distribution of precipitation in time and space.
• Extreme events defined by regional spatiotemporal extent, duration, persistence, and accumulated precipitation show a mean decrease of 16 % in these characteristics, with no significant change in frequency.
• Extreme events defined solely by mean precipitation show a 9 % increase in magnitude and a 61 % increase in frequency.
• ETCCDI precipitation indices generally show weak or negligible statistical evidence for trends, with RX5day (maximum five-day precipitation) being the only index with weak/uncertain but non-negligible statistical evidence, indicating an 8.7 % negative trend.

Contributions

• Provides a detailed characterization of the May 2023 extreme precipitation sequence in Emilia-Romagna using both rain gauge observations and ERA5/ERA5-Land reanalysis datasets.
• Offers a comprehensive 50-year (1974–2023) analysis of long-term trends in sub-daily and multi-day precipitation characteristics (intensity, persistence, spatial extent, frequency) in Emilia-Romagna, focusing on extremes and linking these trends to the characteristics of the May 2023 events.

Funding

• Copernicus Climate Change Service (C3S)
• European Centre for Medium-Range Weather Forecasts (ECMWF)
• Copernicus Emergency Management Service (CEMS)
• National Aeronautics and Space Administration (NASA)
• Jet Propulsion Laboratory (JPL)
• U.S. Geological Survey (USGS)
• European Union's Copernicus Land Monitoring Service (EU-Hydro River Network Database 2006–2012)

Citation

@article{Paola2026May,
  author = {Paola, Francesco Di and Cimini, Domenico and Natale, Maria Pia De and Gallucci, Donatello and Gentile, Sabrina and Geraldi, Edoardo and Larosa, Salvatore and Nilo, Saverio Teodosio and Ricciardelli, Elisabetta and Viggiano, Mariassunta and Volini, Michele and Romano, Filomena},
  title = {The May 2023 flood in Emilia-Romagna and 50-year trends in extreme precipitation based on ERA5-Land},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2025.103070},
  url = {https://doi.org/10.1016/j.ejrh.2025.103070}
}