Nunno et al. (2025) Actual evapotranspiration dynamics in Italy: trend detection and regional clustering analysis

Identification

• Journal: Theoretical and Applied Climatology
• Year: 2025
• Date: 2025-12-12
• Authors: Fabio Di Nunno, Francesco Granata
• DOI: 10.1007/s00704-025-05959-1

Research Groups

• Department of Civil and Mechanical Engineering (DICEM), University of Cassino and Southern Lazio, Italy

Short Summary

This study analyzed actual evapotranspiration (AET) dynamics across Italy from 1950 to 2024 using ERA5-Land data, revealing a north-south AET gradient driven by water availability and widespread statistically significant positive AET trends, particularly in northern and central regions.

Objective

• To systematically investigate the long-term trends and spatial patterns of Actual Evapotranspiration (AET) across Italy from 1950 to 2024 using a physically-based approach and ERA5-Land reanalysis data.

Study Configuration

• Spatial Scale: Italian peninsula, including Sardinia and Sicily, at a horizontal resolution of 0.1° × 0.1° (3394 grid cells).
• Temporal Scale: Monthly values from January 1950 to December 2024.

Methodology and Data

• Models used:
  • Potential Evapotranspiration (PET) estimated using the FAO-56 Penman–Monteith (PM) equation.
  • Actual Evapotranspiration (AET) estimated using a semi-empirical approach adjusting PET with a soil moisture stress coefficient ($fs = \min(1, SM/FC)$, where FC = 0.3 m³ m⁻³) and a vegetation cover coefficient ($f{LAI} = 1 - \exp(-k \cdot LAI)$, where k = 0.5). AET was also constrained to not exceed total monthly precipitation.
  • Clustering analysis performed using K-means, Hierarchical Agglomerative Clustering (Ward’s method), and Gaussian Mixture Models (GMM).
  • Trend detection performed using the Seasonal Kendall (SK) test.
• Data sources: ERA5-Land reanalysis product (gridded monthly values) from the Copernicus Climate Data Store, including daily maximum temperature ($T{max}$), daily minimum temperature ($T{min}$), mean temperature ($T{mean}$), surface solar radiation downward ($Rs$), wind speed at 10 m (Wind), soil moisture in the upper soil layer (Soil Moisture), precipitation (P), and Leaf Area Index for high and low vegetation ($LAI{HIGH}$ and $LAI{LOW}$).

Main Results

• Potential Evapotranspiration (PET) exhibits a clear latitudinal gradient, increasing from approximately 43.5 mm/month in northern Alpine regions to over 94 mm/month in southern Italy, driven by solar radiation and temperature.
• Actual Evapotranspiration (AET) shows an inverse gradient, with higher mean values up to 59.9 mm/month in northern Italy and significantly lower values (below 25 mm/month, as low as 17.1 mm/month) in southern Italy and the islands, primarily limited by water availability.
• Clustering analysis consistently identified three primary hydroclimatic zones: humid northern, transitional central, and arid southern Italy, with K-means providing the most robust spatial partitioning (peak Silhouette score of approximately 0.5 for K=3).
• Statistically significant positive trends in AET were observed over most of the peninsula. Northern Italy showed the highest proportion of area with increasing trends (99.59%), with Z-values reaching 10.73 in Alpine zones.
• Central Italy also exhibited widespread significant increases (99.34% of area, Z-values up to 6.86).
• Southern Italy showed significant positive trends in 99.26% of the area, with Z-values up to 6.59, but with more spatial variability.
• Major islands (Sicily and Sardinia) displayed weaker or non-significant trends, with 23.86% of Sicily and 49.39% of Sardinia showing non-significant trends, including negative trends in southern Sardinia (13.36% of area).

Contributions

• First systematic investigation of long-term AET trends and spatial patterns across Italy (1950–2024) using a physically-based approach and comprehensive ERA5-Land reanalysis data.
• Integration of physically-based AET estimation with unsupervised clustering and robust trend detection methods to delineate coherent hydroclimatic zones and identify evolving evapotranspiration patterns.
• Highlights the contrasting controls of atmospheric demand (PET) and water availability (AET) on evapotranspiration dynamics across Italy, emphasizing the critical role of water limitation in southern and insular regions.
• Provides essential insights for region-specific water resource planning and climate adaptation strategies, particularly for vulnerable areas facing intensifying water stress.

Funding

• The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Citation

@article{Nunno2025Actual,
  author = {Nunno, Fabio Di and Granata, Francesco},
  title = {Actual evapotranspiration dynamics in Italy: trend detection and regional clustering analysis},
  journal = {Theoretical and Applied Climatology},
  year = {2025},
  doi = {10.1007/s00704-025-05959-1},
  url = {https://doi.org/10.1007/s00704-025-05959-1}
}