Goshtasbpour et al. (2026) Areal reduction factors from high-resolution rain gauges in Austria

Identification

• Journal: Natural hazards and earth system sciences
• Year: 2026
• Date: 2026-10-15
• Authors: Golbarg Goshtasbpour, Jannick Alpers, Kai Schröter, Hannes Müller‐Thomy
• DOI: 10.5194/nhess-26-5057-2026

Research Groups

• Department of Hydrology and River Basin Management, Leichtweiß-Institute for Hydraulic Engineering and Water Resources, Technische Universität Braunschweig, Germany

Short Summary

This study empirically derives Areal Reduction Factors (ARFs) for the Feldbach region in southeastern Austria using a high-resolution rain gauge network (WegenerNet) to convert point design precipitation into areal values. The findings reveal that ARFs are necessary even for areas as small as 1 square kilometer, exhibit unique dependencies on return period for sub-hourly durations, and show weak correlation with topography.

Objective

• To empirically derive Areal Reduction Factors (ARFs) using a fixed-area approach for the Feldbach region in southeastern Austria, utilizing high-resolution precipitation data from the WegenerNet.
• To investigate the relationship between sub-hourly ARFs and duration, return period, area, and season.
• To provide new insights into ARF behavior, particularly for short durations and small areas, given the high spatial density of the data.

Study Configuration

• Spatial Scale: Feldbach region, southeastern Austria. WegenerNet climate station network (143 rain gauges). Total area of 330 square kilometers with 75 × 110 grid cells, each 200 meters × 200 meters. Analyzed area sizes range from 1 square kilometer (5 × 5 grid cells) to 81 square kilometers. Elevation range from 250 to 598 meters above sea level.
• Temporal Scale: 2007–2022 (15 years). Temporal resolution of 5 minutes. Durations analyzed: 5, 10, 15, 20, 30, 45, 60, 90, 120, 180, 240, 360, 540, 720, 1080, and 1440 minutes (24 hours). Return periods: 1, 2, 5, and 10 years. Seasonal analysis: Spring (March–May), Summer (June–August), Autumn (September–November), and Winter (December–February).

Methodology and Data

• Models used:
  • Areal Reduction Factor (ARF) calculation: Empirical fixed-area method, defined as the ratio of areal precipitation to point precipitation for given duration and return period.
  • Extreme Value Analysis (EVA): Peak-Over-Threshold (POT) method, selecting approximately 40 largest precipitation values for each duration, ensuring event independence with a minimum inter-event time (MIT).
  • Distribution fitting: Exponential distribution function (F(x) = 1 −e−x/k) fitted to POT samples using a geometric approach (semi-logarithmic plot).
  • Areal precipitation calculation: Arithmetic mean of pixel values within square-shaped areas for each time step.
  • Spatial interpolation for raster product: Inverse distance-squared method (IDSW).
  • Topography analysis: Spearman correlation coefficient (rs) between ARF values and station elevation.
• Data sources:
  • WegenerNet climate station network: High-resolution rain gauge data (143 stations).
  • WegenerNet regionalized raster product: Grid-based precipitation data derived from the station network.
  • Validation data: Official Austrian design precipitation values from eHYD (Electronical Hydrographic Data).

Main Results

• ARFs generally increase with duration and decrease with area, consistent with existing literature.
• Contrary to common assumptions, ARF application is necessary for areas as small as 1 square kilometer, indicating that point measurements and pixel-based precipitation estimates are not equivalent for such small scales.
• For sub-hourly durations (5 to 20 minutes), ARFs partially increase with the return period, a finding that contradicts previous literature and is attributed to the high-resolution data's ability to better represent spatial variability.
• Seasonal variations are prominent, especially for shorter durations: smaller ARFs are observed during spring and summer (linked to convective precipitation), while higher values occur in winter. Autumn shows intermediate values but the largest range.
• The relationship between ARFs and topography (station elevation) is found to be weak, with negligible Spearman correlation coefficients (ranging from 0 to 0.19 for T=1 year, and -0.14 to -0.02 for T=10 years).
• The ARFs for the longest durations (18 and 24 hours) occasionally show lower values compared to 6-hour and 12-hour durations, deviating from the general trend of increasing ARF with duration.

Contributions

• This study is the first to estimate ARFs for sub-hourly durations using a dense rain gauge network like the WegenerNet, providing unprecedented spatial and temporal resolution.
• It offers novel insights into the behavior of ARFs, particularly the observation that for sub-hourly durations, ARFs can partially increase with the return period, challenging established literature.
• The research demonstrates the critical need for applying ARFs even for very small areas (1 square kilometer), questioning the validity of assuming equivalence between point and areal precipitation for small catchments.
• The high network density of the WegenerNet minimizes the risk of missing storm centers, leading to more robust and accurate ARF estimations.

Funding

• This open-access publication was funded by Technische Universität Braunschweig.

Citation

@article{Goshtasbpour2026Areal,
  author = {Goshtasbpour, Golbarg and Alpers, Jannick and Schröter, Kai and Müller‐Thomy, Hannes},
  title = {Areal reduction factors from high-resolution rain gauges in Austria},
  journal = {Natural hazards and earth system sciences},
  year = {2026},
  doi = {10.5194/nhess-26-5057-2026},
  url = {https://doi.org/10.5194/nhess-26-5057-2026}
}