Comparison of precipitation interpolation methods applied to the October 2024 extreme rainfall event in Valencia (Spain)

The Mediterranean region is characterised for its strong spatial and temporal variability in precipitation extremes. Such a variability, driven by complex atmospheric dynamics and intensified by climate change, results in a high exposure to natural hazards, particularly short-duration and high-intensity convective precipitation that usually generate flash floods in small to medium catchments. These phenomena pose significant challenges for precipitation estimation, monitoring, and risk management, especially in highly urbanised or topographically complex catchments.

In October 2024, the eastern Mediterranean coast of Spain was impacted by an extreme meteorological cut-off low associated with intense convective instability and exceptional rainfall accumulations up to 778 mm in 24 h and 79.4 mm in 1 h. The El Poyo Ravine catchment (385 km²) was the most affected area with severe damage to infrastructure, 238 fatalities, 4 500 buildings and 120 000 vehicles. It highlighted the urgent need for reliable methodologies to characterise convective precipitation and support early warning and preventive actions.

This research aims to identify and assess suitable spatial interpolation methods of precipitation for high localised convective storms, with a focus on improving precipitation characterisation for future forecast and operational applications. Based on a comprehensive literature review, eight interpolation methods were selected, including deterministic, geostatistical, and mathematical-formulation-based approaches. Quantitative validation was performed using metrics such as RMSE and the Nash-Sutcliffe Efficiency coefficient between observations and estimates.

The October 2024 flood event in the El Poyo Ravine catchment was selected as case study. Rainfall fields at each time step were generated by using 15-min observations at 12 rain-gauging stations of the real-time system (SAIH) of the Júcar River Basin Authority. Rainfall fields were generated with the eight interpolation methods of precipitation considered in the study.15‑min rainfall observations at 9 rain-gauging stations of the crowdsourced ECOWITT network were used for validation purposes, after undergoing a strict quality control process to discard either poor data or inconsistent stations. The analysed period spanned from 28 October 2024 at 07:00 to 30 October 2024 at 06:45 with a 15‑minute temporal resolution, focusing on the most intense convective phases of the event. Validation considered only time steps with available observations at both networks.

Results show that accuracy and computational cost strongly depend on the interpolation method. While the Inverse Distance Weighting (IDW) method provides results close to observations with low computational cost, local ordinary kriging requires a much higher computational cost to achieve comparable performance. In addition, the mathematical-equation-based method is more suitable for small catchments, as accuracy increases with decreasing catchment size.

Although this exploratory research focuses on a single catchment and one extreme event, the proposed methodology provides a transferable framework that can be applied to additional events and independent datasets. Therefore, this work can contribute to improving precipitation estimation strategies for Mediterranean catchments and supporting more effective hazard monitoring and risk management.

Acknowledgment: This research was supported by INECO (Ingeniería y Economía del Transporte S.M.E. M.P., S.A.) through funding provided under the project ‘Application of stochastic methods for flood assessment in urban areas’. Ángela Zaragoza-Paredes would like to thank the Fundación José
Entrecanales Ibarra for its financial support through a PhD research grant.