This analysis evaluated the skill of the ERIC flash flood products when compared against flash flood observations, the results are used to decide the criteria for issuing flash flood notifications. It was necessary to perform a new skill assessment for EFAS v5.0 to decide these criteria, rather than using the criteria from EFAS v4.1 because of the following changes:
- The new calibration of the LISFLOOD hydrological model which is used within the generation of the ERIC flash flood products
- The ERIC products are now calculated directly from the surface runoff predictions from LISFLOOD, previously precipitation was combined with LISFLOOD predictions of soil moisture to estimate surface runoff
The evaluation was performed for ERIC flash flood predictions between 1st January 2022 - 31st December 2022
Flash Flood Observations
Observations of flash floods during the evaluation period were obtained from three sources:
- EFAS partner feedback
- FloodList.com
- EFAS long term run (river discharge re-analysis)
EFAS Partner Feedback
EFAS partners are able to provide feedback in response to each flash flood notification email they receive, they can inform if the flash flood event did or did not occur. Therefore the feedback received during the evaluation period was selected and events where a flash flood did occur were chosen. 42 flash flood events were reported by EFAS partners during the evaluation period
FloodList.com
Flash flood events were obtained from an API collated by FloodList.com. The API was queried to return events which matched the following criteria:
- Occurred during the evaluation period
- Occurred within the EFAS domain
- Flood event type was recorded as either 'flash flood' or 'minor flood', the latter was included as this refers to flooding with a small spatial and temporal scale which is similar to flash flooding
Some of the extracted flash flood events were recorded as having a duration of several days, whereas normally flash flooding occurs within a 24 hour timescale. This longer duration could be because other types of flooding were recorded during the same event, for example longer lasting riverine flooding may have occurred later on further downstream. Uncertainty in the timing of the reported flash flood event could also explain the longer duration. In these cases it was decided to only record the first date on which the flash flooding was reported as this would coincide with the initial heavy rainfall which is often associated with most flash flooding impacts.
EFAS Long Term Run Re-Analysis
Flash flood observations were also derived from the EFAS v4 historical long term run, which is generated by forcing the LISFLOOD hydrological model with gridded 6-hourly meteorological observations. Flood events were identified at gauging stations with an upstream area between 500 - 2000 km2, this work was performed by the JRC. From these extracted flood events, flash flood events were defined as events where the duration of the 5 year return period exceedance was no greater than 1 day. Events with a longer duration were discarded as they were considered to be longer lasting riverine flood events. Note EFAS version 4 of the historical long term run was used rather than EFAS version 5 , not fully quality checked at the time of the analysis.
Combined Observations
Flash flood observations from each of the above three sources were combined into a single dataset. Events were aggregated onto the same administration regions which are used to issue flash flood notifications. If multiple events were reported in the same administration region on the same day, only one flash flood event was recorded, this reflects that in the same circumstances only one flash flood notification would be issued.
During the evaluation period, the majority of the flash events occur towards the end of 2022 (Fig. 1). The spatial distribution of the reported flash flood events shows that the greatest number of events occurred around the Mediterranean, including Italy and the Balkans (Fig. 2).
Figure 1. Monthly distribution of observed flash flood events.
Figure 2. Number of observed flash flood events per administration region.
ERIC Flash Flood Forecasts
Flash flood forecasts were generated at 00 UTC for each day during the evaluation period from the 1st January to the 31st December 2022. For each forecast, the LISFLOOD hydrological model was forced with the 20 member COSMO-LEPS meteorological forecast, the surface runoff output from the hydrological model was then used to calculate the ERIC surface runoff indicator and generate the forecast products shown on the EFAS website.