Q: What is EFAS-IS?
A: EFAS-IS stands for EFAS Information System and it is the map viewer that presents all the EFAS products.
Q: What is numerical weather prediction (NWP) model?
A: A mathematical model that simulates atmospheric, oceanic, and land processes to predict the weather based on current weather conditions.
Q: What is the hydrological model in EFAS?
A: EFAS uses OS-LISFLOOD hydrological model. OS-LISFLOOD is a physically based, spatially distributed hydrological rainfall-runoff-routing model specifically designed to be used in large scale catchments. Detailed information on its specific characteristics and example applications can be found in the online documentation. The LISFLOOD source code is available from GitHUB. In EFAS, OS LISFLOOD model set-up has 1 arcmin resolution (approximately 1.5 km) and six-hours temporal resolution. More information on EFAS model set-up and calibration is available here.
Q: Which are the meteorological forecast systems in EFAS?
A: A number of meteorological forecast systems are used to generate the hydrological future scenario. These forecasts are either deterministic or probabilistic (ensemble forecasting) and are generated by different providers, e.g. ECMWF and DWD, more information here.
Q: What is ensemble forecasting?
A: A number of simulations (members) using the same meteorological model, but varying the initial conditions and/or the tendencies. The ensemble spread should ideally reflect the model error and is often used to express the uncertainty of a forecast by associating a probability with different forecast values.
Q: What is medium-range forecast?
A: Forecasts produced for a lead time of several days (typically 3-15 days).
Q: How to deal with uncertainties in the forecasts?
A: The EFAS Map viewer contains a number of auxiliary products that allow an estimate of the associated uncertainties, such as the hydrological model performance, forecast skill, the initial conditions and the presence of reservoirs.
Q: What is medium-range forecast?
A: Forecasts produced for a lead time of several days (typically 3-15 days). In EFAS, medium range forecast have maximum lead time 10 days.
Q: What is lead time?
A: The lead time indicates how far into the future the forecast stretches.
Q: What is time step?
A: Time increments for each forecast. A time step of 6 hour means that the forecast has a value every 6 hours until the maximum forecast horizon.
Q: Are flash flood forecasts available every hour?
A: The flash flood forecasts used to issue notifications (ERIC reporting points) are updated twice a day and have a modelling time step of 6-hours. The experimental radar-based flash flood products, in contrast, are updated every hour.
Q: What is EFAS Water balance?
A: Comparison of the EFAS initial conditions against measured discharge.
O: Is the soil moisture content considered for the runoff estimates?
A: Yes, the total amount of soil moisture governs the infiltration capacity. Drier soils can take up more water than wetter soils, thus leading to more or less (surface) runoff.
Q: How to download EFAS data?
A: EFAS partners can download both real-time forecasts and historical data, for example via the CEMS Early Warning Data Store. There is also the possibility to request data via e.g. FTP. The CEMS-Flood Data User Guide provides detailed descriptions for EFAS data access.
Q: Can I integrate the EFAS forecasts into my own system?
A: Yes, EFAS partners can have access to the real-time forecasts via e.g. FTP or using Web Services, thus allowing the integration of the EFAS forecast into own local systems.
O: How are the EFAS thresholds calculated?
A: The modelled long-term discharge series (e.g. 1992-2022 in the case of EFASv5) is used to calculate return period thresholds for every point along the river network, which are used as criteria for issuing notifications. Specifically, EFAS discharge thresholds are the outcome of a statistical analysis based on the Gumbel extreme value distribution. The code is available open source, maps for selected threshold values for the latest operational version can be downloaded from https://confluence.ecmwf.int/display/CEMS/Auxiliary+Data (“EFAS flood thresholds”).
O: Can I retrieve the calculated return period values in EFAS for our hydrological station locations?
A: Yes, if your station is a fixed reporting point, EFAS discharge values for selected return periods are visualized in the pop-out window of the EFAS Map Viewer. Conversely, EFAS discharge thresholds for all pixels of the computational domain can be downloaded here (“EFAS flood thresholds”).
Q: Which threshold level is used for issuing a notification?
A: The criteria for EFAS Notifications are described in detail in the EFAS User Guide. As quick summary, EFAS Formal and Informal Flood Notification are sent when the 5-year return period is exceeded with > 50% total probability (> 40% for Informal Flood Notifications). An EFAS Flash Flood Notification is sent when the probability of exceeding the 5-year return period of the ERIC indicator is ≥ 30%.
O: Why are EFAS notifications not updated when they exceed a higher threshold level?
A: EFAS is expected to issue early warnings, which constitutes an additional source of information for expert users. Notifications are only sent once; after this first information, the EFAS partners are expected to follow the development of the situation on their own. This can be facilitated, if the EFAS forecast are displayed in the partner’s own system (see above).
O: How can I see the geographical area for which a notification is valid for?
A: The “Area of validity” of an EFAS Notification is defined as the upstream and downstream river stretch (same river name) that exceeds the total probability threshold shown in the flood probability layer. The Flood probability layer can be used together with the Major rivers layer to visualize the stretches above threshold. The EFAS Flash Flood Notifications are by definition valid for a larger administrative region stated in the notification email. The ERIC Affected Area layer can be used to see the river network contributing to the ERIC Reporting Point.
Q: What is post-processing in EFAS?
A: For points along the river network where an EFAS partner provides hydrological observations (both historical and real-time), a post-processed hydrograph is produced in addition to the reporting point information (accessed via a second tab in the reporting point pop-up window). The aim of the post-processing methodology is to adjust the CEMS-Flood medium-range ensemble forecasts at specific locations, to provide more accurate predictions of future observed river discharge values.
Q: How is the post-processed hydrograph calculated?
A: The post-processed forecast is represented by a probability distribution that is dependent on recent observations, simulation forced by observations (also known as the CEMS hydrological water balance), and forecasts. The post-processing is composed of two parts; the calibration (offline), and the forecast update (online). A detailed description of the method is found in the EFAS Wiki.
Q: How to include user thresholds in the post-processed discharge forecasts?
A: Since EFAS version 5, thresholds from data providers are included in the EFAS Post-processed Hydrograph following the suggestion from EFAS partners during a workshop at the 17th EFAS Annual Meeting. Up to four river discharge thresholds can be provided and are named in increasing order as TL1 (D) to TL4 (D). More details on how to provide hydrological data (including thresholds) to EFAS is available on the EFAS webpage.
Didn't find what you were looking for? Send us a message at info@efas.eu or go to the EFAS webpage and use the contact form.