ATMOSPHERIC SCIENCE LETTERSAtmos. Sci. Let. 9: 88–94 (2008)Published online 17 April 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/asl.179

EFAS forecasts for the March–April 2006 flood in theCzech part of the Elbe River Basin – a case study

Jalal Younis,1* Maria-Helena Ramos1,2 and Jutta Thielen1

1European Commission, DG Joint Research Centre, Institute for Environment and Sustainability, TP261, 21020 Ispra (Va), Italy2Cemagref, Parc de Tourvoie, BP 44, 92163 Antony Cedex, France

*Correspondence to:Jalal Younis, EuropeanCommission, DG Joint ResearchCentre, Institute for Environmentand Sustainability, TP261,21020 Ispra (Va), Italy.E-mail: jalal.younis@jrc.it

Received: 21 December 2007Revised: 5 March 2008Accepted: 5 March 2008

AbstractIn spring 2006, the Elbe River Basin was hit by severe snowmelt-driven floods. This articleinvestigates the performance of the European Flood Alert System (EFAS) forecasts forthis event in the Czech part of the Elbe River Basin. EFAS runs preoperationally at theEuropean Commission DG Joint Research Centre where it is developed and tested inclose collaboration with the National Hydrological Services. The analysis shows that EFASforecasts were able to detect a signal of probability of flooding up to 8–10 days in advance.The added value of ensemble forecasts to early flood warning is illustrated. Copyright 2008 Royal Meteorological Society

Keywords: floods; Elbe River; forecasting; warning; ensemble predictions

1. Introduction

The European Environmental Agency estimated thatbetween 1998 and 2002, floods in Europe caused about700 deaths and resulted in at least 25 billion Euros ininsured economic losses (EEA, 2003). Much of theselosses can be attributed to the historic Elbe floods inAugust 2002 that affected both the Czech Republicand Germany. In the Czech part of the Elbe RiverBasin, the analysis of extreme flows showed that the2002 flood occurred after a long period (1890–2002)without severe floods (Brazdil et al., 2005; Yiou et al.,2006). Compared with the estimated return periods,in both Vltava and Elbe River Basins up to 100-year flood return periods have been recorded. Highreturn periods (N) were observed in some right-hand tributaries of Vltava, particularly, in Luznice atBechyne stream gauge (N50 years), at Klenovice station(N50–100 years), in Sazava tributary at Nespeky streamgauge (N20–50 years), while in left-hand tributaries ofVltava River the peaks did not reach even a 5-yearreturn period. Due to the Vltava reservoir cascade theflood peak at Prague (Praha-Mala Chuchle gaugingstation) reached almost a 10-year return period. Insome tributaries of Upper Elbe the flood peak reachedup to a 50-year return period, although in somevery upstream tributaries the peak reached up to a100-year flood return period. In the main Elbe, theriver peak discharge did not exceed a 20-year returnperiod, for example, at Nymburk (N10–20 years), Melnik(N5–10 years), Usti nad Labem (N5–10 years), and atDecin (N10 years).

In order to increase preparedness for large floodevents with transnational impact in Europe, theEuropean Flood Alert System (EFAS) activity was

launched at the DG Joint Research Centre of the Euro-pean Commission in 2003 (De Roo et al., 2003; Thie-len et al., 2008; Bartholmes et al., 2008; http://efas.jrc.it/) based on previous research experience (Gouwe-leeuw et al., 2004 and Gouweleeuw et al., 2005).EFAS provides early flood warning products based onseveral medium-range weather forecasts including afull set of Ensemble Prediction System (EPS) (Ramoset al., 2007). The information exchange between theEFAS and the hydrological services on the system’sdevelopment and results takes place within the frame-work of a unique network for operational flood fore-casting across Europe, consisting of National floodforecasting centers and meteorological data providers.At present, 24 hydrological services responsible forthe major transnational river basins in Europe are partof the network.

This article investigates the global performance ofEFAS forecasts for the Elbe 2006 flood event inthe Czech Republic. The study area and the mainfeatures of the flood event are presented in Section 2.Section 3 presents the EFAS setup and flood forecasts.Conclusions are drawn in Section 4.

2. The 2006 flood event in the Czech partof the Elbe River Basin

2.1. The Elbe River Basin

The Elbe River is a transnational river mainly sharedbetween the Czech Republic and Germany. The Czechpart of the Elbe River Basin covers about two-thirdsof the whole Czech Republic, with a total area of51 413 km2, and three main subcatchments (Figure 1):Vltava (28 101 km2), Upper Elbe (13 712 km2) andOhre/Eger (5588 km2).

Copyright 2008 Royal Meteorological Society

EFAS forecasts during the 2006 Elbe flood 89

Figure 1. Overview of the Elbe catchment and a zoom on the Czech part of the Elbe River Basin. The Usti nad Labem (location2) and the Praha-Mala Chuchle (location 1) gauging stations are also indicated.

The Vltava River is one of the major tributariesof the Elbe River (Figure 1). Its hydrologic regimeis affected by a system of eight reservoirs (the ‘Vltavacascade’) located in a chain from the upstream part ofthe catchment up to the vicinity of upstream Prague.Their predominant use is for power generation and, asfor flood control and mitigation, they can only copewith smaller and more frequent floods.

2.2. The March–April 2006 flood event

The spring 2006 flood was essentially driven bysnowmelt combined with precipitation. Temperaturesstarted rising from 20 March onwards and meantemperatures above 10 ◦C were observed from 24 to25 March onwards. Maximum temperatures were ashigh as 20 ◦C on 27 March. The rapid increasing oftemperatures at the end of March caused the melting ofthe snow accumulated during winter. In general, snow-pack layers up to 20–25 cm of snow that was presentduring the week from 13 to 20 March in the catchmentwere completely melted by 27 March. From 25 Marchto 5 April, rainfall was also observed, particularly inthe southern and eastern parts of the catchment, withpeak intensities reaching up to 20–30 mm/24 h on 28and 29 March (see CHMI, Czech HydrometeorologicalInstitute, 2006a,b for more details).

During the event, flood alerts were issued from thenational authorities along the Elbe and Vltava Riversand their tributaries (Table I). Upstream Prague, at

Praha-Mala Chuchle station (location 1, Figure 1), astate of emergency (second level of warning) wasreached at the end of 28 March and was maintained upto 7 April. The peak discharge was slightly below thethird warning level (state of danger). This threshold(1500 m3/s) was not exceeded. At Usti nad Labemstation on the main Elbe, downstream from Prague(location 2, Figure 1), water levels were higher thanthe second warning level from 28 March to 21 Apriland exceeded 700 m3/s to the third warning levelfrom 28 March to 10 April. Observed discharges(reproduced in Figure 4) show water levels startingto rise on 26 and 27 March, reaching a peak between1 and 4 April, depending on location: approximately1400 m3/s upstream at Praha-Mala Chuchle on the 1/2April, and 2500 m3/s downstream at Usti nad Labemon the 3/4 April. Estimates of return periods associatedto peak discharge values indicated a 2–5-year floodperiod at Praha-Mala Chuchle station and a 5–10-yearflood period at Usti nad Labem station (CHMI, CzechHydrometeorological Institute, 2006a,b).

3. EFAS medium-range flood forecasts

3.1. Overview of EFAS setup

The LISFLOOD hydrological model is the core of theforecasting system. The model, designed for large riverbasins (De Roo et al., 2000), is a hybrid between a

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90 J. Younis, M.-H. Ramos and J. Thielen

Table I. Characteristics of the observed flood at Praha-MalaChuchle and Usti nad Labem stations in the Vltava River/Elbecatchment for the March–April 2006 flood event

Praha-Malastation

Usti nad Labemstation

Peak discharge and dateof peak

1430 m3/s on1 April 2006

2540 m3/s on3 April 2006

Estimated return periodof peak discharge

2–5 years 5–10 years

Second local warninglevel (state of emergency)and time period underwarning

1000 m3/s—28 March to7 April

1080 m3/s—28 March to21 April

Third local warning level(state of danger) and timeperiod under warning

1500 m3/s wasnot exceeded

1310 m3/s—28 March to 10April

physical and a conceptual rainfall-runoff model. In theEFAS, LISFLOOD is set up with a spatial grid spacingof 5 km throughout Europe. Initial conditions at theonset of the forecasts are obtained from simulationswith observed meteorological data at a daily time step.Model options such as reservoirs, lakes, dyke breaks,or backwater effects are not activated due to lackof data. Some input parameters are estimated fromphysical properties such as soil depth and texture,vegetation, and land use, while others are estimatedby calibration against measured stream flows (Van DerKnijff and De Roo, 2006).

EFAS is driven with weather forecasts from the Ger-man National Weather Service (DWD) and the Euro-pean Centre for Medium Range Weather Forecasting(ECMWF). Both centers issue weather forecasts twicea day (00 UTC and 12 UTC):

• DWD: 7-day deterministic meteorological forecastwith 7-km grid spacing for the first 3 days offorecast and 40 km for the remaining 5 days.

• ECMWF: 10-day deterministic meteorological fore-cast with 40-km grid spacing.

• ECMWF-EPS: 10-day EPS from ECMWF with 51members and 80-km grid spacing.

Weather forecasts are fed through LISFLOOD asindividual forecasts and the outputs are comparedagainst critical flood thresholds, which were deter-mined for each river pixel from a 14-year (1991–2004)daily run of a LISFLOOD model based on observedmeteorological data (Thielen et al., 2006). Four criti-cal thresholds are considered: Severe (very high pos-sibility of flooding, potentially severe); High (highpossibility of flooding, bankful conditions or higherexpected); Medium (water levels high but no floodingexpected); and Low (water levels higher than normalbut no flooding expected). The threshold defining theSevere level corresponds to the highest simulated dis-charge in the 14-year daily run and therefore reflectsthe extreme of the climatology of this period. Highalert levels of the EFAS are defined by the 99th per-centile of the ranked simulated daily discharges (i.e.1% of the simulated discharges are greater than the

threshold). This threshold corresponds approximatelyto bankful conditions or to a 1–2-year return perioddischarge.

For consistency, the same model version (i.e. thesame parameter setup) used to define the criticalthresholds is used for EFAS forecasts. Flood forecastsbased on deterministic and EPS weather forecasts arecombined and the ensemble forecasts are summarizedin maps and temporal diagrams of forecasted thresholdexceedances (Ramos et al., 2007). For more details,see also, http://efas.jrc.it/.

3.2. EFAS forecasts of critical thresholdexceedances

Figure 2 shows maps of exceedances of EFAS criticalthresholds forecasted on 24 March for the period 30March 00 : 00 to 31 March 00 : 00 (i.e. lead timeof 6–7 days) based on the deterministic (DWD andECMWF) and probabilistic (ECMWF-EPS) weatherforecasts. For EFAS forecasts based on DWD (top)and ECMWF (middle) deterministic forecasts, riverreaches where discharges are predicted to exceed theEFAS Low level are shown in green, Medium levelin yellow, High level in red, and Severe level inpink. EFAS maps of threshold exceedances based onEPS (bottom) show the number of simulations (out of51 members) forecasting discharges exceeding EFASHigh level (i.e. the equivalent of red colors in thedeterministic maps).

It can be seen that exceedances of EFAS Severethresholds were not forecasted. DWD-based fore-casts show EFAS Medium alert levels exceeded inshort reaches of tributaries of the Vltava River. Theexceedance of EFAS High threshold was forecastedin the main Vltava and tributaries by the simulationsbased on ECMWF deterministic weather forecast. Forthe Elbe River, the exceedance of High levels is pre-dicted in upper parts only while in the main river,before and after the confluence with the Vltava River,Low to Medium levels are predicted.

Results based on DWD or ECMWF deterministicforecasts show categorical single solutions, whereasEPS-based forecasts allow quantifying the uncertaintyof the meteorological forecasts on the possibility ofdischarges greater than a given critical threshold.Figure 2 (bottom) shows EFAS forecasts based onECMWF-EPS. They indicate what the possibility ofexceeding the EFAS High threshold is according tothe different weather forecasts: upstream from Praguemore than 41 scenarios (out of 51) indicate theexceedance of the threshold, while downstream fromPrague this number is much lower.

The added value of the EPS-based forecasts ismore clearly illustrated in the temporal evolution ofthe forecasts shown in Figure 3 for the Praha-MalaChuchle location (location 1, Figure 1) from 20 March(2006032000) to 08 April 2006 (2006040800). For thesimulations based on the deterministic forecasts, it isshown in sequential boxes with each box representing

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EFAS forecasts during the 2006 Elbe flood 91

Figure 2. EFAS flood threshold exceedance maps of 24 March00 : 00 applying for 30 March 00 : 00 to 31 March 00 : 00(i.e. lead time of 6–7 days) based on DWD deterministicweather forecast (top), ECMWF (middle), and ECMWF-EPSdata (bottom).

24 h of lead time; the EFAS threshold exceeded ateach forecast (dates in rows). For purposes of sim-plicity, we only represent the 00 : 00 UTC forecasts.The dates for which each forecast applies are shown inthe top of each diagram (columns). In the EPS-basedforecasts, the temporal box diagrams show the num-ber of simulations above the EFAS High alert level(EPS > HAL).

Figure 3 shows that even if High levels were notpersistently simulated for 29–30 March onwards by

the deterministic forecasts (red boxes in the DWD andECMWF diagrams), in the earlier forecasts (startingon 20–21 March for the 10-day ECMWF forecasts,and on 24–25 March for the 7-day DWD forecasts)there was already a signal of a probability of reachingHigh levels in the EPS-based simulations: Already on21 March, 8–17 EPS out of 51 simulations predictedexceedance of high levels for the 29–30 March for thePraha-Mala Chuchle Location.

The signal in EPS-based forecasts was persistent,with increasing probabilities as the forecast date gotcloser to the event. More than 25 simulations exceed-ing the EFAS High thresholds were already forecasted7–8 days in advance. For the same threshold, per-sistence in the deterministic weather forecasts wasachieved only from 29 March onwards, when the eventwas already ongoing or too close to occur.

3.3. EFAS forecasts of flood hydrographs

Since the effects of reservoirs and dykes are not rep-resented in the hydrological model, forecasted dis-charges cannot be directly compared to observed ones.Instead, proxies for observed discharges are used.They correspond to discharges simulated with thesame hydrological model, setup, and parameteriza-tion, but using observed meteorological data as input.These data are collected from European synoptic sta-tions (http://agrifish.jrc.it) and interpolated to the 5-kmgrid resolution of the LISFLOOD model. The proxyresults for observed discharges are then compared tothe EFAS forecasts.

Figure 4 shows the flood hydrographs forecasted bythe EFAS for the simulations based on ECMWF-EPSweather forecasts from 20 March to 23 April 2006and for the two locations indicated in Figure 1. Aconcise visualization is adopted, where the median(50% quantile) from the 51 simulations is plotted, withbars representing the 10 and 90% quantiles. One cansee that the time periods at which the forecasts (lines)and the proxies (dots) show discharges exceeding thecritical EFAS High threshold agree well for bothlocations. In the EFAS forecasts based on deterministicweather forecasts (not shown), the peaks could onlybe well forecasted at shorter lead times, when theforecasted discharges are strongly influenced by theinitial conditions at the onset of the forecasts, and thus,forecasted values are closer to the proxy for observeddischarges. However, the EFAS simulations based onECMWF-EPS could better reproduce the pattern of thehydrographs and forecast, in advance, the increasingof discharges and their peaks.

It is also interesting to look at the magnitude ofthe discrepancies between the forecasted dischargesand the measurements from the gauging stations.When EFAS forecasts and proxies match relativelywell, these differences are expected to be mainly,although not exclusively, related to the hydrologi-cal modeling (physical representation of the relevant

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92 J. Younis, M.-H. Ramos and J. Thielen

Figure 3. History of EFAS forecasted levels at Praha-Mala Chuchle in the Vltava River for forecast dates from 20 March 00 : 00 to08 April 00 : 00 (rows). EFAS forecasts are based on DWD deterministic weather forecast (top), ECMWF deterministic weatherforecast (middle), and ECMWFEPS probabilistic weather forecast (bottom). The dates for which the forecasts apply are shown onthe top of each diagram (columns); each box corresponds to a 24-h lead time. For DWD and EUD, the boxes show the EFASthreshold exceeded by forecasted discharges. For EPS-based forecasts, the number of EPS-based simulations above EFAS Highlevel (EPS > HAL) is shown.

processes and/or coarse resolution in model calibra-tion and simulation). In Figure 4, discharges observedat the gauging stations were also plotted (squares).We can see that differences of 300–800 m3/s can befound between the forecasted peaks and the observedpeaks.

There are several factors that could be responsiblefor this, for instance the small amount of input snowavailable to the rainfall-runoff model for melting, theeffect of coarse resolution (5 km) and/or poor cali-brated hydrological modeling; also the observationsare strongly influenced by the Vltava River reservoir

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EFAS forecasts during the 2006 Elbe flood 93

Figure 4. EFAS forecasted discharges based on ECMWF-EPS midnight 10-day weather forecasts (thin lines), EFAS simulateddischarges with observed meteorological data (dots), national observed discharges (squares), and LISFLOOD simulations with5-km resolution (triangles) and 1-km resolution (crosses) in the Vltava River at (a) Praha-Mala Chuchle station and (b) Usti nadLabem station on the main Elbe from 20 March to 23 April 2006. Thresholds defining EFAS HAL are indicated. Forecasteddischarges are represented by the median value (lines) and the 10 and 90% quantiles (dotted bars) estimated from 51 predictions.

cascade which is not yet taken into account in EFAS.A preliminary investigation on the effect of reso-

lution and calibration was conducted by performingadditional simulations with LISFLOOD using higherresolution data for model calibration and setup, fol-lowing the works initiated by Gierk et al. (2004) andYounis et al. (2004) for the Elbe River. In Figure 4,we plotted:

1. EFAS–LISFLOOD 5-km simulations (triangles):based on revised input maps of river channel widthand gradient, land use and topography, as well as ona higher number of discharge data in an automaticcalibration procedure;

2. EFAS–LISFLOOD 1-km simulations (crosses):same as above, but with the hydrological modelsetup on a 1-km grid.

Both simulations were obtained using observedmeteorological data as input and a daily time step.

One can see that as resolution increases from 5 kmto 1 km, the simulated hydrographs get closer to

the observed ones. This is particularly true to the1-km calibrated model, which reproduces the peakdischarges reasonably well. However, we note that,even for the 1-km calibrated model, the timing ofthe flood peak is not well simulated: There is ageneral tendency of having a simulated peak 2 days inadvance, reflecting too fast a propagation of the floodwave. These differences can be due to the attenuationeffect of the cascade of reservoirs in the VltavaRiver. Further investigation is, however, needed on thehydrological calibration and modeling of transnationalEuropean river basins in the EFAS, which is out of thescope of this article.

4. Conclusions

In this study, preoperational EFAS results based on thecoarsely calibrated 5-km model are presented for theMarch–April 2006 flood event in the Vltava River.The analysis shows that the EFAS forecasts based on

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94 J. Younis, M.-H. Ramos and J. Thielen

weather ensembles were more capable in detectingan earlier and more persistent signal of probabilityof flooding than those based on deterministic weatherforecasts.

Although exceedances of the critical EFAS Highflood threshold were well forecasted for the coreperiod of the flood event (29 March to 8 April), theEFAS forecasts predicted lower discharge peaks andsimulated a faster flood propagation than the one thatwas actually measured by local discharge gauging sta-tions. When applying observed meteorological inputdata to a higher resolution hydrological modeling, withbetter calibrated parameters and improved input infor-mation on river channel and land use, a significantimprovement was observed in the simulations of theflood peak and volume.

Concerning the timing of flood peaks, even the high-resolution and well-calibrated simulations showed anadvanced time to peak of approximately 2 days. Theeffects of the cascade of reservoirs in the Vltava Riverupstream from Prague (river regulation structuresnot included in the simulations presented here) needto be closely investigated and could explain someof the differences between simulated and observedhydrographs.

Acknowledgements

The authors thank the data providers: DWD, ECMWF, and theInstitute for the Security of the Citizen Agrifish unit of theJRC. We also thank the Czech Hydrometeological Institute forthe observed discharge data and for their feedback on the floodevent. The staff from the Weather Driven Natural Hazard actionof the JRC that collaborated to this study is also acknowledged.Special thanks to Meike Gierk, JRC Detached National Expertfrom the Germany.

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