This section briefly describes the EFAS RRA for full details, see, which has been used to estimate the fluvial component of the flood impacts. The method consists of three steps (see also Table ).

Hazard estimation: real-time discharge observations and NWP forecasts are used as input to the LISFLOOD hydrological model. Every 6 h, the model simulates the streamflow over the European drainage network in 5 km resolution.

After this general description of EFAS RRA, we focus now on the particularities of the application of the method in this study. We have decided to substitute the LISFLOOD discharge simulations (step 1 of the method) with the discharges measured by the stream gauges of the Hydrographic Confederation of the Segura . The reasoning behind this decision is described in the following: for the Segura, recent long-term validation of LISFLOOD shows one of the lowest performance scores of all European catchments . This has been attributed to two main sources: firstly, due to the high degree of flow regulation by dams and other hydraulic structures in the Segura Basin (see Sect. ). The rules on which dam operators base their release decisions are typically unknown e.g., usually hindering an adequate representation of the effects of dams in hydrological models. Secondly, large parts of south-east Spain, and in particular the Segura Basin, are situated on a highly karstic topography , in which hydrological simulations generally show high uncertainties . To substitute the LISFLOOD simulations for the DANA event, we have included discharge data from eight stream gauges along the Segura River (Fig. ). The discharge observations have been connected to step 2 of the method as follows: in each river reach, the flood map that corresponded most closely to the measured peak flow has been selected from the set of EFAS flood maps (Fig. ). The resulting mosaic of flood maps represents the maximum of simulated flood extents and depths over the full event duration (11–14 September 2019). To simulate the corresponding flood impacts, the maximum flood depths have been combined with exposure and vulnerability layers. In this last step, the default configuration of EFAS RRA has been applied (see step 3 of the method); however, the impact aggregation has been done at the level of municipalities to enable a more detailed analysis (Table ).

The ReAFFIRM method for full details, see has been used in this study to estimate the flash flood-induced impacts of the DANA event. The method assesses impacts originating from streams with catchment areas between 5 and 2000 km2. Similarly to EFAS RRA, also ReAFFIRM consists of three main steps (Table ):

Firstly, a flash flood hazard module the ERICHA system; uses weather radar observations to estimate the hazard return periods over a gridded drainage network.

Initially applied and tested in Catalonia north-east Spain;, the ReAFFIRM method has now been applied to the hydrographic demarcations (hereafter referred to as basins) of the Jucar and Segura rivers, covering an overall area of almost 62 000 km2 (Fig. ). More than 92 % of economic losses from the DANA event occurred within this domain . The configuration of ReAFFIRM and the datasets used in this region are described in the following (see also Table ).

i.

Rainfall inputs: as rainfall inputs for step 1 of ReAFFIRM, we have used the radar composites from OPERA (Operational Program for the Exchange of weather RAdar information; https://www.eumetnet.eu/opera, last access: 1 February 2022), which are produced in real time and have resolutions of 2 km and 15 min. Although improved by a chain of real-time adjustment algorithms (; see also ), the OPERA rainfall products significantly underestimated the observed rainfall during the DANA event (Fig. a, c). To reduce the bias in the rainfall inputs for the analysis of the event, we have applied the radar–raingauge blending technique proposed by (see also ), using the raingauge measurements of the Spanish State Meteorological Agency (AEMET) with an hourly time step as also done by. The resulting improved rainfall estimates are shown in Fig. b, d. It can be seen that the largest rainfall amounts were observed near the severely affected towns of Orihuela, Los Alcazares, and Ontinyent.

As discussed in detail by , the most pronounced sources of uncertainty affecting the ReAFFIRM impact estimates are the qualities of the employed rainfall inputs and flood maps. Additional important uncertainty sources include the purely rainfall-based hazard estimation and the vulnerability datasets used for translating flood hazard into socio-economic impacts. To account for some of the uncertainties, the default configuration of ReAFFIRM simulates the lower and upper bounds of flood extents and impacts. Throughout this paper, the illustrations of the simulated flood extents in the maps refer to the upper bound of flood extents. The impact estimates listed in the result tables represent the mean values of the lower and upper bounds.

To generate the compound flood impact estimates, the proposed approach combines the impacts of fluvial floods with those of flash floods, estimated by EFAS RRA and ReAFFIRM respectively. Fluvial floods and flash floods mostly occur in different parts of the stream network (fluvial floods in large rivers and flash floods in smaller streams). Hence, for this particular combination of flood types, the compound flood impacts have been approximated as the sum of impacts of the individual flood types. However, to avoid biases, the following consideration has been made: EFAS RRA estimates the impacts in rivers with catchment areas larger than 500 km2, while ReAFFIRM focuses on smaller catchments of 5–2000 km2 size (Table ). This means that in streams with catchment areas of 500–2000 km2, both fluvial and flash flood impacts can be detected at the same time. Moreover, at confluences of large rivers and small tributaries, the simulated impacts of the two methods can overlap. To avoid a systematic overestimation of impacts, we have decided to select in such situations the results from EFAS RRA, since past studies have shown that the impact estimates of ReAFFIRM are subject to increased uncertainties near large rivers .

This decision has enabled a straightforward combination of the two impact assessments: wherever EFAS RRA detects (fluvial) flood extents, the (flash) flood extents and impacts simulated by ReAFFIRM are automatically removed. Then, the (unchanged) fluvial and the (cropped) flash flood extents and impact estimates are instantly merged, resulting in a continuous coverage for catchments larger than 5 km2.

Finally – as also done in the two individual methods (Table ) – the resulting compound flood extents and impacts are aggregated at the level of municipalities.

This section presents the simulated fluvial flood impacts of the DANA event. Using the stream gauge data along the Segura River as input (Sect. ), EFAS RRA estimated the fluvial flood extents shown in blue and purple in Fig. : in the upstream part of the Segura, several minor inundations near the river were identified. In contrast, in the lowlands downstream of the City of Murcia, the simulated fluvial flood extents cover vast areas, reaching up to about 8 km from the river channel (Fig. ). This general image corresponds well to the situation described by the authorities after the DANA event: the Segura overwhelmed the flood protection structures in several locations downstream of Murcia and widely inundated the flat terrain .

Based on the simulated fluvial flood extents and depths (Fig. ), EFAS RRA identified 28 381 people and 17 CI in flooded areas, and economic losses of EUR 422.8 million (Table ). These simulated fluvial flood impacts correspond relatively well to the reported overall impacts (although the reported numbers also include impacts induced by other flood types, which EFAS RRA is not designed to detect). The simulated impacts are distributed over 36 municipalities along the Segura River, of which 10 are listed in Table .

In many of the municipalities flooded by the Segura downstream of Murcia, the quantitative impact estimates are approximately in line with the reported impacts (e.g. in Almoradi, Dolores, Beniel, and Santomera; Table ). However, the impacts were clearly underestimated in the most severely affected municipality of Orihuela, since a significant part of the impacts in this location were caused by flash floods in small tributaries of the Segura (e.g. the two fatalities listed in Table ). Similarly as in Orihuela, flash floods in small tributaries were also responsible for a large share of the impacts in Molina de Segura, explaining the impact underestimation by EFAS RRA in this municipality (Table ). In contrast, the impacts in the municipality of Murcia have been significantly overestimated (Table ): fluvial flooding was reported in the rural areas upstream and downstream of the City of Murcia, but not in the city centre, where local flood protection infrastructure prevented the Segura from flooding urban areas . Since the pan-European flood maps used by EFAS RRA do not account for such local defence structures , the flood extents in the City of Murcia and the corresponding impacts were significantly overestimated (Fig.  and Table ). Similar effects leading to overestimated impacts were observed in Cieza and Blanca in the upstream part of the Segura, and in San Fulgencio close to the river mouth (Fig.  and Table ).

This section presents the flash flood impacts simulated by ReAFFIRM for the DANA event. In the Segura and Jucar basins, ReAFFIRM estimated 43 091 people in flooded areas, EUR 290.2 million in economic losses, and 16 affected CI (Table ). The impacts are spread over a total of 100 municipalities, indicated in Fig. a in red (flood affecting population; 41 municipalities), orange (flood causing economic losses but not affecting population; 38 municipalities), and yellow (flood not affecting population or assets; 31 municipalities). A first visual inspection reveals that the locations of the simulated impacts (Fig. a) correspond very well to those of the reported impacts (Fig. b): simulated impacts appear in most of the municipalities where people were rescued or evacuated. Furthermore, ReAFFIRM identified impacts in the two municipalities with flash flood-related fatalities (Table ), although the signal is small in Caudete, where two persons died in their vehicle on a flooded country road .

ReAFFIRM detected the most significant flash flood impacts in the three parts of the domain indicated by the dashed boxes in Fig. a and shown more closely in Fig. :

In the Jucar Basin, ReAFFIRM detected significant impacts along the Cañoles and Clariano rivers (Fig. a). For the Cañoles River in Almansa (365 km2), the ERICHA system estimated a return period of T = 100 years, resulting in somewhat overestimated impacts in this municipality (Table ). The real flood peak in Almansa was probably lowered by an upstream dam not taken into account by the ERICHA system (see the dam’s location in Fig. a). Further downstream, in Mogente (862 km2), the return period of T = 100 years seems to be in line with the observed flood magnitude , and the relatively low simulated economic losses of EUR 1.1 million in this rural municipality correspond well to the insured losses (Table ). Although the town of Ontinyent (Fig. a) experienced unprecedented flooding from the Clariano River , the estimated return period in this location is only 5–10 years. This low hazard estimate stems from a rainfall underestimation in the small catchment (160 km2): a few kilometres upstream of the town, three local raingauges recorded 333–344 mm on the day of the flood 12 September 2019;, whereas for the same day the radar (blended with the national raingauges) estimated only 233–250 mm in the raingauge locations. This rainfall underestimation propagated down to the impact estimates in Ontinyent (Table ).

Around the town of Orihuela (Fig. b), the ERICHA system estimated return periods of up to T = 500 years in the small tributaries of the Segura River, resulting in significant flood extents simulated by ReAFFIRM (Fig. ). These results seem to be in line with the reported fatalities and evacuations along these tributaries (Fig. b). The overall impacts, however, seem to be underestimated in Orihuela (Table ). This is due to the fact that – in addition to the flash floods – exceptional fluvial flooding from the Segura River also affected the municipality Sect. ;. For the Segura River itself, no flash flood hazard has been estimated, since the catchment area of around 15 000 km2 at Orihuela is far above the limit of the ERICHA system (2000 km2, see Sect. ).

Severe flash floods also affected the south-eastern part of the domain (Fig. c). The ERICHA system identified return periods of T = 100–200 years in the municipalities of Torre-Pacheco, Los Alcazares, and S. Javier, in ephemeral streams with flat catchments areas in the order of 10–100 km2 (Fig. a). These high return periods correspond well to the exceptional reported impacts in these three municipalities (Table ). In Los Alcazares and Torre-Pacheco (Fig. c), the flood extents during the event were recorded by the satellite-based Copernicus Rapid Mapping Service Fig. b. The satellite image in Torre-Pacheco dates from the morning of 13 September 2019 (i.e. only a few hours after the flood peak). It can be seen that the simulated flood extent in the south-eastern part of the municipality corresponds reasonably well to the recorded flood extent (Fig. b). In the rural lands west of the town, ReAFFIRM underestimated the flood extents since the employed flood maps did not include the small streams in this area (Fig. a). Overall, however, the simulated impacts in Torre-Pacheco correspond well to those reported (Table ). Also in Los Alcazares, the simulated and observed flood extents are similar (Fig. b). However, the flood extent observed at this location was obtained from satellite observations taken 5 days after the peak of the event, suggesting that the real flood extent in Los Alcazares was significantly larger than recorded. Furthermore, civil protection authorities reported that in reality the municipality of Los Alcazares was inundated in its entirety . This means that ReAFFIRM underestimated the flood extent in Los Alcazares (and thus the impacts; Table ). One reason for this underestimation is that – similarly as in Torre-Pacheco – the employed flood maps cover only part of the municipality (Fig. a). In flat areas such as this part of the domain, the flood maps are subject to high uncertainties due to the increased complexity of the underlying hydraulic simulations. Similar uncertainties in flat terrain also appeared further south in the municipality of Cartagena (Fig. c), where the flood maps of T = 50 years show widespread flooding along a few streams. This resulted in overestimated flood extents and impacts in the city centre of Cartagena and in a few smaller towns upstream that, in reality, suffered lower impact (Table ).

To estimate the compound flood extents and impacts, the simulation results of EFAS RRA (Sect. ) and ReAFFIRM (Sect. ) have been combined by following the simple procedure described in Sect. . The resulting compound flood extents in the areas along the Segura River are illustrated in Fig.  in red and blue. Also in this part of the domain, we have compared the simulated flood extents to satellite observations from the Copernicus Rapid Mapping Service Fig. . The satellite image in this location dates from 14 September 2019 17:52 UTC, i.e. about 30 h after the measured discharge peak in the Segura passed the most severely affected town of Orihuela . Even though the flood had already mostly receded at that time, many areas located several kilometres from the Segura still appear inundated in the satellite observations. The locations of these flooded patches indicate how far the water from the Segura must have reached during the peak of the event. From around Beniel to the river mouth, the flooded patches line up relatively well with the outlines of the simulated flood extents (Fig. ), which indicates a good general correspondence between the simulated and the real flood extents in the downstream part of the Segura (although the flood extents were somewhat underestimated north-east of Dolores). The inundated areas north-west of Dolores (Fig. ) originated not from the Segura but from a small tributary catchment and were correctly identified by ReAFFIRM (see the estimated return periods of T = 10–50 years in this location in Fig. b). Along the other tributaries of the Segura affected by flash floods, the inundations had already fully receded at the time of the satellite image acquisition. For instance, the flood peak in the stream north of Orihuela, where the two fatalities occurred (Fig. b), was observed on 13 September 2019 at 08:15 UTC , about 34 h before the satellite image was recorded (Fig. ). At 08:00 UTC, ERICHA detected a return period of T = 500 years in this stream (Fig. b), indicating a good timing of the hazard signal in this location.

Over the two analysed river basins, the combination of EFAS RRA and ReAFFIRM identified 70 278 people and 31 CI located in flooded areas, and EUR 668.9 million in economic losses (Table ). These numbers correspond relatively well to the reported impacts over the domain (Table ). When analysing the results at the municipality level, the uncertainties affecting the compound impact estimates are more apparent (see, e.g. the large differences between simulated and insured economic losses in Los Alcazares or Murcia in Table ).

To clearly illustrate the complementarity of the two methods, we have analysed in detail the 15 municipalities with more than EUR 10 million in either simulated or insured losses (Fig. ). As expected, EFAS RRA detected the losses induced by fluvial flooding along the Segura River (e.g. in Almoradi, Dolores, Beniel, or Cieza), whereas ReAFFIRM identified the losses in the municipalities that experienced flash floods (e.g. Los Alcazares, San Javier, Torre-Pacheco, Cartagena, or Almansa; Fig. ). While the individual impact assessments of the two methods detected only the impacts induced by the specific flood type they are designed for, the compound impact estimation identified significant losses in all of the severely affected municipalities in the domain (Fig. ). In other words, the false negatives (misses) in these 15 municipalities have been reduced to zero through the combination of the two methods. However, the false positives (false alarms) caused by the uncertainties in the individual methods cascaded down to the compound impact estimates. For instance, the significant overestimations of losses caused by EFAS RRA in Blanca (see Sect. ) and by ReAFFIRM in Almansa (see Sect. ) also appear in the simulated compound losses (Fig. ).

Significant losses were simulated by both methods in only 3 of the 15 analysed municipalities (Orihuela, Murcia, and Santomera; Fig. ). We have been able to confirm that these three municipalities were indeed affected by both fluvial and flash floods. In Orihuela, the estimated compound losses are lower than the real losses, since the combination of the methods identified only inundations of the agricultural lands and settlements surrounding the town but not in the severely affected town centre Fig. . One reason for the flood extent underestimation in this location might by the high uncertainty of the EFAS flood maps in urban areas due to limitations of the underlying elevation data see. In the municipality of Murcia, EFAS RRA significantly overestimated the fluvial flood impacts (see Sect. ) and this overestimation propagated down to the compound impact estimates (Fig. ). Also ReAFFIRM identified impacts in Murcia (Fig. ) and it could be verified that flash floods occurred in some of the estimated impact locations . The impacts in Santomera were mostly induced by fluvial flooding from the Segura River, as correctly identified by EFAS RRA (Fig. ). The flash flood impacts in this municipality were overestimated, since the real discharge peak in the affected tributary was significantly lowered by a dam not taken into account by the ERICHA system (see the location of the dam in Fig. b). The dam’s buffer capacity prevented a catastrophic flash flood in this tributary , but the critically high storage level required the evacuation of more than 2200 people in the town of Santomera, situated between the dam and the confluence with the Segura River Fig. b.

To also evaluate the simulated impacts from a quantitative perspective, we have conducted a correlation analysis of the loss estimates with the insurance claim database over the 907 municipalities in the domain (Table ). The Spearman (ρ) and Kendall (τ) rank correlation coefficients have been used to avoid an overly strong penalisation by large differences in single data points as, e.g. in the Pearson correlation; see. These two coefficients measure to what degree the relationship between two datasets is monotonic, i.e. how well one variable can be expressed as a monotonic function of the other. Intuitively, values of ρ or τ = 1 correspond to a perfect correlation, whereas values of ρ or τ = 0 mean that the datasets are uncorrelated . Values of ρ > 0.5 are commonly considered to show a strong correlation see, e.g. and values of ρ are generally higher than those of τ see, e.g.. The results of ρ and τ computed for our analysis show a moderate but significant correlation of the separate loss estimates from EFAS RRA and ReAFFIRM with the insurance claim database (Table ). Furthermore, the correlation of the compound loss estimates with the insured losses is stronger than for those generated by the separate two methods (Table ). This illustrates how the integration of EFAS RRA and ReAFFIRM into one compound flood impact estimation has improved the agreement between the simulated loss estimates and the reported insured losses.