Low river flow signal during Europe’s dry summer

Louise Arnal
Shaun Harrigan
David Lavers
Fredrik Wetterhall
Christel Prudhomme

 

The summer 2018 in Europe was remarkable from a hydroclimate perspective, with many regions experiencing persistent dry or drought conditions, a series of heatwaves with record-breaking temperatures and numerous wildfires. The European Flood Awareness System (EFAS) seasonal hydrological outlook, which is produced operationally at ECMWF as part of the Centre’s work for the Copernicus Emergency Management Service (CEMS), managed to capture the intensifying low-flow signal throughout the summer with lead times up to two months.

A developing drought

The summer’s extreme weather was caused by persistent blocking high-pressure systems over northern and central Europe, which were part of a summer variation of the North Atlantic Oscillation (NAO) pattern. These meteorological conditions had severe hydrological impacts. According to the European Drought Observatory (EDO), much of central and northern Europe was affected by drought, resulting in very dry soil and low river, groundwater and reservoir levels. A remarkable feature over this period was the growth in the spatial extent of low flows across the European river network. According to the daily EFAS discharge simulation, the low flow area covered about 10% of the extended EFAS domain river network at the beginning of June (primarily in southern Sweden and northern Africa) and increased to a maximum of 41% on 24 August. For details on the recently upgraded EFAS system used to produce the simulation, see the separate article on the EFAS upgrade in this Newsletter.

Low river flow prediction

The EFAS seasonal hydrological outlook issued on 1 July, valid from 1 July to the end of August, showed a low-flow signal in large parts of Europe up to two months in advance. The predictability of this event can be largely attributed to the dry initial hydrological conditions at the start of the forecast, carried over into the future through the land surface memory. The lack of predictability in western Europe is likely due to the wet anomaly predicted for August by ECMWF’s SEAS5 seasonal forecast issued on 1 July. The next SEAS5 forecast, issued on 1 August, showed a drier signal in this part of Europe. Moreover, the hydrological model used to produce these outputs has known limitations on the Iberian Peninsula. Looking back at this event and similar ones (e.g. the summer of 2003 and of 2015) in more detail should help us understand the sources of predictability and uncertainty in the EFAS seasonal hydrological outlook and should lead to improved forecasts of future events.

EFAS seasonal hydrological outlook for the last week of August 2018. The shading indicates different levels of elevated probabilities of low river flow, based on the EFAS seasonal discharge forecast issued on 1 July for the last week of August 2018. ‘Low’, ‘medium’ or ‘high’ probability indicates that 50–75%, 75–90%, or more than 90% of the ensemble distribution is below the 10th percentile of the EFAS discharge simulation weekly climatology. The red river network shows the low flow situation from the EFAS discharge simulation for 24 August as a proxy for the truth.

A more detailed version of this article was published on 6 September 2018 as a blog post on the HEPEX website: https://hepex.irstea.fr/summer-2018-in-europe/