The influence of snow cover

The analysis of snow cover's influence on the flood will focus on the period with the most precipitation, the days from 26 May to 2 June 2013. Only the Alps had appreciable amounts of snow at this time.

Fig. 3.6 depicts snow water equivalent, with the values in Austria and Switzerland reaching 400 mm and more. Analysis of snow cover's effects on the flood is only necessary in the Danube drainage basin. The calculations come from the SNOW model from Deutscher Wetterdienst. You will find more information here:
http://www.dwd.de/EN/specialusers/water_management/water_management_node.html

Figure 3.6: The SNOW4 model depicts the snow water equivalent (in mm) of the snow cover on 26 May 2013. Source: (1)

Precipitation, which the snow cover will not hold back, combines with snowmelt and intensifies the flood.

This process was at its strongest on 28 May. The difference between runoff water and precipitation in this area reached 415 billion liters. Fig. 3.7 shows positive values everywhere in the Alps within the covered region.

Figure 3.7: Difference between total runoff and precipitation, 28 May 2013, in the catchment area of the Danube. Source: (1)

Figure 3.8: Difference between total runoff water and precipitation, 31 May 2013, in the catchment area of the Danube. Source: (1)

If precipitation occurs in the form of snow or rainfall is retained within the snow cover, the runoff is reduced. This process saw its maximum on 31 May, when no snowmelt occurred (see Fig. 3.8). In the catchment area of the Danube 187 billion liters of water were retained in the snow cover.

In summary, snowmelt increased the total runoff by approximately 5%, which means the snow cover played a small part in this flood event.

Summary for the June 2013 flood:

  • The soil was saturated in most parts of Germany and could not absorb more water.
  • Runoff caused by melting snow was only important near the Alps.
  • The snow cover had a small contributing role on the flood.
  • The snow cover would have played a more significant part if the event took place during the winter.