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The rise and fall of Alpine hydrological balance

Eylert Ellefsen
Archived blog post. This blog post has been transferred from our previous blogging platform. Links and images may not work as intended.

Precipitation levels have fluctuated massively across the Alps this autumn, as a dry November followed a very wet October. In this blog post, you will see how these two months compared to the climate years of 1980-2019 (40 years).

Key findings

  • October precipitation came out at 168% of normal for the region
  • The October precipitation was close to the 95%-tile for 1980-2019 period
  • We estimate total November precipitation will be around 25% of normal
  • This would be an all-time low precipitation level for the 1980-2019 period

So what happened?

During the first days of October the Alpine region was hit by thunderstorm Alex (see our blog from 6 October).

During week 43 (Oct 20-26) precipitation levels were again very high, raising the French hydrological balance close to a surplus of 5 TWh, up from zero (normal situation) at the beginning of October (see chart below).

Hydrological balance in France late September to early December 2020.

November, contrary to October, turned extremely dry, and the hydrological balance in France will decline to nearly -2 TWh, which equals a 6-7 TWh reduction since late October.

EQ has dived more into the hydrological data for Alpine region for October and November and made some comparisons to the climatic years 1980-2019 (40 yrs).

The charts below show the monthly precipitation in 2020 compared to the 1980-2019 climate data:

Alpine total precipitation in October 2020 vs. 1980-2019 (TWh).
Alpine total precipitation in November 2020 vs. 1980-2019 (TWh).

Alpine region totals

We have also looked at the total precipitation for the Alpine region (France + Switzerland + Austria + Italy) during October and November – both daily and accumulated for the period:

Deviation for accumulated precipitation from the normal (grey) for the course of October and November 2020.

Key takeaways are:

  • The accumulated curve shows the accumulated deviation from normal.
  • We see the strong incidents in the first and latter part of October. The accumulated deviation hits about 13000 GWh/13 TWh by end of October, but falls dramatically during the extremely dry November.

In the following graph, we show the hydrological balance curve for the region. The hydrological curve is nearly in parallel to the accumulated precipitation curve:

Development in Alpine hydrological balance during October and November 2020.

We estimate that the hydrological balance will decrease by about 16 TWh during November. This is nearly 10 % of the yearly normal hydro production average in the region, which clearly illustrates how dry November has been.

Hydrological data for each country

Precipication energy (TWh) on a country-basis in October and November 2020.

From the table above we see that the precipitation conditions have been rather equally distributed between the Alpine countries for this period.

  • The regional surplus in October was 17.4 - 16.4 = 11.0 TWh
  • The regional deficit in November was 18.9 - 4.8 = 14.1 TWh

Final words

Precipitation levels in the Alpine region were historically exceptional in the October-November period. This could probably be related to climate change as weather conditions are expected to become ever more extreme.

After a wet October, the hydro reservoir fillings were higher than normal in the region, but keep a close eye on inflow conditions going forward as the snow/groundwater levels are falling below normal levels after a dry November so far.

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