Wetter trifft Klima – das Jahr 2018 im Zeichen der #Trockenheit

Das Sächsische Landesamt für Umwelt, Landwirtschaft und Geologie und der Deutsche Wetterdienst haben für Sachsen – wie jedes Jahr – eine sehr profunde Zusammenschau auf die Witterung des vergangenen Jahres inkl. einer klimatologischen Einordnung vorgestellt. Dominierendes Thema war natürlich die Trockenheit.

Die wichtigesten Kernaussagen finden sich komprimiert in zwei Leitvorträgen des LfULG bzw. des DWD. Für Sachsen war das Jahr 2018 (gegenüber dem Zeitraum 1961/1990) beispielsweise 2,2 K zu warm und hatte 33 % zu wenig Niederschlag.

Im dem genannten Bericht gibt es auch umfassende gewässerkundliche Ausführungen, wie die folgenden Beispiele belegen. Eine Lektüre des Berichts sei Interessierten also unbedingt empfohlen.

Anteil sächsischer Pegel im Niedrigwasser (Wasserführung unterhalb MNQ(Jahr) zum Stichtag Dienstag der dargestellten Wochen) von Mitte Mai 2018 bis Anfang Januar 2019; zwischenzeitliche Rückgänge der Betroffenheit sind durch Niederschlagsereignisse verursacht (Quelle: LfULG-Bericht).

Im Folgenden sind einige Durchfluss-Klimatologien für ausgewählte Pegel dargestellt (Quelle und weitere Erläuterungen: siehe LfULG-Bericht):

Die Auswirkungen der Trockenheit 2018 sind immer noch zu spüren. So ist beispielsweise für schwere Böden mit größeren Mächtigkeiten (wie in Nordsachsen) nach wie vor ein beträchtliches Bodenwasserdefizit zu verzeichnen. Auch ist die allgemeine Tendenz der Grundwasserstände bestenfalls (auf niedrigem Niveau) stagnierend. Das Niederschlagsdargebot der nächsten Monate wird daher entscheidend für den weiteren Verlauf in 2019 sein.

Wichtige Punkte zur Einordung:

  • Trockenheiten gab es immer wieder und wird es auch zukünftig geben
  • Es gab in der (bekannten/jüngeren) Vergangenheit “schlimmere” Trockenheiten in Sachsen (z.B. 1976)
  • Trockenheit ist vor allem abhängig vom Niederschlagsdargebot/-defizit und zu einem geringeren Grad eine Funktion anderer Faktoren, wie Temperatur, Wind, Strahlung etc.
  • Auf Basis einschlägiger Klimaprojektionen ist derzeit nicht sicher zu sagen, wie zukünftig das mittlere Niederschlagsdargebot in Sachsen aussehen mag; die meisten Klimamodelle tendieren hier eher in Richtung Zunahme des mittleren jährlichen Dargebots
  • Generell können aus Einzelereignissen keine klimatologisch belastbaren Informationen abgeleitet werden
  • Auf Einzeleireignisse bezogene Aussagen wie “Die Trockenheit 2018 ist eine Folge des Klimawandels” sind daher nicht zulässig

#EuroDrought18: brief update for Saxony, Germany

This is how the SPI-180d evolved in 2018 for Saxony, Germany; very likely that the start of the upcoming year 2019 will be significantly drier than the beginning of 2018. HOWEVER, (even deeper) droughts of course occurred in former times. So, lining out a direct connection from the 2018 drought to climate change is a bit speculative (one cannot infer climatologically robust information from single events)…

Remarkable low flow of River Elbe during #EuroDrought18

N.B.: This is only to shed a light on hydrological conditions at a specific location and a specific day. #EuroDrought18 isn’t over yet for larger regions that were affected up to now. More in-depth analyses need to follow. Additionally, flow in the River Elbe is strongly affected by dams, barrages, locks, etc., located in the Czech part of the catchment, foremost since the 1960’s, where a number of larger dams went into operation. Dams etc. affect the flow regime in both ways; they can sustain the flow by an additional release of water. On the other hand, flow is sometimes retained by technical operations for a certain time, e.g., in order to collect water for a more efficient energy production or to prepare releasing a “wave” to the downstream channel to support navigation, for instance. Therefore, a direct comparison of current conditions to historical extremes (i.e., prior the 1960’s) is not permitted.

Yesterday’s remarkable low flow was caused by a sunk due to the operation of Střekov barrage weir. The barrage was operated to retain water which was later released to improve power generation at Střekov and to support tourist-steamer navigation in and around Dresden on the upcoming weekend. Thus, the current low flow situation in the Elbe clearly is a consequence of #EuroDrought18. However, yesterday’s quite extreme low flow was also influenced by man-made operations.

First of all, here are some impressions of the situation on the evening of yesterday’s August, 23 (all pics taken by myself; feel free to share):

As already mentioned, a comparison with past data should be carried out with caution. Taking data starting from 1965 (data can be downloaded here) and comparing to current data (which can be found here) leads to the conclusion, that yesterday’s mean flow of 73.8 m³/s (mean water level: 43 cm) is the lowest daily mean flow since at least 50 years:

Daily mean flow of Elbe at Dresden from January 01, 1965 to August 23, 2018 (data can be found here and here; note the logarithmic scale of the ordinate axis).

Ripping up the data a bit and comparing it to empirically drawn flow percentiles gives the following image:

Flow climatology for Dresden / Elbe drawn from daily mean flow values from January 01, 1965 to December 31, 2017 and compared to current mean hourly flow values of the years 2017 and 2018 (solid lines). The R-Script used for data analysis and dataviz is courtesy of WSL and can be downloaded here.

The big question is, if the current drought will still last for a while, or not… Seasonal forecasts indicate near-normal conditions for the next months. If they are right, the upcoming rainfall would not fully compensate the current rainfall deficit until the end of the year, though.

ECMW seasonal forecast (Sept-Oct-Nov) of mean precipitation anomaly (source).

Up to now, the present drought seems to be a major event with more severe characteristics than the droughts of 2003 and 2015, at least for parts of Europe. However, this claim needs to be verified by further analyses, which will be carried out by the hydrology community in the future, for sure! In the meantime, feel free to follow the #EuroDrought18 hash tag on Twitter to stay up to date on the topic…

Last not least, here is some visual comparision of the 2015 with the 2018 situation:

Augustus Bridge over River Elbe in Dresden with indicated extreme water levels for selected flood and low flow events (as of August 23, 2018).


Colleague Massimiliano Zappa of WSL proposed via his Twitter channel to connect the scientific community via the hashtag #EuroDrought18 (at least the ones, who are on Twitter):

The proposal faced very broad agreement. The further development of the current drought situation will be interesting since seasonal precipitation forecasts show no fundamental relief of the situation (except parts of Southern Europe):

Seasonal precipitation anomaly forecast of DWD/MPI/UHH for August to October 2018; (c) DWD, MPI-M and UHH; Source: https://www.dwd.de/DE/leistungen/jahreszeitenvorhersage

Other seasonal forecasting systems point out the same direction; so, there is some evidence that there will be further coverage of #EuroDrought18

Seasonal precipitation anomaly forecast of ECMWF for August to October 2018; (c) ECMWF; Source: https://www.ecmwf.int/en/forecasts/charts/catalogue/

By now, there has been a lot of interesting coverage going on with Twitter. So please feel free and follow and contribute to #EuroDrought18!

Study on the 2015 European Drought published in HESS

Almost one year ago, I wrote a number of posts on the drought situation in Germany/Saxony, e.g., here or here. For parts of Europe, e.g., the upper Elbe basin, the 2015 event was even more severe than the 2003 drought. For the Czech Republic and parts of Eastern Germany (e.g., Saxony) the situation did only slightly improve by now. For instance, there was no significant groundwater recharge in Saxony since the 2013 flood event.

A nice analysis of last-year’s drought, paired with a follow-up investigation is now freely available from HESS. The results suggest, that the drought situation was ongoing for parts of Europe as of February 2016. In the meantime, some of these parts (e.g., Southern Germany) received way too much precipitation at once (e.g., see here). Also for other parts of Europe, the situation did relax (see here or here).

However, specifically for the Czech Republic or Saxony, there was no complete turnaround regarding especially the groundwater balance til today. Schau’n ‘mer mal…