2018 was a very hot, but above all extremely dry year. If things continue like this in 2019, groundwater levels and water resources will have no chance of recovering. However, the biggest loser of the drought is the climate.
The term “Heißzeit” (hot age) hit the headlines nationwide in Germany in 2018 and was choosen as “word of the year”. It is a wordplay on the German term “Eiszeit” (ice age). The summer had indeed been unusually warm – with an average of 10.4 degrees Celsius, 2018 was even the warmest year since the beginning of Germany-wide weather records in 1881. For farmers in particular, however, the biggest problem was not the heat, but the extreme drought that began in June.
“Our measuring stations in the meadows and forests of the Eifel as well as on a field in Selhausen near Jülich recorded an annual average of up to 23 per cent less precipitation than usual in 2018,” reports Dr. Alexander Graf from the Institute of Bio- and Geosciences (IBG-3). The three stations belong to the Helmholtz initiative TERENO, a network for earth observation stretching from the North German lowlands to the Bavarian Alps. The researchers measure how the surface of the land and the atmosphere exchange substances and energy – from precipitation and evaporation to carbon dioxide.
Compared to the Jülich locations, rainfall in other German regions was even lower (see chart here). According to the German Drought Monitor, summer and autumn have never been drier than in 2018 compared to the average since 1951. The drought continued even after the great heat had passed in September: the peak of the strain on the soil was not reached until early December as the soil and groundwater systems have a delayed reaction to the absence of rain. “The 2018 drought is still in the system,” says Graf.
“The 2018 drought is still in the system. If it continues to rain too little this year, the situation could worsen further.”
Dr. Alexander Graf, Institute of Bio- and Geosciences (IBG-3)
“At the beginning of 2019, soil moisture was significantly lower in most regions than in the previous year and so were groundwater levels. If it continues to rain too little this year, the situation in the affected areas could worsen further. Some consequences become apparent later on: in forests, for example, such weather events can lead to pest epidemics years later.”
According to Alexander Graf, the biggest loser of the 2018 drought was the climate: “Every summer like the one in 2018 is a lost opportunity for our biosphere to reduce the carbon dioxide load in the atmosphere,” says the Jülich expert. A rule of thumb is that an area that evaporates less than usual also absorbs less carbon dioxide than usual – either because the plants have dried up anyway or because they close their stomata in the leaves and perform less photosynthesis, in which sugar and oxygen are produced from carbon dioxide, light and water. This means that the plants do not grow as well and absorb less carbon dioxide. More CO2 in the atmosphere, however, intensifies the greenhouse effect and thus advances global warming – and as a result, climate change. Global warming, in turn, means more drought on average – a vicious circle.
Precipitation in 2018 at selected locations (in millimetres)
longterm annual average
In 2018, forests, meadows, soils and fields reacted very differently to too much sun and too little rain. Evaporation played an important role.
During evaporation, water is converted into water vapour on the earth’s surface – above water (1) and soil (2) as well as by plants (3). The water vapour rises (4), cools down in the process and condenses to clouds (5). It then returns to the land surface as precipitation (6), where the precipitation seeps into the groundwater (7) and supplies lakes, rivers and oceans with new water (8). The cycle then begins anew.
The field in Selhausen, at which Jülich researchers continuously collect measurement data, evaporated a quarter less water in 2018 than in a normal year. The reason: due to the warm temperatures, the farmer was able to harvest his grain earlier than usual. As a result, the area lay fallow much earlier. Groundwater and soil water had subsequently dropped to such depths that the so-called capillary forces in the soil were not sufficient to bring the water to the earth’s surface. Another effect: “Such surfaces, which do not emit any cooling water vapour, contribute to making an already warm summer even warmer,” explains Graf.
In contrast to the previous year, both the forest and the grassland in the Eifel evaporated up to seven per cent more water despite the drought. This happened mainly through the plants. Their roots were deep enough to make use of remaining water reserves. These surfaces were thus able to at least partially cover the atmosphere’s greater water vapour demand. But this plus had a price: in 2018, the two investigated sites supplied 22 per cent (forest) and 38 per cent (meadow) less water for rivers and groundwater recharge.
The water levels of various rivers fell to record depths by autumn. For example, the level of the Rhine in Cologne was 69 centimetres in October. This was 12 centimetres less than the previous all-time low of September 2003. Because of the low water levels, many inland waterway vessels had to stop traffic for weeks or travel with little freight.
hours is how long the sun shone in Germany in 2018. This is the highest average value since the beginning of the measurements in Germany in 1951.
“The lowering of the groundwater level in 2018 is clearly visible – not only in Germany, but throughout Central Europe,” as Prof. Stefan Kollet from the Institute of Bio- and Geosciences (IBG-3) has found. He and his team have modelled Europe from the groundwater to the atmosphere and calculated the groundwater level depths for Central Europe from 1996 to 2018.
Photos: Jasper Suijten/shutterstock.com, Forschungszentrum Jülich/Sascha Kreklau, Graphic: SeitenPlan/ArtMari/Shutterstock