Is Germany drying out?

22 August 2022

An analysis of the current drought period in context of long-term climate development.

Monica Ionita, Patrick Scholz and Klaus Grosfeld (AWI)

Over the last two decades, most months have been the warmest in the history of observations, both globally and in Germany. Globally speaking, climate change has expressed its impact in the warmest decades since the beginning of the continuous recordings. The last seven years have been the warmest since 1881. Precipitation, on the other hand, has a different trend. In Germany, precipitation amounts have not decreased, but their distribution has changed. Throughout the last two decades there have been large amounts of precipitation falling in relatively short intervals, followed by large intervals without precipitation. This creates the need to expand water storage in drought years and increases the risk of flooding and dam failure during periods of extreme rainfall. The Ahr valley experienced a dramatic example of the consequences of extreme precipitation events a year ago. To combat the damaging effects of drought, both in the short and long term, governments and other decision-makers need to identify vulnerability to drought and improve their resilience. Decision factors can help prepare for both, future droughts and climate change by practicing and promoting water conservation and improving water efficiency in landscapes, urban planning and water infrastructure. They can also identify alternative sources of water, create drought contingency plans and encourage farmers to plant drought-resistant crops.

For the next few decades, anthropogenic climate change and the resulting changes in the global water cycle will lead to significant increase in the frequency of droughts in the northern hemisphere. Climate simulations (produced by the latest generation of models for three different emission and development scenarios) show that the risk of summer drought will change significantly in the northern hemisphere regions. In this respect, drought risk will increase particularly in subtropical areas: if global temperatures rise by more than 4° C by 2100, regions such as the Mediterranean are likely to face severe droughts year after year (Balting et al., 2021).   

Over Europe, droughts occur primarily in summer, as their occurrence depends largely on three factors: air temperature - the warmer the air, the more moisture it can absorb, theoretically leading to more evaporation; rainfall (mostly absent); and the amount of moisture evaporating from the Earth's surface. Higher temperatures increase evaporation, which reduces surface water, drying out soils and vegetation. Warmer temperatures in winter mean also that less snow falls, including in some high-altitude regions. Decreasing snow cover can be a problem, even if total annual precipitation remains the same. This is because many water management systems rely on spring snowmelt.

Drought can be defined as "a shortage of precipitation over a long period of time (usually a season or more), leading to a shortage of water". Indicators of drought include precipitation, temperature, streamflow, groundwater and reservoir levels, soil moisture and snow cover. Depending on the parameters on the basis of which we define drought, we can distinguish different types of droughts: meteorological drought, agricultural drought, hydrological drought, ecological drought and socio-economic drought. Depending on the region and the type of climate, very severe droughts occur in years when less than 50 % of normal rainfall falls during the growing season. Severe droughts occur when rainfall is 60-70 % of normal and moderate droughts occur in years when rainfall is 70-80 % of normal.

The first seven month of the year 2022 were characterized by a significant rainfall deficit over Germany, especially in spring and summer. For the ninth year in a row, spring was drier than normal, recording ~ 67 % of its multi-year average. Overall, it remained considerably dry in March and May (Figure 1). In April, precipitation was above the norm compared to previous years. July and June 2022 were also drier than normal, over large parts of the county (Figure 1). This rainfall deficit led to extreme dry soils at the end of spring and beginning of summer which in turn have significant consequences on the development of extreme dry conditions in June and July.

The average temperature in spring 2022 was 1.4° C above the climatological mean (i.e. 1961 – 1990) (Figure 2). In June 2022 the average at country level was 18.4° C, which translates into 3.0° C above the climatological period 1961 to 1990. The highest temperature was recorded in the southern half part of the country (Figure 2b). In June 2022 new temperature records were observed in Saxony and Brandenburg. On the 19^th of June, the German Meteorological Office measured 39.2° C in both Dresden-Strehlen and Cottbus. These were the highest values in Germany. In July 2022, the average temperature was 19.2° C, meaning 2.3° C above the climatological reference period 1961 to 1990. The second part of the month was characterized by a long-lasting heatwave, with record-breaking temperatures throughout the country. In the southern half of the country, the temperatures were above the climatological mean most of the month. Up to 29 summer days (maximum values of ≥ 25° C) were counted. Temperatures over 40° C were recorded in more Federal Sates, breaking the local state’s all-time record: Bad Mergentheim 40.3° C, Hamburg-Neuwiedenthal 40.1° C, Barsinghausen 40.0° C and Huy-Pabstorf 40.0° C, among others.

Consequences of the dry and hot spring – mid-summer

The extremely dry months of March and May corroborated with the above-average temperatures, caused the soils to dry out severely throughout the country. Extremely high temperatures increase evapotranspiration and trigger higher water demand; thus, they contribute substantially to soil depletion and drought severity. The most severe drying, at soil level, started to be visible in June and continued also in July, when the whole country was affected by a soil moisture deficit (Figure 3). Soils severely dried out particularly in the east and over Rhine’s catchment area, in the southwest of the country, but in the far north and south regions, soil moisture did not reach quite as low levels. The ongoing drought caused increasing impacts on agriculture and inland waterway transport, but the overall damages are going to be visible in autumn, when we might hopefully see a relief of the current conditions.

The extreme ongoing drought can be seen also when we look at pre-defined drought indices, in this case the Standardized Potential Evapotranspiration Drought Index (SPEI) accumulated over different months (e.g., 1-month and 3-months). A 1-month SPEI indicates the water availability over 1-month, while a 3-month SPEI indicates the water availability integrated over three months. By taking into account different accumulation periods, this index has a multiscale feature, and hence gives us the possibility to approximate agricultural, hydrological and socioeconomic drought by adjusting the accumulation period for a particular region. A positive index means periods wetter than normal, while a negative index indicates a period drier than normal. The higher the amplitude of the index, the wetter/drier the observed conditions. In April, the drought conditions were not visible, mainly because April 2022 was wet enough to prevent the development of potential dry conditions (Figure 4a). The situation changed rather drastically in May, when drought started to develop especially on the side of the Rhine catchment area (Figure 4b). In June, due to a combination of rainfall deficit and record-breaking temperatures, the drought continued to develop and extend all over the country, with a special focus on Thuringia, Saxony and the south-eastern part of Lower Saxony (Figure 4c). In July 2022, the drought continued to amplify, but the most affected areas were in the south-western part of the country, with a focus on the German side of the Rhine catchment area (Figure 4d).

A remarkable feature of the ongoing drought is the growth in the spatial extent of low flows across the European river network. The European inland waterways offer more than 40,000 km network of canals, rivers and lakes connecting cities and industrial regions across the continent. The German inland waterway network – an integral part of the trans-European waterway system – comprises about 7,350 km, of which approximately 75 % are rivers and 25 % canals. In Germany, almost 6% of the total transported goods are transported per inland waterway vessel (as of 2017). A large part of this happens at around 78 % via road traffic, while railway and maritime transport account for around 9 % and 7 %, respectively. However, for individual goods divisions such as coal, oil and natural gas, coking, as well as petroleum and chemical products, inland waterway transport is responsible for 10 % to 30 % of the transport volume and is thus of significantly greater importance (Ionita and Nagavciuc, 2020). Any slowdown in the navigation time leads to production hindrances in downstream production stages. Furthermore, inland navigation is also important for foreign trade.

The daily streamflow reached minimum values in most central European rivers over the last few weeks. Over Germany, long sections of the Rhine, Elbe and Weser rivers and their tributaries reached the critical levels for low flows. The daily hydrograph for 2022, at Kaub gauging station, indicates that at the beginning of August, the water levels of Rhine River were at critical low levels, making the inland waterway transport extremely difficult (Figure 5a). On the 17^th of August 2022, due to the extreme ongoing drought, the water level of the Rhine in Emmerich, shortly before the Dutch border, has reached a historic low of -3 cm. The previous low, at Emmerich gauging station, was reportedly on October 30, 2018, with 7 cm as a daily average. Extremely low flows are currently observed also for the Elbe River (Figure 5b).

Drought 2022 at European level

Starting with the spring months of 2022, large regions in Europe had already been affected by drought (Figure 9a). The drought continued to develop and intensify at the beginning of summer (Figure 9b), reaching unprecedented levels over some regions throughout the observational record (Figure 10). This extended and extreme drought had significant effect on the water levels, agriculture, biodiversity, leading to water restrictions in some countries (e.g., France, Italy, Netherlands). Moreover, aquaculture and fisheries are also suffering as a result of the declining water levels in fish ponds and rivers, and for some regions shipping restrictions are about to be implemented. Reduced power supply and, occasionally, power outages are the result of hydroelectric power facilities that are not operating or operating only partially.

At the end of July 2022 - 23.98 % of Europe was affected by extreme drought (for the months May/June/July - SPEI3 July), 36.37 % of the European region was under severe drought condition and 47.25 % was under moderate drought conditions (Figure 9c). The most affected regions were Spain, France, Italy, Germany and eastern part of Europe (Figure 9b). In terms of extreme drought, the 2022 drought event is characterized by largest domain affected by extreme drought since 1950.

Next to the precipitation deficit, the current extreme drought could have been exacerbated by the extremely high temperatures observed from May onwards (Figure 11). Less cloud cover, more radiation, and drier air are typically associated with higher temperatures, which increases evaporation and boosts the air's capacity to absorb water (for every 10 °C rise in temperature, the air's capacity to absorb water doubles). In addition, vegetation begins to grow sooner at higher temperatures, and the later-developed plants may draw considerably more water from the soil, which further depletes the soils of water. Positive temperature anomalies lead most of the time to higher evaporation than normal and, corroborated with lower precipitation during the growing season, can lead to drier conditions in the soil. A recent study by (Ionita and Nagavciuc, 2021) shows that especially for the Mediterranean region and the central part of Europe, the increase in the mean temperature started to play a significant role in the occurrence and magnitude of drought events, especially after the 1990’s. The temperature contribution to drought risk has implications especially concerning the future occurrence of drought events, given that potential evapotranspiration, which is directly influence by the temperature, is projected to increase under a warming climate.

In terms of what is going to happen in the upcoming months, the seasonal climate prediction system of the Germany Weather Center predicts drier conditions for the next three months (July-September) in large parts of Europe (except in the northern part of Europe and Greece) compared to the multi-year average from 1991-2020. The most affected regions by the drier conditions are going to be the western and central part of Europe. Hence, the current situation will last at least for the upcoming month and no all- clear signal can be given, unfortunately.

           
          
                      
                         
References:

Balting, D.F., AghaKouchak, A., Lohmann, G., Ionita, M., 2021. Northern Hemisphere drought risk in a warming climate. npj Clim. Atmos. Sci. 4, 61. doi.org/10.1038/s41612-021-00218-2

Ionita, M., Nagavciuc, V., 2021. Changes in drought features at the European level over the last 120 years, Natural Hazards and Earth System Sciences. doi.org/10.5194/nhess-21-1685-2021

Ionita, M., Nagavciuc, V., 2020. Forecasting low flow conditions months in advance through teleconnection patterns, with a special focus on summer 2018. Sci. Rep. 10, 13258. doi.org/10.1038/s41598-020-70060-8

Contact:

Monica Ionita, Patrick Scholz und Klaus Grosfeld
(Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven)