WW-I-4: Low water

The picture shows a river course with strong low water, in the foreground the dry river bed with three rotten technical objects, in the middle ground a transport ship in the remaining navigation channel and behind it the steeply rising opposite bank with groups of trees and bushes.Click to enlarge
Low water occurs mainly in summer and early autumn, when there is a prolonged absence of rainfall.
Source: Photograph: © Comofoto/stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents


WW-I-4: Low water

In recent years, the low-water regime in Germany’s river basins was characterised to a considerable degree by distinct low-water years. Especially in the years 1991, 2003 and 2015 and latterly in 2018, prolonged drought periods have led to a strong decline in water levels of Germany’s rivers.

The stack column graph shows the mean number of low-water days in the hydrological winter half-year (October to March) and in the hydrological summer half-year (April to September) from 1960 onwards. Both time series show no trend. There are strong fluctuations between the years.
WW-I-4: Low water

The stack column graph shows the mean number of low-water days in the hydrological winter half-year (October to March) and in the hydrological summer half-year (April to September) from 1960 onwards. Both time series show no trend. There are strong fluctuations between the years.

Source: Run-off measurement of federal states

Low-water events – no climate-change related clusters found

Low-water events, in the same way as floodwater, are part of a natural run-off regime. In alpine catchment areas low-water events may occur in winter owing to the storage of precipitation as snow. However, in river basins in upland areas and in the case of rivers in lowlands and plains, low-water levels occur especially in summer and early autumn owing to high evaporation compared to the precipitation. Prolonged meteorological droughts, i.e. periods of low or no precipitation, tend to exacerbate seasonal low-water levels, especially during the summer months.

Changes associated with climate change may influence the time, duration and intensity of low-water events in many ways. The projected decrease in precipitation in the summer half-year and a higher evaporation requirement of the atmosphere may lead to a decrease in run-off in the summer half-year.

The consequences of low-water events influence both the ecology of waters and their utilisation. Owing to low flow rates at low water levels, the water warms up faster. This entails increased plant growth, especially in terms of algae, thus causing reduced oxygen concentrations, especially in river lakes. Moreover, if run-off decreases, inputs into the water bodies are diluted to a lesser degree thus leading to higher concentrations of nutrients or harmful substances. Both processes have far-reaching impacts on aquatic creatures as well as on water quality.

For various types of use, sufficient run-off or adequate supplies of water are fundamental prerequisites. In the absence of adequate amounts of water, which is specific to each river, the potential for shipping is restricted. Furthermore, lack of run-off can jeopardise the abstraction of water for cooling purposes or for agricultural irrigation. It can also lead to quantity restrictions being imposed on the discharge of wastewater.

For the time series illustrated, the run-off values for 80 levels of German rivers were examined in order to establish in respect of the water regime, on how many days in the summer half-year (1st April to 30th September) and in the winter half-year (1st October until 31st March) of the subsequent year, low-water levels were recorded. A low-water day is defined as a day on which the mean annual daily run-off is lower than the mean low-water run-off (MNQ) calculated for a level in the period of 1961 to 1990. The MNQ is calculated on the basis of the lowest run-off rates of individual water regime years (NQ). When the number of low-water days is averaged across the levels recorded, it becomes clear that again and again there have been individual years with an extreme accumulation of low-water days. In the course of the past 30 years, such accumulations occurred in particular in the years 1991, 2003 and 2015. This affected in particular the river basins of Rhine, Elbe and Weser, and, to a lesser extent, also the Danube. Regarding the river basin districts of Eider/Schlei, Schlei/Trave and Warnow/ Peene, the water regime years 1996 and 2008 recorded a high number of low-water days. Owing to the fact that low-water events can usually be attributed to stable high-pressure fronts, the ensuing consequences typically extend over wide areas.

In 2015 low-water levels prevailed for more than six months. The impacts on eastern Germany were particularly severe and prolonged, as here the summer drought lasted well into autumn whereas the situation in parts of the south-western catchment areas of Rhine and Danube were mitigated by considerable precipitation in the Swiss Rhine area and in the south of the Upper Rhine. Although the summer of 2015 was generally less extreme than the summer of 2003, the drought period lasted for an extraordinarily long time. On several reaches of the river shipping was considerably restricted in 2015.

2018 was another extreme low-water year with impacts on all German river basins. In the Rhine area, owing to months of drought, low-water levels and run-off rates were reached which had not been recorded for many years, consequently exposing rock formations and gravel banks in the river basin which had not been seen in living memory. Very low-water levels were also recorded for Elbe, Danube and Weser. All German waterways experienced shipping restrictions some of which lasted for some considerable time.

For the entire time series no significant trends are discernible either for the winter or the summer halfyear. This may be due partly to the fact that the extreme low-water situations in the 1960s and 1970s are still exerting considerable influence on developments.

Nevertheless, the seasonal distribution of low-water days indicates a significant trend. Compared to the low-water days in winter, there is a noticeable decline during summer. As in the case of mean run-off values, this suggests that there are changes taking place in the seasonal availability of water.



EW-R-4: Water efficiency of thermal power plants

VE-I-2: Low-water restrictions to shipping on the Rhine