WW-I-6: Low water | Umweltbundesamt

WW-I-6: Low water

In recent decades, the low-water regime in Germany’s river basins has been characterised to a considerable degree by individual years with distinctly low water levels. However, the hot and dry summers of the period from 2018 to 2020 subsequently led to three extreme years.

Record low-water levels in drought years

Low-water events, in the same way as flood events, are part of a natural discharge 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 when phases of low or a complete lack of precipitation coincide with high evaporation. In particular, prolonged meteorological droughts, in other words, periods of low or no precipitation, tend to exacerbate seasonal low-water levels, above all 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 discharges in the summer half-year.

The impacts of low-water events influence both the ecology of water bodies and their utilisation. The low water volumes prevailing at times of low water levels result in water warming up faster (cf. Indicator WW-I-10). Both directly and indirectly, this leads to an intensification of algal growth, especially in river lakes. The subsequent decomposition of dead algae produces oxygen depletion and reduced concentrations of oxygen. Moreover, when discharge decreases, any inputs into the waters are diluted to a lesser extent thus leading to higher concentrations of nutrients and harmful substances. Both processes have far-reaching impacts on aquatic creatures as well as water quality.

For various types of water utilisation, a sufficient discharge quantity and / or adequate availability of water are fundamental prerequisites. In the absence of adequate water levels – a variable specific to each river – the potential for shipping is restricted (cf. Indicator VE-I-2). For various types of water utilisation, a sufficient discharge quantity and / or adequate availability of water are fundamental prerequisites. In the absence of adequate water levels – a variable specific to each river – the
potential for shipping is restricted (cf. Indicator VE-I-2). Furthermore, lack of discharge and / or excessive water temperatures can jeopardise the abstraction of water for cooling purposes; in other respects, the low water volume can affect the availability of water for agricultural irrigation (cf. Indicator LW-R-6). Moreover, such conditions may necessitate the imposition of restrictions on the discharge of waste water.

For the time series illustrated, the discharge values measured at 76 gauging stations of German rivers were examined in order to establish on how many days in the water balance of 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 discharge is lower than the mean low-water discharge (MNQ) calculated for the relevant gauging station in the period of 1961–1990. The MNQ is calculated on the basis of the lowest discharge rates of the individual water balance years (NQ). By averaging the number of low-water days across the levels recorded, it becomes clear that again and again there have been individual years with an extreme accumulation of low-water days. Looking back at the past four decades, such accumulations occurred in particular in the years 1991, 2003 and 2015 and lately in 2018 to 2020. 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 balance years 1996 and 2009 as well as the period of 2018 to 2020 recorded a high number of low-water days. Typically, low-water events can also be due to stable high-pressure systems. Accordingly, the impacts tend to be rather large-scale.

Up to that time, a sequence of several extremely dry years – such as latterly in the period of 2018 to 2020 – had been extremely unusual. The drought period began in April / May of 2018. Both months had very little precipitation, especially in the north of Germany. In the summer months and well into September, several stable high-pressure systems resulted in heat and drought throughout Germany. For example, the German part of the Rhine catchment, had just roughly half the usual precipitation⁶⁰. Owing to the resulting extreme low-water levels, this exposed rock formations and gravel banks which had not been seen since the extreme low-water levels of 1921. Very low-water levels were also recorded for the rivers Elbe, Danube and Weser. All German waterways experienced shipping restrictions some of which lasted for some considerable time. Likewise, the summer months of 2019 were too dry throughout, and the summer of 2020 again caused droughts in many regions of Germany, such as the Rhine area. Despite the fact that the winters of 2018 / 2019 and 2019 / 2020 were overall on the wet side, this did not suffice to offset the shortage of water, partly because in individual regions such as Thuringia, there was not enough precipitation in the winter months either.

The extreme drought of recent years has characterised the findings of the statistical trend analysis. The trend of low-water levels in terms of the water balance of the summer half-year is significantly rising. Furthermore, a statistical ‘breakpoint analysis’ makes it clear that the situation in both half-years regarding the water balance has become distinctly more acute since the mid-2010s. Beforehand, the number of low-water days in the winter half-year used to be slightly regressive in general. However, in recent years this number has increased, especially around the rivers Weser and Elbe. In the summer half-year, the number of low-water days increased slightly, but not significantly. In the course of recent years the increase became more pronounced – and this substantial increase in low-water days in summer is reflected in all river districts considered.

^{60 - IKSR – Internationale Kommission zum Schutz des Rheins (Hg.) 2020: Bericht zum Niedrigwasserereignis Juli-November 2018. Bericht, Band 263. www.iksr.org/de/oeffentliches/dokumente/archiv/fachberichte/fachberichte-einzeldarstellung/263-bericht-zum-niedrigwasserereignis-juli-november-2018}