WW-I-2: Mean run-of

The picture shows a water level gauge in a calm body of water.Click to enlarge
The values for summer run-off are in decline. This reflects a diminished availability of water.
Source: Photograph: © Jodocos /stock.adobe.com

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

Table of Contents

 

WW-I-2: Mean run-of

The average run-off value across 80 levels measured in German river basins show clear variations from year to year. The amount of run-off during the hydrological winter half-year has been decreasing, albeit not significantly, since 1961 However, a significant decline in the mean run-off value for summer has been observed, which suggests a change in the availability of water in summer.

The column chart shows the discharge height on the German federal territory in the hydrological winter half-year (November to April) and in the hydrological summer half-year (May to October) from 1961 onwards. The discharge height in the hydrological winter half-year shows no trend, that in the hydrological summer half-year shows a significantly decreasing trend.
WW-I-2: Mean run-of

The column chart shows the discharge height on the German federal territory in the hydrological winter half-year (November to April) and in the hydrological summer half-year (May to October) from 1961 onwards. The discharge height in the hydrological winter half-year shows no trend, that in the hydrological summer half-year shows a significantly decreasing trend.

Source: Run-off measurement of federal states
 

The availability of water is changing

In the greater part of Germany, the natural run-off regime of watercourses is determined by precipitation. During the warm season, the degree of evaporation also plays a role. This leads to high mean values of run-off in winter and early spring, compared to low run-off values in late summer and autumn. Especially in the southern parts of Germany, winter snow cover plays a crucial part alongside run-off and rain volumes. In alpine catchment areas with major rivers such as Iller, Isar, Lech and Inn, winter precipitation often accumulates as snow; therefore run-off is lowest at that time of year. As a result of snow melt in spring and early summer, often accompanied by rainfall, the run-off maximum occurs in the middle of the year. This is termed a nival run-off regime.

Apart from precipitation, the catchment area’s topography plays an important role; it is crucial for the speed at which precipitation actually affects the run-off regime.

Where climate change affects the precipitation and temperature conditions, it will also affect the run-off regime. This may include impacts on the volume of the total run-off of water and on the seasonal distribution of run-off.

For an analysis of run-off regimes and their development, a total of 80 levels across Germany’s river catchment areas were selected. They represent mean catchment area sizes at a scale of 250 to 2,500 km². These are levels which are, as far as possible, unaffected by anthropogenic influences, i.e. where run-off levels were observed which are not strongly marked by, for instance, water transfer or water retention systems.

The mean water run-off (MQ) or the annual run-off volume (Ah) derived in relation to the size of the catchment area concerned acts as an indicator for the availability of water. This indicator shows the availability of water in principle, including water which is available for cultivation and for a variety of surface water applications such as cooling water or shipping. Changes in the mean runoff values can also entail changes in groundwater levels in riparian areas, thus affecting supplies of drinking water and process water.

Although the time series from the 1960s onwards shows a slight decline in the mean run-off value for the hydrological half-year period from early November to end of April, this cannot be regarded as a statistically significant trend. However, during the hydrological summer halfyear, i.e. from early May to end of October, a significant falling trend can be discerned. This is due to reduced precipitation in summer and temperature-related higher evaporation during those months. This development suggests that changes in the fundamental availability of water in both half-years, i.e. winter and summer, are already beginning to take place.

The mean value calculated across Germany, as well as – owing to the effects of precipitation and evaporation – the winter run-off values, are clearly higher in general than the run-off values for the summer half-year. The low-water year 1972 is the only year in the time series observed where summer run-off was higher than winter run-off. Since then there has not been one year when this happened again. Nevertheless, the relationship between medium run-off in the summer half-year and the winter half-year does not suggest any changes that can be described as statistically significant.

In the Danube river basin where the run-off regime is characterised by a predominantly nival run-off, there have been – from 1960 until the late 1980s – just as many years in which the values for summer run-off were higher, as there were years where the winter run-off values were higher. Since 1990 the years in which winter run-off values exceeded summer run-off values, have increased in frequency nationwide. This suggests that the influence of the snow cover on the run-off regime is diminishing.

 

Interfaces

WW-I-1: Groundwater level

WW-I-3: Floodwater

WW-I-4: Low water