Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 79

Looking at the daily or monthly mean runoff values over several years, the run-
off is characterised by a seasonal curve depending on the analysed river and the
particular stretch of the river. Depending on the river and the meteorological con-
ditions such annual cycles might be visible within one year or on the long-term
average only. As an example for such characteristic annual cycles Fig. 2.45 shows
mean monthly runoffs at selected gauges of Austrian rivers. According to this the
Danube as well as the Gurgler Ache shows significant differences in the runoff
between different months. The maximum runoff is given here during summer
because of the snow melting on the higher altitudes of the Alps. This is not the
case for the Mattig; the peak runoff here occurs during spring because the catch-
ment area of this stream is located in lower mountains where the snow melts ear-
lier in the year compared to the Gurgler Ache.

3.5

Danube close to
Vienna

3.0

Mattig close to
Ja h r sd orf

2.5

Gurgler Ache
close to
Ob ergu rgl

2.0

1.5

1.0

0.5

0.0

123456 7891011 2
Time in months

Fig. 2.45 Examples for average mean monthly runoffs at selected gauges in Austria (ac-
cording to /2-13/)

But the long term average of the annual cycle is subject to significant changes
between different years. Therefore Fig. 2.46 shows the mean values of the
monthly mean through-flows over many years and the corresponding average and
maximum fluctuation range using the example of a water level at Rockenau for
the Neckar river and at Maxau for the Upper Rhine. According to this plot the
variations that might occur between different years on a monthly average are tre-
mendous. In some months the maximum measured runoff is 10-times higher com-
pared to the average for the Neckar river and some 4-times for the Rhine. Such
flood events have often the unwanted effect of flooding villages and cities as well
as agricultural land.

Reservoirs. In line with the topographic conditions, the possibilities of storing
water vary substantially with regard to different regions.
In the high mountains, conditions are in most cases quite favourable. There-
fore, for example in the Alps, there are already quite many reservoirs. The