Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 70

Commonly, precipitation is expressed in terms of the precipitation level (i.e. as
the ratio of precipitation volume and surface ( P S / S ) in mm). This also applies to
the runoff level describing the share of the overall precipitation level that is effec-
tively drained and does not evaporate, or flows off the observed area with the
groundwater stream. The so-called flow coefficient, defined as the ratio of flow
and precipitation, describes which proportion of the precipitation is finally
drained. Together with a number of other parameters, the flow coefficient is par-
ticularly dependent on the precipitation and the condition of the analysed area (i.e.
vegetation, permeability, topography). However, the flow coefficient generally
increases with an increasing precipitation level.
On the basis of these interdependencies, the flow characteristics in a certain
area can be described. With the knowledge of precipitation, evaporation and re-
tention, the flow regime can also be explained, at least qualitatively. The flow
regime is thus defined as the timely behaviour and the discharge volume of a
creek or river throughout the course of one year within a certain area. In order to
obtain this value, the catchment area needs to be unambiguously assigned to the
corresponding outflowing stream or river. The "inflow" streaming towards a de-
fined point along the course of a stream or a river is thereby equal to the "outflow"
of the respective catchment area. It shows time variations that generally fluctuate
significantly, in dimension and course, within one year and from year to year.
The flow of a potential catchment area is, among other factors, dependent on
the size of the area and the precipitation. It has to be taken into consideration that
the watershed (above ground) defining the catchment area can be very different
from the watershed below ground in the case of sloping, impermeable layers. This
can significantly enlarge or reduce the flow of a certain catchment area.
Precipitation and runoff of a particular catchment area are only indirectly
linked, as only parts of the falling rain only flow off immediately. In times of high
precipitation the runoff is delayed due to the formation of reserves, whereas in
times of low precipitation there is more flow due to these reserves being used. At
temperatures below the freezing point, additional delays of the runoff occur due to
the water stored in form of snow and ice. Additionally, parts of the precipitation
are completely lost through i.e. immediate evaporation, indirect evaporation
caused by plant growth, or through increased evaporation due to irrigation meas-
ures. Thus, no direct conclusion can be drawn from the amounts of rain to the
runoff.
Snowfall is also important for the runoff activity of a particular area, as the wa-
ter stored in snow only reaches the runoff with a certain delay. The snow blanket
is influenced, among other factors, by air temperature, global radiation, wind and
topography. Sudden melting of snow - as well as heavy rain exceeding the stor-
age capacity of an area - can lead to flooding. Extreme floods therefore often
occur when melting snow and heavy rainfalls occur at the same time (like it is
often the case in early spring in some parts of Central Europe).