Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 74

− The through-flow of a pipeline can moreover be measured with Hydrometric
vanes firmly installed into a pipeline, driving a meter via a mechanical connec-
tion (e.g. water meters).
Distribution and variations of precipitation. The instruments for measuring the
precipitation (such as the pluviometer) have been in use worldwide since centu-
ries. The measured values serve to calculate the annual precipitation. If the yearly
precipitation is averaged over many years, areas of similar precipitation can be
identified. Fig. 2.38 shows these values on a worldwide scale.
< 250
250 - 500
500 - 1,000
1,000 - 2,000
> 2,000
Values in mm/a
Fig. 2.38 Distribution of the average annual precipitation (time period between 1961 and
1990; some of the data points within the ocean are probably from weather buoys; data
according to /2-11/)
According to this map, areas with a high precipitation are primarily located
close to the equator. On the other side, the deserts existing on earth are notorious
for being regions with very low rainfall.
Such maps of the average annual precipitation can only provide a first rough
estimation about the given coherences on a worldwide scale. To analyse the given
dependencies in more detail Fig. 2.39 shows the distribution of precipitation ex-
emplarily for Germany. Here, the annual rainfall is mainly influenced by the oro-
graphy and the increasing continentality of the climate towards the East. Precipita-
tion varies here between approximately 500 mm in the Northern part of the Rhine
Valley and over 2,500 mm at the rim of the Alps. There are also very rainy areas
in the low mountain ranges.
The mean values of precipitation levels over many years shown in Fig. 2.39
can be subject to large differences throughout one year. This reveals Fig. 2.40
showing the annual precipitation levels at four different sites (Hamburg, Hanover,
Stuttgart, Munich; all sites are located in Germany) between 1961 and 1995. The
annual precipitation levels in this period over 35 years fluctuated within +/- 25 %
in Munich for example and within +/- 50 % in Hanover by the respective mean
value.