Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 67

2.4.1 Principles

Water reserves of the earth. The water reserves of the earth available in solid
(ice), liquid (water) and gaseous (water vapour) condition have a total volume of
scarcely 1.4 10 9 km 3 /2-23/.
Table 2.5 shows the global distribution of the different occurrence modifica-
tions of water in terms of volume. Accordingly, with 0.001 % water vapour within
the atmosphere only accounts for very little of the entire water reserve of the
earth. with 2.15 % the proportion of ice is also comparably small. Thus, the over-
whelming majority of the global water reserves is liquid water with around
97.8 %, mainly concentrated in the oceans.

Table 2.5 Water reserves on the earth /2-23/

Volume in 10 3 km 3 Volume proportion in %

Water vapour in the atmosphere (volatile)
Water (liquid) in Rivers and streams
Fresh-water lakes
Groundwater
Oceans
Ice (solid) in polar ice and glaciers

ca. 13
ca. 1
ca. 125
ca. 8,300
ca. 1,322,000
ca. 29,200

ca. 0.001
ca. 0.00001
ca. 0.009
ca. 0.61
ca. 97.2
ca. 2.15

Total ca. 1,360,000 100.0

Water cycle. The described water reserve on the earth is continuously cycled by
the incident solar energy. This global water cycle is mainly fed by evaporation of
water from the oceans and, among other factors, the plants and continental waters
(Fig. 2.36). This evaporated water is transferred within the atmosphere as water
vapour by global and local wind circulation, and afterwards precipitates as e.g. in
the form of rain, snow, soft hail, or dew. Above oceans a little less water rains
down than is evaporated. This leads to correspondingly higher precipitation levels
on land and results in a net import of water from the oceans to the continents. The
resulting precipitation feeds snow fields, glaciers, streams, rivers, lakes and the
groundwater.
If the global total precipitation is related to the surface of the earth, a mean an-
nual precipitation of around 972 mm (1 mm equals 1 l/m 2 ) is obtained. But de-
pending on the local conditions these values may vary significantly. For example,
it never rains in certain desert areas for many years (e.g. Sahara Desert, Kalahari
Desert). At exposed sites however, for example at mountain ascents, up to
5,000 mm and more rainfall can be observed in the course of one year. On aver-
age, on the continents the mean annual precipitation is approximately 745 mm. Of
the water volume precipitating for example under Central European conditions
approximately 62 % evaporate directly or indirectly; the remaining 38 % flow off
as surface or groundwater. But these proportions are subject to change under other
conditions.