The Geographical Journal (Journal of the Royal Geographical Society): Volume X, No. 6 [‎114v] (81/186)

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POTAMOLOGY AS A BRANCH OF PHYSICAL GEOGRAPHY.
considerably (four or five tenths) less than the amount of precipitation. We must
assume, therefore, that only a part of the rainfall on the land is derived directly
from the sea. The other comes from the evaporation of rain-water already fallen
on the land. The proportion between the total rainfall of the region and the
quantity of river water escaping from it shows how many times the water of oceanic
origin has been precipitated and reprecipitated in that region. Taking, for example,
Bohemia, where the yearly rainfall is 17’3 inches, and the height of run-off is
4'8 inches, we must assume that 4'8 inches of water have been brought during the
year from elsewhere to Bohemia, and have fallen there three and a half times as
precipitation. Therefore high scientific interest is attached to the knowledge of
this proportion. On the other hand, the proportion between river water and the
rainfall of a region has high practical value. It allows one to determine the quan
tity of river water from the precipitation.
Hydro technologists have laid much stress on this proportion, the so-called
“ coefficient of run-off,” and it is generally thought that this might be a constant
factor for a distinct river. This is not correct. Recent investigations by Mr.
Newell, of the Geological Survey of the United States, and by myself have shown
that the coefficient of run-off varies not only from river to river, but also from
year to year for the same river. It varies with precipitation : the higher the rain
fall is, the higher also the coefficient of run-off'.
In Central Europe the height of run-off is zero up to a certain height of
precipitation. That corresponds to the well-proved fact that many dry countries
have no rivers. Over that height (nearly 17 inches), the height of the run-off
grows with that of the rest of precipitation, and is nearly three-quarters of the
latter. This rule holds good for different kinds of rivers, for those coming from
impermeable soil (Moldau in Bohemia, Enns), as well as for rivers from permeable
regions (Iraun, upper Elbe in Bohemia). It is correct for rivers coming from
woodlands (upper- March) and for rivers coming from steppes (lower March); the
quantity of run-off seems thus to be independent of the soil and of the covering
of a land with vegetation. 1 his is contrary to the ruling opinions, and the quantity
of river-water of a certain region seems only to be dependent on its climate, and
especially on its rainfall.
I his rule has been established only for a very limited part of the land surface.
Our knowledge of the quantity of water running in the rivers is restricted to the
smaller rivers in Central Europe and North America and to the Mississippi. The
huge volume of water in the St. Lawrence has, so far as I know, never been exactly
measured. We know nothing about the water masses in the Amazons and Congo.
It would be very desirable if geographical investigations were extended in this
direction, and if river-gaugings were made at the mouths of the larger watercourses.
I his will not only be of scientific interest. The bulk of water in a river multiplied
by its fall-height gives a measure of its energy. The more electricity will be
used, the more the watercourses will gain value as motive powers, and every
measurement of their volume will help to determine a force of nature.
Ihe fact that the quantity of river-water changes with the climate is very well
known. Prof. Woeikof has worked out a valuable classification of the rivers based
on the dependence of their high water on rainfall and the melting of the snow.
1 his classification is mainly based on facts taken from European and Asiatic rivers,
and it would be very desirable if data on the annual rise and fall of the rivers of
other continents would be gathered. Prof. Bruckner has shown how the mean
annual height of rivers changes in a cycle of thirty-five years. Indeed, the height
of a river may be taken as corresponding to its volume, but it is also influenced°by
the condition of its bed. Climatic changes will be shown more clearly by the