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THROUGH NORTH ARMENIA TO ERIVAN 189
the outlet, which would make it possible to lower the level
of the lake when desired, while the sluice in the dam would
regulate to a nicety the amount that flowed out ; in this way
a large reserve of water would be under complete control,
ready to be let out in specially dry seasons.
But it was strange to see how small the river Zanga was ;
where it flowed into this enormous lake it was no more than
a big brook, and this seemed all the more remarkable when
there was so much snow lying on the mountains around.
Moreover, the rainfall there is not inconsiderable. According
to measurements tåken at six different stations over periods
varying from three to thirteen years—at three of them the
periods covered eleven to thirteen years—the average for the
whole area is calculated at 477 millimetres. lam inclined to
think this calculation too low, as the rainfall in the moun
tains is certainly greater than that recorded at these stations.
The explanation must be that the lake’s area of precipitation
is small in proportion to its surface, from which the evapora
tion is bound to be considerable. By map-measurements
I flnd that the surface-area is 1,360 square kilometres,1 while
the whole area of precipitation, including the lake, does not
exceed 4,590 square kilometres, i.e. 3-4 times the size of the
lake itself ; and if wc subtract the latter the land area is only
21 times the size of the lake. With an evaporation that is
probably rather rapid in such a climate all the year round,
including the winter,2 one could hardly expect that there would
be much surplus water to run away. A little water might also
escape by percolating down through the porous rock at the
bottom of the lake, though the amount would scarcely be
appreciable. That the evaporation from the surface of a lake
1 According to the calculation of sevcral Armenian and Russian investigators
the area of the lake is about 1,445 square kilometres at the highest water
observed, and about 1,400 square kilometres at the lowest water observed.
» It is probable that the evaporation from the ice-free surface of this lake is
very considerable, even in winter. As a rule, the temperaturc of the water will
then be a good deal higher than that of the cold air above it. The lower layers
of air will be warmed by the water and rise, carrying aqueous vapour with them.
In this way constantly rising currents of air will be formed, which will carry
off the vapour from the surface of the lake and in turn make way for new and
drier air. The evaporation from the lake will thus be considerable. Moreover,
the rainfall in winter is comparat vely small, and the water in the lake is then at
its lowest.
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