But this information will be of but little service to the engineer
without an investigation of the loss due to evaporation and absorption,
varying with the season of the year and the more or less degree of
saturation of the soil; the amount of absorption depending upon the
character of the ground, dip of strata, etc., the hydrographic area
being, as a rule, by no means equal to the topographic area of a given
basin. From this cursory view of the preliminary investigations necessary
can be realized what difficulties must attend the design of dams for
reservoirs in newly settled or uncivilized countries, where there are no
data of this nature to go on, and where if maps exist they are probably
of the roughest description and uncontoured; so that before any project
can be even discussed seriously special surveys have to be made, the
results of which may only go to prove the unsuitability of the site under
consideration as regards area, etc. The loss due to evaporation,
according to Mr. Hawksley, in this country amounts to a mean of about 15
in.; this and the absorption must vary with the geological conditions,
and therefore to arrive at a satisfactory conclusion regarding the amount
of rainfall actually available for storage, careful gaugings have to be
made of the stream affected, and these should extend over a lengthened
period, and be compounded with the rainfall. A certain loss of water, in
times of excessive floods, must, in designing a dam, be ever expected,
and under favorable conditions may be estimated at 10 per cent. of the
total amount impounded.

As regards the choice of position for the dam of a reservoir, supposing
that it is intended to impound the water by throwing an obstruction
across a valley, it may be premised that to impound the largest quantity
of water with the minimum outlay, the most favorable conditions are
present where a more or less broad valley flanked by steep hills suddenly