pact
rocks, like granite, gneiss, or slates. Localities where the formation
consists of porous rocks, sandstones or limestones, are not desirable
on account of the great loss from absorption.
Steep
and denuded slopes are always the best, as but little water will
escape. The greatest slope will give the largest available quantity of
water. The configuration of the ground influences and affects
evaporation, and vegetation causes a large amount of absorption. The
losses due to absorption and evaporation are reduced to a minimum where
the site of a reservoir is in a compact formation with steep sides, and
the surface area is consequently small. Evaporation, varies with the
season of the year and the weather (being most active in summer), while
percolation, depending on the soil, varies from year to year.
Percolation is greatest during melting oi snows, and especially when a
thaw follows small falls of snow. From reliable experiments made in
France and England, the ratio of evaporation to rainfall was determined
(1839 to 1852) in the former to have been 76.57 per cent., and in the
latter, subsequently, 77.27 per cent.*
Finally,
it must be added that the rule for estimating the total quantity
available for storage varies in different districts. In some localities
two-thirds of the total amount is estimated to be serviceable, and in
others one-third. At the Bowman reservoir 75 per cent, of the total
rainfall and snowfall, reduced to rain, is stored.
Reservoir Grange.—
In the construction of reservoirs the location selected must be
sufficiently large to hold a supply necessary to meet a maximum demand.
The exact area of the reservoir should be determined, and a table
showing its contents for every foot of depth made, so that, from an
inspection of the gauge and reference to the table, the amount of water
available for service can always be known. A longitudinal section
through the centre ol the reservoir, with cross-sections and contour
lines, five
* Harcourt, " Rivers and Canals," p. 3.
