(2) The water-supply from all creeks and springs, and the catchment area.
(3) The amount of rain and snowfall.
(4) The formation and character of the ground, with especial reference to the amount of absorption and evaporation.
All
of these points must be thoroughly investigated and determined. It is
supposed that the catchment area has been ascertained, and that it is
sufficiently large for its minimum discharge to supply all requirements.
Elevation.—The
elevation of a reservoir depends upon the location of the mines and the
altitude and extent of the country which it is proposed to cover with
the ditch. The reservoir should be located below the snow belt wherever
possible, and so situated as to obtain the largest water-supply from
the catchment area.
Streams.—All the streams should be gauged carefully to determine the minimum and the average supply.
Rainfall.—In
new and unexplored localities the water-supply due to rainfall can be
determined only by actual measurement. It cannot be too earnestly
impressed upon the engineer that for all such information he must
depend on his own observations, which in some cases may require a
prolonged 'stay of a season or more in the field. Under any
circumstances rainfall data cannot be relied upon, unless based on many
decades of observation.
The
rainfall is always greater in mountain districts than in the lowlands.
It is greatest on the slopes facing the direction from which the moist
winds blow. Definite data of the rainfall of any catchment area can be
obtained only by establishing rain gauges at different points, where
the observations should be made daily during the season.
Snowfall.—The measurement of the snowfall must be taken on a level, and a given amount of snow reduced to water and calculated for rain.
Absorption and Evaporation. — In reference to the ground, the most desirable formation is that of com-
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