ses all of a stream, will be absorbed by the ground and by the
vegetation and will never be delivered into the stream which drains an
area. On large streams it is fair to assume that, on the average, only
one half of the rainfall on the area will reach the stream, while with
sandy soils this may be as small as 20 per cent. From December to May
inclusive, when the ground is frozen, when there is no vegetation to
absorb the water, and when evaporation is very light, practically all of
the rainfall reaches the streams. From June to August, on the other
hand, when the soil becomes rapidly parched, when vegetation is most
active, and when evaporation is high, frequently no rainfall reaches the
streams and the ground water sinks lower and lower, so that often
streams themselves dry up. It is necessary, therefore, in providing for
a definite quantity of water to be taken from a reservoir built on a
small stream, to make the reservoir large enough to furnish water from
June to September without being supplied with rain. This does not call
for a very large dam or a very large storage, and three months' supply
will usually be ample.

We have already estimated above that the quantity of water needed for
three months will be 45,000 gallons, or about 6000 cubic feet. If the
reservoir is built in a small gulley or ravine, its width may be
twenty-five feet. If the length of the reservoir or pond formed by the
dam is 240 feet, then the reservoir will furnish 6000 cubic feet for
every foot of depth, and a reservoir of that size holding one foot of
water will tide over a dry season.

Evaporation during these same three months will use up about a foot and
a half in depth over whatever area the reservoir covers, so that two
and a half feet in depth must be provided above the lowest point to
which it is desirable to draw off the water. It would be well to allow a
depth of at least ten feet in order to avoid shallow, stagnant pools,
and if this depth is provided, even more than the two-and-a-half foot
depth mentioned might be withdrawn in extremely dry seasons, though
perhaps at some reduction in the quality of the water.

_Deficiency from well supplies._

A large number of water-supplies in the country, perhaps the largest
number, at present comes from wells, either dug or drilled. It often
happens that after plumbing fixtures have been installed with a pump to
raise the water to the necessary elevated tank, the increased
consumption ca