t to be, assuming a family of five persons using water at the
average rate of 25 gallons per head per day or 125 gallons each day.
Doubling this amount to take care of emergencies and of the extra water
used in hot weather, let us say that 250 gallons a day must be provided,
or 7500 gallons a month. If we could be sure of starting at the
beginning of any month with the tank full and that exactly thirty days
would be the period of no rainfall, then a tank holding 7500 gallons
would be the proper size. Unfortunately, with any month, as August, in
which the rainfall may be practically zero, the preceding month may also
have been so short of rain that the consumption was equal to or even
more than the rainfall, and the month of August would start with no rain
in the tank.

But if we take a three-month period, those inequalities will be averaged
and the supply will be, so far as one can foresee, ample in amount; that
is, we shall take the supply required in three months, namely, 22,500
gallons, and subtract from it the amount of water furnished in the three
months, which is presumably two thirds of the average rainfall on the
area contributing to the tank. The normal rainfall in three months is
three times 3-3/4 inches, or 11-1/4 vertical inches, and if this falls
on a roof area of, say, 2000 square feet, the total amount of water is
1850 cubic feet or 13,875 gallons, and two thirds of this is 9250. The
tank, then, must hold the difference between the 22,500 gallons and
9250, or 13,250 gallons, whereas a month's supply would be 7500 gallons.
The actual tank, therefore, is made to hold a little less than two
months' supply. Such a tank would be ten feet deep and fourteen feet
square, a good deal larger tank, of course, than one ordinarily finds
with a rain water-supply; but the estimate of the use of water has been
high and a long period of rainfall has been assumed, so that there is
little likelihood of a house with this provision being ever without
water.

_Computation for storage reservoir on a brook._

In determining the quantity of water that may be taken from a small
stream the area of the watershed answers the same purpose as the area of
the roof which delivers water into a tank, the only difference being
that from the roof all the water is always delivered, except a small
proportion that evaporates at the beginning of a rain in summer. From
the surface of a watershed, on the contrary, a large amount, and in
some ca