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6 Chapter 1 Introduction to Water Systems
icebergs move an average of 17 km (about 10 mi) a day. These icebergs originate from gla-
ciers, and may have an interior temperature of 15 to 20 C (5 to 4 F).
Municipal supplies may be derived from more than one source, the yields of available
sources ordinarily being combined before distribution. Dual public water supplies (see
Chapter 8) of unequal quality are unusual in North America. However, they do exist, for
example, as a high-grade supply for general municipal uses and a low-grade supply for
specific industrial purposes or firefighting. Unless the low-grade (nonpotable) supply is
rigorously disinfected, its existence is frowned on by health authorities because it may be
cross-connected, wittingly or unwittingly, with the high-grade (potable) supply. A cross-
connection is defined as a junction between water supply systems through which water
from doubtful or unsafe sources may enter an otherwise safe supply.

1.4 RAINWATER

Rain is rarely the immediate provenance of municipal water supplies. Instead, the capture
of rainwater is confined to farms and rural settlements usually in semiarid regions devoid
of satisfactory ground or surface waters. On homesteads, rainwater running off roofs is led
through gutters and downspouts to rain barrels or cisterns situated on or in the ground.
Storage transforms the intermittent rainfall into a continuous supply. For municipal serv-
ice, sheds or catches on ground that is naturally impervious or made tight by grouting, ce-
menting, paving, or similar means must usually be added.
The gross yield of rainwater is proportional to the receiving area and the amount of
precipitation. However, some rain is blown off the roof, evaporated, or lost in wetting
the collecting surfaces and conduits and in filling depressions or improperly pitched
gutters. Also, the first flush of water may have to be wasted because it contains dust,
bird droppings, and other unwanted materials. The combined loss may be high. A cut-
off, switch, or deflector in the downspout permits selective diversion of unwanted water
from the system. Sand filters will cleanse the water as it enters the cistern and prevent
its deterioration via the growth of undesirable organisms and consequent tastes, odors,
and other changes in attractiveness and palatability.
The storage to be provided in cisterns depends on the distribution of rainfall. Storage
varies with the length of dry spells and commonly approximates one-third to one-half the an-
nual consumption. If rainfalls of high intensity are to be captured, standby capacity must exist
in advance of filtration. Because their area is small, roofs seldom yield much water. A careful
analysis of storm rainfalls and seasonal variations in precipitation is, therefore, required.

EXAMPLE 1.1 CALCULATING THE VOLUME OF RAINFALL THAT CAN BE
COLLECTED FROM A BUILDING ROOF
Make a rough estimate of the volume in gallons or liters of water that can be caught by 3,000 ft 2
2
(278.7 m ) of horizontally projected roof area (the average area of American farm buildings) in a
region where the mean annual rainfall is 15 in. (38.1 cm).

Solution 1 (U.S. Customary System):
2
3
Gross yield 3,000 ft (15/12 ft) 7.48 gal/ft 28,100 gal annually 28,100 gal/
365 days 77 gpd.
Net yield approximates two-thirds gross yield 18,800 gal annually 51 gpd.
3
About half the net annual yield, or 9,400 gal 1,250 ft , must normally be stored to tide the
supply over dry spells.