Page 68 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch02_029-060.qxd 8/2/10 9:14 PM Page 31

2.1 Sources of Surface Water 31

The Water Cycle

Water storage
in ice and snow
Water storage in the atmosphere
Condensation
Sublimation
Precipitation Evapotranspiration

Evaporation
Surface runoff
Snowmelt runoff
to streams Stream flow
Infiltr
Infiltration Evaporation
ation
Infiltration
Spring
Freshwater
storage
Water storage
in oceans
Groundwater discharge
Groundwater storage

Figure 2.2 The Water Cycle
(Courtesy of USGS)

by evapotranspiration. The significance of these relations to water supply is illustrated in
Fig. 1.1. Where surface-water and groundwater sheds do not coincide, some groundwater
may enter from neighboring catchment areas or escape to them.
Communities on or near streams, ponds, or lakes may withdraw their supplies by con-
tinuous draft if stream flow and pond or lake capacity are high enough at all seasons of the
year to furnish requisite water volumes. Collecting works include ordinarily (a) an intake
crib, gatehouse, or tower; (b) an intake conduit; and (c) in many places, a pumping station.
On small streams serving communities of moderate size, intake or diversion dams can cre-
ate a sufficient depth of water to submerge the intake pipe and protect it against ice. From
intakes close to the community the water must generally be lifted to purification works and
thence to the distribution system (Fig. 2.3).
Because most large streams are polluted by wastes from upstream communities and
industries, their waters must be purified before use. Cities on large lakes must usually
guard their supplies against their own and their neighbors’ wastewaters and spent indus-
trial process waters by moving their intakes far away from shore and purifying both their
water and their wastewater. Diversion of wastewaters and other plant nutrients from lakes
will retard lake eutrophication.
Low stream flows are left untouched when they are wanted for other valley purposes
or are too highly polluted for reasonable use. Only clean floodwaters are then diverted into
reservoirs constructed in meadowlands adjacent to the stream or otherwise conveniently
available. The amount of water so stored must supply demands during seasons of unavail-
able stream flow. If draft is confined to a quarter year, for example, the reservoir must hold
at least three-fourths of a community’s annual supply. In spite of its selection and long
storage, the water may still have to be purified.
In search of clean water and water that can be brought and distributed to the commu-
nity by gravity, engineers have developed supplies from upland streams. Most of them are