Page 43 - Water Engineering Hydraulics, Distribution and Treatment

Chapter2
Water Sources: Surface Water
Figure 2.1 shows where water is and how it is distributed
with stream flow moving water toward the oceans. Runoff and
on Earth. The bar on the left shows where the water on Earth A portion of runoff enters rivers in valleys in the landscape,
exists; about 97% of all water is in the oceans. The middle groundwater seepage accumulate and are stored as freshwater
bar shows the distribution of the rest 3% of all Earth’s water, in lakes.
which is freshwater. The majority, about 69%, is locked up Not all runoff flows into rivers, though. Much of it soaks
in glaciers and ice caps, mainly in Greenland and Antarctica. into the ground as infiltration. Some water infiltrates deep
You might be surprised that of the remaining freshwater, into the ground and replenishes aquifers (saturated subsur-
almost all of it is below our feet, as groundwater. No matter face formations), which store huge amounts of freshwater for
where on Earth one is standing, chances are that, at some long periods of time. Some infiltration stays close to the land
depth, the ground below is saturated with water. Of all the surface and can seep back into surface-water bodies (and
freshwater on Earth, only about 0.3% is contained in rivers the ocean) as groundwater discharge, and some groundwater
and lakes—yet rivers and lakes are not only the water we are finds openings in the land surface and emerges as freshwater
most familiar with, but also where most of the water we use springs. Over time, though, all of this water keeps moving;
in our everyday lives exists. some reenters the oceans, where the water cycle “ends”—or,
For a detailed explanation of where Earth’s water is, where it “begins.”
look at the data in Table 2.1. Notice how of the world’s
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total water supply of about 326 million mi (1360 million 2.1 SOURCES OF SURFACE WATER
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km ), more than 97% is saline. Also, of the total freshwater,
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about 7 million mi (29 million km ) is locked up in ice In the United States by far the largest volumes of municipal
and glaciers. Another 30% of freshwater is in the ground. water are collected from surface sources. Possible yields vary
Thus, surface-water sources only constitute about 30,300 directly with the size of the catchment area, or watershed, and
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mi (126,300 km ), which is about 0.009% of total water. with the difference between the amount of water falling on it
What interconnects groundwater and surface water is the and the amount lost by evapotranspiration. The significance
water cycle (Fig. 2.2). The water cycle has no starting point, of these relations to water supply is illustrated in Fig. 1.1.
but we will begin in the oceans, because that is where most of Where surface-water and groundwater sheds do not coincide,
Earth’s water exists. The sun, which drives the water cycle, some groundwater may enter from neighboring catchment
heats water in the oceans. Some of it evaporates as vapor into areas or escape to them.
the air. Ice and snow can sublimate directly into water vapor. Communities on or near streams, ponds, or lakes may
Rising air currents take the vapor up into the atmosphere, withdraw their supplies by continuous draft if stream flow
along with water from evapotranspiration, which is water and pond or lake capacity are high enough at all seasons of
transpired from plants and evaporated from the soil. The the year to furnish requisite water volumes. Collecting works
vapor rises into the air where cooler temperatures cause it to include ordinarily (a) an intake crib, gatehouse, or tower;
condense into clouds. Air currents move clouds around the (b) an intake conduit; and (c) in many places, a pumping
globe; cloud particles collide, grow, and fall out of the sky station. On small streams serving communities of moderate
as precipitation. Some precipitation falls as snow and can size, intake or diversion dams can create a sufficient depth
accumulate as ice caps and glaciers, which can store frozen of water to submerge the intake pipe and protect it against
water for thousands of years. Snowpacks in warmer climates ice. From intakes close to the community the water must
often thaw and melt when spring arrives, and the melted generally be lifted to purification works and thence to the
water flows overland as snowmelt. Most precipitation falls distribution system (Fig. 2.3).
back into the oceans or onto land, where, due to gravity, Because most large streams are polluted by wastes
the precipitation flows over the ground as surface runoff. from upstream communities and industries, their waters

Water Engineering: Hydraulics, Distribution and Treatment, First Edition. Nazih K. Shammas and Lawrence K. Wang.
© 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc.

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