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Chapter 2

Water Sources: Surface Water

Figure 2.1 shows where water is and how it is distributed on Earth. The bar on the left
shows where the water on Earth exists; about 97% of all water is in the oceans. The middle
bar shows the distribution of that 3% of all Earth’s water that is freshwater. The majority,
about 69%, is locked up in glaciers and ice caps, mainly in Greenland and Antarctica. You
might be surprised that of the remaining freshwater, almost all of it is below our feet, as
groundwater. No matter where on Earth one is standing, chances are that, at some depth,
the ground below is saturated with water. Of all the freshwater on Earth, only about 0.3%
is contained in rivers and lakes—yet rivers and lakes are not only the water we are most
familiar with, the are also where most of the water we use in our everyday lives exists.
For a detailed explanation of where Earth’s water is, look at the data in Table 2.1.
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Notice how of the world’s total water supply of about 326 million mile (1,360 million km ),
more than 97% of it is saline. Also, of the total freshwater, about 7 million mile 3
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(29 million km ) is locked up in ice and glaciers. Another 30% of freshwater is in the
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ground. Thus, surface-water sources only constitute about 30,300 mile (126,300 km ),
which is about 0.009% of total water.
What interconnects groundwater and surface water is the water cycle (Fig. 2.2). The
water cycle has no starting point, but we will begin in the oceans, because that is where
most of Earth’s water exists. The sun, which drives the water cycle, heats water in the
oceans. Some of it evaporates as vapor into the air. Ice and snow can sublimate directly
into water vapor. Rising air currents take the vapor up into the atmosphere, along with
water from evapotranspiration, which is water transpired from plants and evaporated
from the soil. The vapor rises into the air where cooler temperatures cause it to condense
into clouds. Air currents move clouds around the globe; cloud particles collide, grow,
and fall out of the sky as precipitation. Some precipitation falls as snow and can accumu-
late as ice caps and glaciers, which can store frozen water for thousands of years.
Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted
water flows overland as snowmelt. Most precipitation falls back into the oceans or onto
land, where, due to gravity, the precipitation flows over the ground as surface runoff. A
portion of runoff enters rivers in valleys in the landscape, with stream flow moving water
toward the oceans. Runoff, and groundwater seepage, accumulate and are stored as
freshwater in lakes.
Not all runoff flows into rivers, though. Much of it soaks into the ground as infiltration.
Some water infiltrates deep into the ground and replenishes aquifers (saturated subsurface
formations), which store huge amounts of freshwater for long periods of time. Some infiltra-
tion stays close to the land surface and can seep back into surface-water bodies (and the

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