Sustaining Our Freshwater Resources
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After reading this chapter, you should be able to
Describe how New York City worked with nature to improve its water supply.
Illustrate the water cycle and how the planet’s water is distributed.
Define different types of water use.
Analyze the methods used to meet global water demand.
Describe the potential for global conflict over water.
Describe different types of water pollution and ways to manage that pollution.
Differentiate between the hard path and soft path approaches to water management.
Discuss the role of forests in water management.
When viewed from space, Earth is a watery planet, with oceans covering over 70% of the planet’s surface and
glaciers, ice caps, lakes, rivers, and streams covering another 10%. Yet water shortages and access to clean, safe
drinking water are a serious problem in virtually every region of the world. The abundance of ocean water is
too salty for human use, and much of the freshwater is either polluted or inaccessible.
Given its importance and critical role in all human life, it is remarkable how poorly managed water is as a
resource. We regularly use rivers, streams, and the oceans as a dumping ground for our wastes and allow
contaminants like spilled oil and agricultural chemicals to pollute critical groundwater supplies. We dam rivers
and use massive amounts of energy to pump water hundreds of miles to irrigate golf courses and suburban
lawns in the middle of deserts. And we pay little attention to how the management—or mismanagement—of
natural capital resources like forests, wetlands, and other open spaces impacts water quality in surrounding
This chapter will examine issues of freshwater management and consider the challenges of both water quantity
and water quality. The next chapter will examine issues and challenges associated with our oceans.
We will first discuss issues of water quantity, which involve ensuring that there are adequate supplies and that
mismanagement of water does not result in flooding. Only a tiny fraction of water on the planet is accessible
and suitable for human consumption, making wise water management a critical priority. We’ll also see that just
as with other critical resources like food and energy, water use varies greatly in different regions of the world.
We will then consider issues of water quality, which involve ensuring that water is safe to use. Lastly, we will
look at ideas and approaches for water conservation and sustainable water management, including efforts both
to increase the availability of water on the supply side and to reduce usage on the demand side.
5.1 Case Study: New York City’s Water Supply
New York City has long prided itself on the quality of its municipal drinking water, with some residents and city
boosters going so far as to call it the “champagne of tap water.” Over the years the city has garnered awards for
the quality of its water relative to other major cities in the United States, and chefs and food experts have
debated whether the city’s water might have something to do with the quality of its pizza and bagels. A
Southern California–based pizza business even goes so far as to spend $10,000 a year to have New York City
tap water trucked across the country to use in making dough for its New York–style pizza.
The story of why New York City’s water quality is so good and how the city addressed contamination can help
us begin to understand the issues discussed in this chapter and the importance of sustaining freshwater
Building a Water Supply System
As far back as the 1830s, city leaders in New York knew that, in order for the city to grow and thrive, they
needed to do something about their water supply situation. At the time, the city drew its water from a
patchwork of ponds, springs, and underground wells, but overuse and poor waste management were affecting
both the quantity and the quality of the city’s water supply. Massive fires burned through wood-framed
buildings because water pressure was too low to fill fire hoses. Overpumping of wells led freshwater levels to
fall below sea level, allowing the nearby ocean to seep in and contaminate groundwater supplies. The raw
sewage and animal waste being dumped in the streets ran off and contaminated ponds and small reservoirs.
After a cholera epidemic (due in large part to poor water quality) killed thousands in 1832 and the Great Fire
of New York burned 17 city blocks in 1835, city leaders embarked on a massive water development project that
would change the course of New York City history. A dam was built on the Croton River north of the city, and a
65-kilometer (40-mile) covered aqueduct was built to carry water from there to the middle of Manhattan,
where Central Park is located today. When the new water supply system opened in 1842, it carried 340 million
liters (90 million gallons) of clean water every day to the thirsty city.
Sixty years later, the system was expanded on as city officials
sought to prevent water shortages and inadequate supply while
New York City grew and expanded. Water development projects
were undertaken further north and west of the city in the
Catskill Mountain region. An entire series of dams, reservoirs,
aqueducts, and tunnels were constructed in the early 1900s,
and by 1915 the Catskill Aqueduct was in operation.
Today New York City’s water supply system is still based almost
entirely on the projects from the 1800s and early 1900s. Each
day over 4.5 billion liters (1.2 billion gallons) of water are
delivered to New York City’s 9 million residents, with 10% of
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this water coming from the Croton portion of the system and
90% originating from the Catskill portion. The Catskill The Ashokan Reservoir in the Catskill
watershed region, over 160 kilometers (100 miles) away from Mountains is one of several to provide
the city, draws water from 19 reservoirs and 3 lakes spread out New York City with its water supply.
over a 500,000-hectare (2,000-square-mile) area. A watershed
is an area of land where sources of water (streams, creeks) flow together to a single destination. These lakes
and reservoirs are connected to the city by 10,000 kilometers (over 6,200 miles) of pipes, tunnels, and
aqueducts. Because of differences in elevation, almost the entire system moves water through gravity, with a
drop of water taking anywhere from 3 months to 1 year to travel from an upstate lake or reservoir to a
customer in the city. As the water approaches the city, it’s treated with chlorine to kill germs and pathogens, as
well as fluoride for dental health and a couple of other chemicals to prevent corrosion of pipes.
Unlike most major urban water systems, New York City’s drinking water is not filtered. In fact, New York has
the largest unfiltered drinking water system in the United States. New York’s water supply reservoirs were
built in upstate areas that were covered in forests and that also had vast areas of intact wetlands. These forests
and wetlands act as natural sponges and filters, absorbing rainfall and snowmelt and purifying the water in the
process. Many other cities that draw their drinking water from nearby lakes and rivers need to have expensive
filtration systems to remove sediment and other particles and contaminants before distributing water to
Learn More: New York City’s Water Supply
To get a sense of how vast the Catskill watershed region is, visit the following link:
Expanding Ecosystem Management
By the 1990s, however, things began to change for the worse in terms of New York City’s drinking water.
Increased development, road building, suburban sprawl, and other activities in the Catskill region were having
a negative impact on water quality in surrounding reservoirs and lakes. U.S. Environmental Protection Agency
(EPA) inspectors warned the city that it might have to build a $10 billion water filtration plant to address the
Instead, New York City decided to take a different approach. The 1997 Watershed Memorandum of Agreement
(MOA) was negotiated between New York City, New York State, the EPA, environmental groups, and
municipalities and townships in the Catskills region. The MOA committed New York City to spend just under $2
billion on a range of initiatives intended to improve water quality in the Catskill reservoirs. These initiatives
included purchasing and protecting lands surrounding reservoirs and lakes, as well as paying nearby
landowners who agreed not to develop their lands commercially. In addition, the city helped upstate
communities improve wastewater treatment plants, assisted dairy farmers with manure management, and
worked with road departments to ensure that runoff from roads and highways was not entering reservoirs.
Lastly, the city provided funding for upstate home owners to upgrade septic systems and for forest landowners
to improve forest management practices. Collectively, these approaches are known as ecosystem
management because they focus on maintaining water quality at the source rather than cleaning the water as
it reaches its destination. Over the past 20 years, the ecosystem management initiatives undertaken as part of
the MOA have proved effective enough that the EPA has granted New York City a series of “filtration avoidance
determinations” that allow the city to operate its water system without a filtration plant.
The ecosystem management approach has been supplemented with high-tech features, including a network of
hundreds of robotic buoys deployed across reservoirs to continually test and monitor water quality. These
robotic water quality monitors test over 1.9 million water samples each year. In addition, the city has recently
put in place the world’s largest ultraviolet water disinfection facility. Water passes through containers mounted
with ultraviolet lights that kill any microorganisms that might contaminate the water and make consumers
While New York City water officials must always be vigilant in ensuring the quality of the city’s water, the
success of the MOA initiatives points to the importance of “source management” as an approach to meeting our
water needs. Rather than spend $10 billion building a water filtration plant to treat polluted water at the back
end of the system, New York City spent one fifth of that amount to ensure that its drinking water was not
polluted at the source in the first place. Essentially, New York City has been investing in the natural capital
resources of forests and wetlands in the Catskills region and letting this natural infrastructure provide the
ecosystem service of keeping the city’s water clean.
5.2 Freshwater Systems
Water is perhaps the most critical resource to human well-being and survival. Our bodies are made up of as
much as 60% water, and while healthy individuals can survive weeks without food, they would last only a few
days without water. We also rely on water to grow food, produce energy, and manufacture just about
everything imaginable. In addition, we depend on and benefit from a range of ecosystem functions and services
provided by water, including transportation, recreational activities, and wildlife habitat. We regularly rely on
rivers, streams, and oceans to dilute and purify our waste products, although this use frequently conflicts with
the other ecosystem functions and services that water provides. Despite all the ways we depend on water, we
seldom give much thought to where it comes from and how it gets to us.
It’s been said that we live on a “blue planet,” since water covers nearly three fourths of the Earth’s surface.
However, when we account for where water is located and what condition it is in, we realize that water is not
only a critical natural capital resource but also a scarce one. How can it be that such an abundant resource can
also be scarce at the same time?
Imagine the world’s water as 1 million individual 1-gallon containers. (In reality, there are 370 million trillion
gallons.) For starters, about 970,000 (97%) of those containers would be filled with salty ocean water
unsuitable for human consumption. It was this reality that inspired the line from The Rime of the Ancient
Mariner, “water water everywhere, nor any drop to drink” (Coleridge, 1919/1990, lines 121–122). Another
26,100 gallons (2.61%) would be filled with ice and snow—nearly all of it from ice caps and glaciers in the
Arctic and Antarctic regions, far from major human populations. Roughly 3,600 gallons (0.36%) would be filled
with groundwater, with much of this (but not all, as we will learn) consisting of salt water also unsuitable for
Out of the 1 million gallons we started with, only 300 gallons remain. Some of those 300 gallons consist of
water vapor in the atmosphere, water found in saline or salty lakes, or water in the soil, leaving just about 180
gallons (0.018%) of fresh surface water—water on the surface of the Earth, found in rivers, wetlands, lakes,
and reservoirs. Because this fresh surface water is the primary source of water for most people on the planet,
we can see just how scarce and precious this resource actually is. (See Figure 5.1.)
Figure 5.1: Water distribution
Only 0.6% of the world’s freshwater—0.018% of all water
on Earth—is readily available as surface water for human
Source: Data adapted from “Where Is Earth’s Water?” by US Geological
Survey, n.d. (https://www.usgs.gov/media/images/distribution-wate
Thankfully, nature has a way of constantly recycling, replenishing, and purifying water sources. In fact, unlike
other resources (such as fossil fuels) that are permanently “consumed,” global water supply is more or less
fixed. This is because of the global hydrologic cycle. The hydrologic cycle, or water cycle, describes the
movement of water between the planet’s surface, atmosphere, soil, oceans, and living organisms. If we think
again of our 1 million gallon containers, the water cycle is constantly moving water among the different
containers, although human activities are increasingly interfering with this process and further complicating
effective water management.
The global water cycle is driven primarily by solar energy. Heat from the sun causes water to evaporate from
surface waters and land surfaces and enter the atmosphere as water vapor. For example, it’s estimated that
solar energy evaporates roughly 425,000 cubic kilometers (km3) of ocean water each year. To put that in
perspective, just 1 cubic kilometer of water is equivalent to a tank of water that is 1,000 meters (3,280 feet) tall,
wide, and long, or 1 trillion liters (265 billion gallons). The amount of energy it takes to move this much water
from the ocean to the atmosphere is massive. Roughly one third of all the solar energy striking the Earth each
day is used to drive evaporation.
In addition to evaporation, plants draw massive amounts of water from the soil and release some of that water
to the atmosphere as water vapor through a process known as transpiration. Evaporation and transpiration are
together known as evapotranspiration. As water vapor from evapotranspiration rises into the atmosphere, it
cools and condenses to form clouds (condensation) before falling back to Earth as rain and snow
(precipitation). Evaporation, transpiration, condensation, and precipitation form the basis of the water cycle
(see Figure 5.2).
Figure 5.2: The water cycle
The basis of the hydrologic cycle is condensation, precipitation, and evapotranspiration.
Once water reaches the ground, it either runs off into nearby bodies of water or
infiltrates the surface, where it reaches the water table and underground aquifers.
Source: Based on “Ground Water and Surface Water a Single Resource,” by US Geological Survey, 2013 (https://p
ubs.usgs.gov/circ/circ1139/ (https://pubs.usgs.gov/circ/circ1139/) ).
The processes of evaporation and condensation purify water naturally because only water molecules are pulled
into the atmosphere, leaving any salts, contaminants, or pollutants behind. This is basically the same as making
distilled water by boiling water and condensing the vapor. Roughly 90% of the ocean water evaporated each
year falls back as precipitation over the oceans, where it mixes again with salt water. However, about 10% of
that moisture falls over land surfaces as freshwater precipitation.
An even larger amount of freshwater precipitation is provided by evapotranspiration from plants and forests.
In tropical forests as much as 80% of all precipitation comes from the direct recycling of evapotranspiration
from plants. This feedback loop—more trees leading to more transpiration leading to more precipitation
leading to more trees—is a key reason why forest management is so tightly linked with water management.
Overall, of the 110,000 km3 of precipitation that falls over land surfaces each year, it’s estimated that roughly
one third comes from moisture drawn from ocean waters and two thirds from moisture from
evapotranspiration from plants. This 110,000 km3 of precipitation ends up doing one of three things. First,
about two thirds of that water evaporates back into the atmosphere from land surfaces or through plant
transpiration. The other one third either flows over land and enters rivers, streams, and lakes (surface water)
or gradually percolates through soil and rock to enter underground aquifers (groundwater). It’s this relatively
small amount of water, roughly 37,500 km3 per year, that replenishes the tiny sliver of fresh surface water
illustrated in Figure 5.1 and represents the total renewable supply of fresh surface water on the planet. As with
most other resources, this freshwater supply is unevenly distributed around the world. Atmospheric
circulation patterns, topography, and proximity to water sources and forests are all factors that influence the
amount of precipitation in a given location.
Learn More: Water Cycle
This animated video reviews the water cycle in more detail.
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Human Impact on the Water Cycle
Human activities can also affect precipitation patterns and what happens to that precipitation after it falls to
Earth. Under normal conditions, as precipitation reaches the ground, some of it is pulled below the surface by
gravity through a process known as infiltration. This water eventually reaches the water table, a depth below
ground where soil and rock are completely saturated with water. The saturated area immediately below the
water table is known as an aquifer, an area of permeable rock and sediment from which water can be
Many communities, private home owners, factories, and farmers use pumps to pull groundwater from aquifers
to the surface. As long as rates of infiltration are the same or greater than rates of extraction, the water level in
the aquifer will be maintained. However, this is often not the case, and overpumping is resulting in aquifer
depletion in many locations, such as with the Ogallala Aquifer in the U.S. Midwest (recall Chapter 4). As New
York City discovered in the 1830s, overpumping of water from aquifers near the ocean can ...
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