Water Supply, Use, and
Management
A Perspective on Water
-97% of water on Earth is located in oceans
-2% of water is freshwater contained in
icecaps and glaciers
The Earth’s water problems are not derived
from lack of water, but from the quality
and location of the available water.
The amount of water available for all of the
people, plants and animals is less than 1%
of the total.
The consumption rate of freshwater on
Earth is accelerating rapidly.
1975: Total Human use of water was about
3,850 km3/yr
Today: Total Human use of water is about
6,000 km3/yr
Water: a very cheap resource
Water has always been a favorite resource for
man, and has been used in great quantities.
Because of its great abundance, water has been
a very cheap resource, and fully exploited.
Ex: Las Vegas, Nevada
It was always cheap enough to subsidize large
canals and aqueducts to carry water to where
we needed it.
Ex: Roman aqueducts to carry water long
distances from wells to cities.
Recent events have shown that water is
NOT an unending supply, and that our
misuse of the resource is causing us to
lose our supply of drinking water.
The question now is, how do we manage
our dwindling resource, to maintain supplies
large enough?
Wars have been fought over the use of
water, and will only get worse if we don’t
get it right.
The U.S. Water resources Council estimates
that water use in the U.S. will exceed our
surface supplies by 13% in 2020.
That means that we are increasingly going
to rely on groundwater as a source of our
drinking water.
Groundwater generally refers to water below
the water table, where saturated conditions
exist. ( i.e. deep in the ground)
Water from the surface evaporates, runs off,
or soaks in and is transported underground.
Areas where water infiltrates the ground are
called recharge zones.
Areas where water seeps out of the ground,
such as springs, are called discharge
zones.
An aquifer is an area underground with
spaces to hold water, such as in gravel ,
sand, or fractured rock deposits.
Effluent vs. Influent Streams
Effluent streams are maintained during
dry seasons by seepage from groundwater.
Influent streams are entirely above the
water table, and flow only in response to
precipitation.
Influent streams are also sometimes called
ephemeral streams, because they don’t
flow all year.
Groundwater and Surface Water Interactions
Because groundwater and surface water are
so interrelated, pollution of one usually
causes the pollution of the other.
The quality of both is reduced by urban and
agricultural runoff, which add nutrients and
fertilizers, oils from roads, and nutrients
from treated waste.
Pollution within a groundwater resource may
last a very long time, even if the pollution is
stopped, due to the slow turnover rate of
groundwater.
Groundwater Overdraft
In areas where water is being removed faster
than it is being replaced, a condition called overdraft exists.
Overdraft can lead to a variety of ecological
problems, such as damage to river and wetland
ecosystems, and land subsidence.
In the Texas-Oklahoma High Plains area, pumping
of water for agriculture has lowered the aquifer
more than 15 meters in some areas.
Water Level
Changes Due
To Overdraft
Of the Ogallala
Aquifer.
Trends in Water Usage
Withdrawals of surface water in the U.S. far
exceed withdrawals of groundwater.
Water usage reached a peak in 1980, and
has leveled off, despite an increase in total
population.
This suggests an increase in efficiency and
better management and conservation.
Water Use
-Off stream: water removed from its source
-often returned after use
(industrial cooling)
Consumptive Use: similar to off-stream, but
is used up by plants, animals, or industry
In-stream: navigation, hydroelectric
generation, fish and wildlife habitat
Water Diversion:
How much can be removed from a body of
water without damaging the ecosystem?
EX: Pacific Northwest, where salmon and
trout are declining due to water diversions
for agriculture.
EX: the Aral Sea, where water was diverted
from two rivers for cotton agriculture.
Trends in United States fresh groundwater and
surface-water withdrawals and human population
(1950-1995).
The greatest consumers of water are agriculture and the thermoelectric (power
generation) industry.
Although the use by agriculture and industry
has leveled off, use for public water supply
has risen.
This rise is due to the increased population.
Fig 20.11 Trends in United States water withdrawals
by water-use category and total withdrawals (19601995).
Man’s Impact on Water Movement
Channelization: straightening, deepening,
clearing and widening of an existing stream.
May also involve lining the canal with cement
stone or metal to prevent erosion.
Effects:
Water movement is much faster, with less
structure for aquatic habitat.
Lower numbers of aquatic insects, fish and
Birds.
The channelization of a stream results in a
degraded ecosystem, more prone to flooding
and erosion.
Increased downstream flooding, due to the
increased water flow, and lack of things to
slow the water down.
2003: Parts of the Brandywine, which were
Channelized, are returned to their natural
Channels, and rocks are returned.
FROM
TO
Canals and the environment
Canals are not the same hydrologically as streams
or rivers.
Canals tend to be straighter, with steep sides,
and are designed to carry water from one place
to another.
The water in canals often moves quickly, and they
can present a safety hazard to people and animals
The development of canals has led to the movement of unwanted organisms from place to place
Ex: snails that carry disease (schistosomiasis)in
Egypt. Theses snails were flushed out each year
by flooding of the Nile, but canals have left them
with areas that are not flushed, so they stay.
Ex: The Panama Canal has allowed species from
the Atlantic and pacific to mix, reducing diversity
of marine areas around both ends of the canal.
Building on a Floodplain
Floodplains are areas along rivers where flood
Waters can go, and lose their sediment.
They are areas which should NOT be built on,
because eventually they WILL flood.
They are generally designated by the number of
years between anticipated flood.
EX: A “100 year flood plain” is expected to be
flooded every 100 years or so.
Building on a floodplain also contributes to
flooding downstream , as the additional water
runoff is added to the stream.
Areas which previously did not flood will now
flood due to the increased water volume,
which should have been absorbed by the
floodplain upstream.