1.
2.
List at least TWO
examples of
density dependent
limiting factors.
Using the graph to
the right, predict
what will happen
to Hungary’s
population.
Why is water so important to us, and how can
we preserve it?

Recall our earlier studies of the water cycle.
We examined the water cycle as a means of
recycling fresh water on Earth.
◦ In the water cycle, water evaporates from the
planet’s bodies of water (mainly the oceans). Water
then condenses and forms clouds; eventually, it
falls back to Earth in the form of precipitation.
◦ This runoff either forms surface or groundwater
sources, or it returns to the oceans.


While fresh water is a renewable resource, it is
still quite limited. 70% of the Earth’s surface is
covered by water, but only 3% of all water is
freshwater.
Furthermore, only 30% of all freshwater is
accessible – most of it is frozen in glaciers and
icecaps. Most of our accessible freshwater
consists of groundwater.


Surface water sources include lakes, rivers,
streams and wetlands.
Historically, cities were placed in close
proximity to water sources, especially rivers.
Even today, most large cities depend on
surface water for operation.
◦ Water is used not just for drinking, but for
agricultural and industrial processes, as well as a
source of food and transportation. We will discuss
this in greater detail later.


Rivers and streams form from runoff, melting
snow, and groundwater. Streams eventually
combine to form tributaries and rivers.
A network of flowing water is called a river
system. Each river system drains the land
around it; the area of land being drained is
called a watershed.


Most of our available fresh water is located
underground. Groundwater is formed when
water percolates through the soil, and
collects within sedimentary and rock
formations.
As water collects in groundwater sources, it
begins to saturate the rock and soil around it.
The water table is the boundary between fully
saturated and unsaturated earth.


Despite its name, the water table is not level,
and it is not static. It changes due to geography,
rainfall conditions and human interaction.
When the water table reaches the surface,
springs, rivers, lakes and oases can form.

A fully underground formation of
groundwater is called an aquifer. Aquifers
are often found in rock, sand and gravel,
since these materials often have space
where groundwater can accumulate.

Many different types of rocks have small
pores, where water can leak through.
Porosity is the percentage of a rock’s
volume that is made up of pores and empty
space.
◦ The greater a rock’s porosity, the more water it
can hold before it becomes saturated.

Permeability is the ability of rock or soil to
allow water to pass through them.
◦ For instance, gravel is permeable, since it permits
water flow. In contrast, clay and granite are not.

As a result, aquifers tend to form within
permeable materials, and can be contained
around impermeable materials.


The land right above an aquifer (where
water trickles down into the aquifer) is
called the recharge zone.
Recharge zones are particularly vulnerable,
since pollution typically enters the aquifer
at this point.


Human development can also block up the
recharge zone. This is common with parking
lots, which contain impermeable materials.
Note that aquifers often take a long time to
recharge. As such, it is vital to protect recharge
zones to the best of our ability.


People have three major uses for water:
residential, agricultural and industrial.
Although most fresh water worldwide is used
for irrigation, local water usage does
fluctuate with availability of water, local
population size, and economic conditions.
◦ For instance, compare Asia and Europe’s water use;
Asia uses over 80% of its fresh water for agriculture,
while Europe only uses 34% of its fresh water for
this purpose.


Residential water use includes all personal
use of water, both inside (drinking, cooking,
cleaning) and outside (grounds keeping,
maintenance) the home.
Residential water use is a small proportion of
total water use all other the world. Latin
America and Oceania use the highest
percentage of water for residential use
(~20%), while Asia and Africa use the lowest
percentage (~10%).

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Most water for residential use that has been
treated to the point it can be safely drank is
considered potable.
Water treatment removes pathogens –
organisms that can cause disease – as well as
hazardous materials, like mercury, arsenic
and lead.
We will discuss water treatment in greater
detail when we look at water pollution.

Worldwide, 19% of all
fresh water is used
for industrial
processes. Industrial
water use contains
three major factors:
◦ Manufacturing of
goods
◦ Waste disposal
◦ Generation of
power/electricity

Most industrial water
is used to cool power
plants. Water is often
pumped from a river
or lake, and is
pumped back after it
runs through a
cooling tower.
◦ While this water is
typically clean and
reusable, it can also be
a major source of
thermal pollution.

Two-thirds of all fresh water around the
world is used for agriculture. Irrigation is the
process of providing water to plants (outside
of typical rainfall).
◦ However, many irrigation systems are not efficient.
In a poor system, as much as 80% of the irrigation
water evaporates, never reaching a root system.
◦ High-pressure sprinklers are a common example of
poor irrigation.

Many irrigation techniques are used today,
often depending on crops and geography.

Water management projects are meant to
meet the needs of human water supply.
◦ This can include anything from providing potable
water for consumption, to providing irrigation water
for crops in an arid region, to providing
hydroelectric energy.

Water management projects are vital for
allowing human communities to exist in
regions relatively far from obvious water
sources.


Water diversion refers to channeling all or
part of a river into man-made canals. This is
done to tap the water for consumption,
irrigation or energy production.
Excessive diversion can result in drying up
the river, and as a result, drying up wetlands
and floodplains that depend on the river’s
water flow to exist.
◦ For example, the Colorado river has been so heavily
diverted that the river no longer reaches the Gulf of
California, except in the wettest months.

This is a
photograph of a
diversion dam
along the Colorado
River. Note how it
draws water away
from the river’s
natural water flow.



Dams, as discussed previously, are used to
limit a river’s natural flow. Once a river is
dammed up, a reservoir (artificial lake) is
formed behind the dam.
Reservoirs can provide water for residential,
industrial and agricultural use. By hindering
the river’s flow, they also provide a means of
flood control.
However, they also disrupt communities and
ecosystems behind and in front of the dam.

Water conservation
is the act of
rationing water
sources, either by:
◦ Limiting water use
◦ Increasing the
efficiency of
agricultural,
residential or
industrial processes
◦ Reusing or recycling
water sources


While households use the least amount of
fresh water overall, it is easiest for humans to
reduce water use through household water
conservation.
Water-efficient technology helps to reduce
the water used in common processes. Good
examples are efficient toilets, shower heads
or washing machines.


As one-third of a typical household’s water
use is spent on landscaping, xeriscaping –
the act of designing a landscape with minimal
water use – can significantly reduce water
intake.
When watering landscapes or plants, it is best
to do so in the early morning or evening, to
prevent water loss due to evaporation.



As water intake continues to increase, people
are beginning to tap new resources.
Desalination is the act of removing salt and
heavy minerals from “hard” (mineral) water.
Hard water is often brackish, making it
unsuitable for human use.
Desalination plants can treat ocean water, as
they do in the Middle East – or they can treat
mineral groundwater, such as in El Paso!