materials to the freshwater supply. This use may lead to pollution of our water resources
and cause harm not only to humans but to many life forms, reducing our biodiversity. Most
importantly, as our water supplies become scarce, there will be conflicts between individuals
who have enough clean water and those who do not (Figure21.12). As with any limited
resource, this conflict could produce warfare.
Two of the most serious problems facing humans today are shortages of fresh water and the
lack of safe drinking water.
Humans use six times as much water today as we did a hundred years ago. As the number
of people on Earth continues to rise, our demand for water grows. Also, people living in
developed countries use more water per person than individuals in lesser developed countries.
This is because most of our activities today, such as farming, industry, building, and lawn
care, are all water-intense practices, practices that require large amounts of water.
Droughts occur when for months or years, a region experiences unusually low rainfall
(Figure21.13). Periods of drought naturally make water shortages worse. Human activ-
ities, such as deforestation, can contribute to how often droughts occur. Trees and other
land plants add water back into the atmosphere through transpiration. When trees are cut
down, we break this part of the water cycle. Some dry periods are normal and can happen
anywhere in the world. Droughts are a longer term event and can have serious consequences
for a region. Because it is difficult to predict when droughts will happen, it is difficult for
countries to predict how serious water shortages will be each year.
Water shortages hurt human health, agriculture and the environment. What happens when
water supplies run out? In undeveloped regions in the world, people are often forced to
move to a place where there is water. This can result in serious conflicts, even wars, between
groups of people competing for water.
Water disputes happen in developed countries as well. Water-thirsty regions may build
aqueducts, large canals or pathways to import water from other locations. For example,
several cities inaridregions of the United States import water from the Colorado River. So
much water is taken from the river that it can end as just a trickle when it reaches Mexico.
Years ago, Mexico could depend on the river supplying water for irrigation and other uses.
Today that water resource is gone from importing water upstream.
Some of the biggest legal battles in the United States have been over water rights, including
access to the Colorado River. Water disputes may have lead to some of the earliest wars
known.