IB Environmental Systems and Societies

1. Topic 4
Water and aquatic food production
systems and societies
4.2 Access to freshwater

2. Earth’s water budget
• Only a small fraction of the Earth’s water supply is
freshwater (about 2.5% by volume)
• Atmospheric water vapour is about 0.001% of the
total by volume
• Total water supply is refered to as the
hydrosphere
• Only about 0.3% of total water supply is surface
water (freshwater easily available to humans
from rivers and lakes)

3. Access to water
• Distribution of freshwater throughout the
world is not equitable
• LEDCs lack the technology to make some
sources of freshwater readily available
• Different parts of the hydrosphere have very
different turnover times

4. Access to water
• Some water is more easily replenishable than others
• Turnover time is the time it takes for a drop of water to leave and then
return to the same part of the system
Location Typical Turnover Time
Polar Ice caps 10 000 years
Ice in permafrost 10 000 years
Oceans 2 500 years
Groundwater 1 500 years
Mountain glaciers 1 500 years
Large lakes 17 years
Bogs 5 years
Upper soil moisture 1 year
Rivers 12 days
Atmosphere 12 days
Living things 16 days

5. Sustainability
• The use of global resources at a rate that allows
replenishment and minimises damage to the
environment
• Water use in MEDCs and LEDCs are increasing (due to
population increase and greater per capita need)
• In MEDCs water is exploited for many non-essential
uses, such as:
– Washing cars
– Watering gardens
– Frequent bathing
– Swimming pools

6. Case Study – Mexico City
• Mexico City is in a closed basin and has a population of around 21.4
million people
• Water availability is around 165m3/inhabitant/year
• Most of this is extracted from groundwater (which is overexploited
by 120%)
• Ground water availability is decreasing and there is a big problem of
subsidence, especially in Centro Historico
• The government is trying to regulate water use
– Car washes and watering public green spaces use secondary-treated
wastewater
– The remaining wastewater is reused by being channeled to the Tula
Valley where it is used on the largest wastewater-irrigated area in the
world
– Here it recharges the local aquifer and brings huge economic benefits.
However there is also a risk of local health problems

7. Questions
1. List possible storages of global water
2. List possible flows of global water
3. List and evaluate human impacts on the
water cycle