Download Ch38_Water_Limiting factor

Ch38: Water as a limiting Factor
Higher Human Biology
Earth’s Water Supply
Not available because
stored in:
• polar ice caps
• glaciers
• ground water
Water Cycle
Water is one of the Earth’s
most precious resources.
We depend on our water
supply. Although it is
recycled constantly, so is
considered to be
renewable, it is a
finite resource so could
act as a limiting factor
on the human
population.
Uneven Demand for Water
Which country uses the most water? Why?
Each person needs ~2l per day to enjoy a
reasonable quality of life.
Average in
developed
country
~ 500 l per day
Average in the poorest
developing countries
< 10 l per day
Increasing Demand for Water
Increasing
as
technology
advances
Water
used as a:
•Solvent
•Cleanser
•Coolant
7%
Domestic
Need for
irrigation
is
increasing
annually
Meeting the Increased Demand
for Water
population
increases
demand for water
increases
Water management programmes try to increase
efficiency of water consumption rather than
increase the water supply.
90% Water re-used
Irrigation is very
inefficient - only 35%
is absorbed by plants
Meeting the Increased Demand
Micro-irrigation for Water
Very efficient, drip-feed method using pipes with
tiny holes that give regular, small doses of water
Dams
• Allows control of area’s water supply –allows
growth of crops downstream
• Prevents seasonal flooding of farmland
• Provides water for:
Industry
Recreation
Drinking
Irrigation
Hydro
electric
Disadvantages of Dams
• Silt from the upland fields
clogs up the dam – these
would previously fertilise
the downstream fields so
instead expensive fertilisers
are needed.
• Causes habitat destruction upstream
• Irrigation channels can carry harmful or even
lethal parasites.
• Shallow water at the edge of a hydroelectric
power reservoir gives idea conditions for malariacarrying mosquitoes to breed.
Unpredictability of Supply
Developed Country
• Continuous supply of
clean piped water
• sewage disposal via
mains sanitation
• Water rationing –
very rare!
Developing Country
• Unreliable water supply
• Often seasonal due to rain
• Differs from year to year
• Permanent shortage in some
areas
Unpredictability of Supply
Climate – the long term prevalent weather
conditions of a region
- caused by complex interactions
between
• atmosphere • oceans
• geographical location
• land mass
• Changes over millions of
years e.g. Ice age
• Short, extreme changes of
climate occur periodically and
can devastate regions
Unpredictability of Supply
El Nino – a warm ocean current
In 1982-83 rapid warming of part of the
Pacific Ocean caused an extreme El Nino.
Flooding
Drought
(India & Bangladesh) (Africa & Indonesia)
Unpredictability of Supply
Erosion – loss of fertile top soil due to wind or
water
• Occurs where land is misused & in extreme
climatic conditions.
When vegetation covers
slopes = no Erosion
• Canopy reduces force
of rainfall
• Roots bind soil
• Soil is more porous
Terracing e.g. rice paddies &
• Soil litter acts as a
ploughing along contours
sponge
prevents erosion
Deforestation
Forest cleared and not replanted
Caused by clearing
land for:
• agriculture
• Subsistence farming
• Disrupts water cycle
• Causes infertile soil
• Leads to droughts &
flash floods
Forests & Water Cycle
Effects of Deforestation
Effect
Deforestation
Trees help regulate flow
of water to rivers
Rapid run-off
Fertile top-soil eroded
by fast flowing water
Less evaporation &
transpiration
Loss of Carbon sink
Consequences
Irregular flow of water in
rivers
Flash floods in lowlands
(usually farmland or inhabited
areas)
Reduced soil fertility, silt
block dams & water
undrinkable
Less rainfall
Extra CO2 contributing to
‘enhanced greenhouse effect’
Desertification
In some parts of the world the
area of desert is increasing,
mainly due to human activities.
Before Desertification
Typical scene at edge of a desert in 1960s:
Although the rainfall is irregular it can still sustain
the plants & water table.
Agriculture is traditional and fields are allowed to
lie fallow (unplanted) to regain their fertility
Desertification
• Traditional farming abandoned
• Land not allowed to lie fallow fertility decreases
decrease in crop yield more land needed
• Marginal land used for growing crops infertile  desert
• Overgrazing  Plants die  Desert
• Forests cleared for crops, firewood & houses  desert
Desertification
Typical scene
in 1990s:
• Land lost to desert
• Very little chance of it ever being cultivated
• No food, no water
• People try to cultivate more marginal land
 accelerates desertification