1. Water as a limiting factor
Chapter 38

2. Water & the water cycle 97.4% of water on earth is saltwater
2.6% freshwater – 2.586% in groundwater, ice caps
Only 0.014% free in rivers, lakes & soil
Is replenished continuously by recycling
Water cycle – involves evaporation & precipitation, transpiration etc.
Water is a finite resource, vital for human existence
Global use (4,000km3/yr) short of need (10,000km3/yr)

3. Water demand
73% for agriculture, 22% for industry, 5% for domestic use
Minimum of 80 litres per person
To meet demand, water use needs to be more efficient
90% of domestic & industrial water can be reused
Irrigation – only 35% re-used
Micro-irrigation & dams can aid efficiency of use
Risks (with dams):
- dam clogged up by silt
- parasites & mosquitoes
- habitat destruction

4. Unpredictability of supply
World’s water supply unevenly distributed
Climate can have a vast impact on water availability
E.g. ‘El Nino’ – flooding (India) & droughts (Africa)
Erosion
– loss of top soil by water or wind
- minimised by forest coverings
- terraces & using contours
Deforestation – irreplaceable loss of natural forest
Effects – loss of ‘sponge’ effect, erosion of fertile top soil, increasing CO2 levels, less water vapour in atmosphere

5. Desertification Human activity causing desert land
Accelerated by: - increased food demand
- Fallow periods abandoned
- Marginal lands used
- Clearing of forest for fuel or materials
- Grazing land used for crop growing
- Dry top soil suffering wind erosion
- Severe drought