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 the poorest developing countries < 10 l per day Average in developed country ~ 500 l per day

Domestic 7% Increasing Demand for Water Increasing as technology advances Water used as a: Solvent Cleanser Coolant Need for irrigation is increasing annually

Irrigation is very inefficient - only 35% is absorbed by plants Water management programmes try to increase efficiency of water consumption rather than increase the water supply. Meeting the Increased Demand for Water 90% Water re-used population increases demand for water increases

Irrigation Meeting the Increased Demand for Water Micro-irrigation 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: Drinking Hydro electric RecreationIndustry

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. Irrigation channels can carry harmful or even lethal parasites. Shallow water at the edge of a hydroelectric power reservoir gives idea conditions for malaria- carrying mosquitoes to breed. Causes habitat destruction upstream

Unpredictability of Supply Developed Country Continuous supply of clean piped water sewage disposal via mains sanitation Water rationing – very rare! Unreliable water supply Often seasonal due to rain Differs from year to year Permanent shortage in some areas Developing Country

Unpredictability of Supply Climate – the long term prevalent weather conditions of a region - caused by complex interactions between atmosphere land mass oceans geographical location Changes over millions of years e.g. Ice age Short, extreme changes of climate occur periodically and can devastate regions

El Nino – a warm ocean current In rapid warming of part of the Pacific Ocean caused an extreme El Nino. Unpredictability of Supply Flooding (India & Bangladesh) Drought (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 Soil litter acts as a sponge Terracing e.g. rice paddies & ploughing along contours prevents erosion

Deforestation Forest cleared and not replanted Disrupts water cycle Causes infertile soil Leads to droughts & flash floods Caused by clearing land for: agriculture Subsistence farming

Forests & Water Cycle

Effects of Deforestation

Trees help regulate flow of water to riversDeforestationEffect Consequences Rapid run-off Fertile top-soil eroded by fast flowing water Less evaporation & transpiration Loss of Carbon sink 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 CO 2 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