Enviromentsl igcse unit 2

The Hydrosphere

1: The water cycle and water availability on Earth

Water on Earth

• No water = No life.
• Fresh water is the main resource on Earth. Plants and animals. Humans: drink, grow food, operate industries, provide energy.
• Ecosystems in salt water seas and oceans support diverse species, which are additional sources of food.
• Oceans: 71% of the Earth´s surface.
• Oceans and seas: 97% of the water.
• Fresh water: 3% (25% ground water; 74.5% ice; 0.04% atmosphere, 0.3% lakes; 0.03% rivers; 0.06% soil).

Water cycle

• One of the Earth's great natural systems: very important because surface fresh water stores are small.
• Evaporation: water from oceans, seas, and water surfaces on land, is changed from water droplets to water vapour in the atmosphere. Heat.
• Transpiration: water lost from trees and plants (leaves).
• Evapo-transpiration: water losses from land surfaces (water and vegetation covered surfaces).
• Condensation: water vapour is carried upwards by air currents and it is cooled, leading to condensation. The water vapour is turned back into a liquid or a solid.
• Precipitation (rain, sleet, snow, hail): further cooling leads to precipitation, which is all the moisture that reaches the surface.
• Interception: water is prevented from falling directly on to the ground by trees and plants.
• Run-off: water that flows over the ground surface and finds its way into streams and rivers.
• Evapo-transpiration: immediate water loss back to the atmosphere in hot days from both surfaces.
• Infiltration: The remainder of water seeps underground, downwards.
• Groundwater flow: The remainder of water seeps underground, sideways.

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The role of water within ecosystems

Ecosystem: living communities in an area (biotic elements: plants, animals) and its non-living natural environment (abiotic elements: climate and soils). All elements are interlinked, affect and are affected by the other elements.

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What is needed for life on Earth?

• Light from the sun: primary source of energy, which is captured by green plants and transformed into chemical energy by photosynthesis, that needs light, chlorophyll, carbon dioxide, warmth and water.
• Carbon dioxide is absorbed through stomata in the leaves of higher plants. This reacts indirectly with water taken up by the roots when temperatures are high enough to form carbohydrates (food for the plant). The energy needed comes from sunlight which is trapped by chlorophyll.
• Plant roots absorb water containing soluble nutrients from the soil. The leaves absorb carbon dioxide and water vapour from the atmosphere that are converted  to carbohydrates with the help of solar energy. Leaves lose water to the atmosphere by transpiration. Between roots and leaves: stems (hold up the leaves and reproductive parts). Cells: shape and structure to help survive environmental conditions (thick barks or leaves with waxy surfaces to prevent excessive water loss by transpiration).
• All plants need water to survive (amount: depends on the species). Climate: controls how much water is available (if precipitation is lower or greater than the rate of evapo-transpiration).
• Rainfall decreases away from the Equator but temperatures remain high, causing rates of evapo-transpiration to be greater than precipitation.

Effects of vegetation cover on the water cycle

Where forests have been cleared on a big scale over large areas: the amount of precipitation has gone down. Rapid run-off (water soon leaves the area where it has fallen) and less evaporation (less water vapour feeding moisture to the atmosphere).

How natural availability of water varies from place to place

Precipitation replenishes water supplies by filling surface rivers and topping up surface and underground stores. It varies greatly from place to place.

Water-rich countries: majority is located in the hot wet tropics. plentiful fresh water supplies. Main reasons. large countries that have plenty of land for the rain to fall on, some of the world's great rivers flow through these countries. However, big area: no guarantee of big water resources.

• Brazil
• Russia
• Canada

The world's wettest place: Cherrapunji-India.

Water-poor countries: located in the desert lands in the Middle East and the northern half of Africa. Desert countries except Singapore and Mauritius (high precipitation but have small areas for rain to fall on). Likely to suffer from water stress (worries over present and future water supplies).

• Saudi Arabia: desalinated water production.  Desert climate - limited surface sources (little precipitation). Three quarters comes from underground aquifers (filled up when the climate was wetter). Aquifers in 2010 contained 40% less water than in 1985. There is a lot of demand and it is used for agriculture, domestic use and industry. Would be better off importing more of its food needs.
• Mauritius
• Oman

The world's driest place: Atacama Desert-Chile.

Competition for water supplies

World fresh water availability: * 1950 - 16000 m 3 per person

                                * 2000 - 7000 m 3 per person

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