4.1

• The importance of water in supporting life on the planet


  Scientists believe that water is the key to understanding the evolution of life on Earth, as it provides a medium that allows all organic molecules to mix and form more complex structures.
• Importance

  Oceans

  Oceans cover approximately 71% of the Earth's surface. Oceans help to moderate the Earth's temperature by absorbing heat, storing it and releasing it slowly.

  This means that it keeps temperatures less extreme as it has a higher specific heat capacity.
• Importance

  Clouds

  Water droplets in clouds are also vital in moderating the Earth's climate. Clouds are made up of tiny water droplets and ice crystals which reflect about a fifth of incoming solar radiation and lower surface temperatures. At the same time, water vapour is a potent gas, so it helps to keep the planet warmer than it would be without it. Greenhouse gases absorb long wave radiation from the Earth, which keeps our planet temperatures 15 degrees higher.
• Importance

  Flora (plants)

  Plants need water for all their functions, including photosynthesis, respiration and transpiration.

  For photosynthesis to occur, plants take in CO2, sunlight and water and convert this into glucose and starch. Respiration then converts this energy through reacting with oxygen.

  Plants also need water for rigidity and to transport nutrients from the soil through the plant.
• Importance

  Fauna (animals)

  All chemical reactions in animals body's occur in water, including the circulation of oxygen and nutrients. Sweating is an example of where water is evaporated off the surface of the skin to cool us down.
• Importance

  People (economy)

  Water is needed for a number of economic activities. It is used to generate electricity, irrigate crops, provide recreation and in drinking water and sewage disposal. It is also used in a range of manufacturing industries such as paper making.
• The importance of carbon to life on Earth

  Apart from carbon's biological significance, it is also used as an economic resource. Fossil fuels such as coal, oil and natural gas power the economy.
• Importance

  Element

  Carbon as an element is found everywhere. It is in our foods as carbohydrates and proteins, in the air as carbon dioxide (CO2) and methane (CH4), dissolved in the oceans, found in plants and animals and carbonated rocks.

  An example of a carbonated rock is limestone.

  Both carbon dioxide and methane are examples of a greenhouse gas and therefore help to regulate Earth's temperature and keep it warm enough for life.

  Carbon dioxide is used in photosynthesis to create glucose and starch, which eventually turns into energy through respiration.

  Therefore carbon is also important in providing crops and trees for food, paper and so on.
• Importance

  People (economy)

  Just like with water, carbon is an important economic resource.

  Fossil fuels such as coal, oil and natural gas power the global economy. These contain carbon from the dead remains of plants and animals, when burned, they release carbon dioxide (CO2) into our atmosphere. Not only do fossil fuels provide electricity, but they are used as raw materials in other products. For example, oil is used in the manufacture of plastics and paint.
• The water and carbon cycle as open and closed systems

  Systems are groups of objects and the relationships that bind the objects together.

  On a global scale the water and carbon cycles are closed systems driven by the Sun's energy (which is external to the Earth. Only energy (and not matter) crosses the boundaries of the global water and carbon cycles - hence we refer to these systems as 'Closed".

  At smaller scales (e.g. drainage basin or forest ecosystem), materials as well as the Sun's energy cross system boundaries. These systems are therefore open systems.
• The water cycle

  Reservoirs and stores ...

  Reservoirs and stores ...

  Oceans = 97% of global water

  Polar ice and glaciers = 2% of global water

  Groundwater (aquifers) = 0.7 % of global water

  Lakes = 0.01% of global water

  Soils = 0.005% of global water

  Atmosphere = 0.001%

  Rivers = 0.0001% of global water

  Biosphere = 0.00004% of global water
• Global reservoirs and stores info

  The oceans contain 97 per cent of all water on the planet and dominate the global water cycle.

  Fresh water comprises only a tiny proportion of water in store and three-quarters is frozen in the ice caps of Antarctica and Greenland. Meanwhile, water stored below ground in permeable rocks amounts to just one-fifth of all freshwater.

  Given its pivotal role in the water cycle, it is perhaps surprising that only a minute fraction of the Earth's water is found in the atmosphere. This paradox is explained by the rapid flux of water into and out of the atmosphere: the average residence time of a water molecule in the atmosphere is just nine days.
• Inputs and outputs of water

  According to US Geological Survey (USGS) estimates, the global water cycle budget circulates around 505,000 km of water a year as inputs and outputs between the principal water stores.
• Input

  Water Vapour

  Inputs of water to the atmosphere include water vapour evaporated from the oceans, soils, lakes and rivers, and vapour transpired through the leaves of plants. Together these processes are known as evapotranspiration.
• Output

  Precipitation

  Moisture leaves the atmosphere as precipitation (i.e. rain, snow, hail, etc.) and condensation (e.g. fog).
• Output

  Ablation

  Ice sheets, glaciers and snowfields release water by ablation (melting and sublimation).
• Output

  Surface run-off

  Precipitation and meltwater drain from the land surface as run-off into rivers. Most rivers flow to the oceans though some, in continental drylands like southwest USA, drain to inland basins. A large part of water falling as precipitation on the land reaches rivers only after infiltrating and flowing through the soil.
• Output

  Percolation

  After infiltrating the soil, water under gravity may percolate into permeable rocks or aquifers. This groundwater eventually reaches the surface as springs or seepages and contributes to run-off.
• Carbon cycle

  Principle carbon stores

  Principal carbon stores ...

  The global carbon cycle consists of a number of stores or sinks connected by flows of carbon. The principal stores are: the atmosphere, the oceans, carbonate rocks, fossil fuels, plants and soils.

  
  

  Carbon moves between these stores in an unending cycle.

  Atmosphere = 600 (Carbon store in billion tonnes)

  Oceans = 38, 700

  Sedimentary (carbonate) rocks = 60, 000 - 100, 000, 000

  Sea floor sediments = 6, 000

  Fossil fuels = 4, 130

  Land plants = 560

  Soil/peats = 2, 300
• Carbonate rocks

  Carbonate rocks, such as limestone and chalk, and deep-ocean sediments are by far the biggest carbon store. Most of the carbon that is not stored in rocks and sediments is found in the oceans as dissolved CO Carbon storage in the atmosphere, plants and soils is relatively small. However, these stores play a crucial part in the carbon cycle. They also represent most of the carbon in circulation at any one time.
• What is the slow Carbon cycle?

  Carbon stored in rocks, sea-floor sediments and fossil fuels is locked away for millions of years.

  The total amount of carbon circulated by this slow cycle is between ten and 100 million tonnes a year.
• Inputs and outputs in the slow Carbon Cycle

  CO, diffuses from the atmosphere into the oceans where marine organisms, such as clams and corals, make their shells and skeletons by fixing dissolved carbon together with calcium to form calcium carbonate (CaCO,).

  On death, the remains of these organisms sink to the ocean floor. There they accumulate and over millions of years, heat and pressure convert them to carbon-rich sedimentary rocks.

  Typical residence times for carbon held in rocks are around 150 million years.

  Some carbon-rich sedimentary rocks, subducted into the upper mantle at tectonic plate boundaries, are vented to the atmosphere in volcanic eruptions. Others exposed at or near the surface by erosion and tectonic movements are attacked by chemical weathering.

  Chemical weathering processes such as carbonation are the result of precipitation charged with CO, from the atmosphere, which forms a weak acid. The acid attacks carbonate minerals in rocks, releasing CO, to the atmosphere, and in dissolved form to streams, rivers and oceans.

  On land, partly decomposed organic material may be buried beneath younger sediments to form carbonaceous rocks such as coal, lignite, oil and natural gas. Like deep-ocean sediments, these fossil fuels act as carbon sinks that endure for millions of years.
• What is the fast Carbon Cycle

  Carbon circulates most rapidly between the atmosphere, the oceans, living organisms (biosphere) and soils. These transfers are between ten and 1000 times faster than those in the slow carbon cycle. Land plants and microscopic phytoplankton in the oceans are the key components of the fast cycle.
• Inputs and outputs of the fast Carbon Cycle

  Through photosynthesis they absorb CO, from the atmosphere and combine it with water to make carbohydrates (sugars/glucose). Photosynthesis is a fundamental process and the foundation of the food chain.

  Respiration by plants and animals is the opposite process and results in the release of CO.

  Decomposition of dead organic material by microbial activity also returns CO, to the atmosphere.

  In the fast cycle, carbon exchange also occurs between the atmosphere and the oceans. Atmospheric CO, dissolves in ocean surface waters while the oceans ventilate CO, back to the atmosphere.

  Through this exchange individual carbon atoms are stored (by natural sequestration) in the oceans for, on average, about 350 years.
• The processes of the Water Cycle

  The water balance

  The water balance ...

  The water balance equation summarises the flows of water in a drainage basin over time.

  It states that precipitation is equal to evapotranspiration and streamflow, plus or minus water entering or leaving storage:

  Precipitation (P) = Evapotranspiration (E) + Streamflow (O) +/- Storage
• The processes of the Water Cycle

  The principal flows in the water cycle that link the various stores are: precipitation, evaporation, transpiration, run-off, infiltration, percolation and throughflow.
• Precipitation

  Precipitation is water and ice that falls from clouds towards the ground. It takes several forms: most commonly rain and snow, but also hail, sleet and drizzle.

  Precipitation forms when vapour in the atmosphere reaches its dew point and condenses into tiny water droplets or ice particles to form clouds. Eventually these droplets ice particles aggregate, reach a critical size and leave the cloud as precipitation.
• How precipitation varies in character and how this then impacts the water cycle at a a drainage basin scale

  Most rain on reaching the ground flows quickly into streams and rivers. But in high latitudes and mountainous catchments, precipitation often falls as snow and may remain on the ground for several months. Thus there may be a considerable time lag between snowfall and run-off.

  Intensity is the amount of precipitation falling in a given time. High-intensity precipitation (e.g. 10-15 mm/hour) moves rapidly overland into streams and rivers.

  Duration is the length of time that a precipitation event lasts. Prolonged events, linked to depressions and frontal systems, may deposit exceptional amounts of precipitation and cause river flooding.

  In some parts of the world (eg. East Africa, Mediterranean) precipitation is concentrated in the rainy season. During this season river discharge is high and flooding is common. In the dry season rivers may cease to flow altogether.
• Transpiration

  Transpiration is the diffusion of water vapour to the atmosphere from the leaf pores (stomata) of plants.

  It is responsible for around 10 percent of moisture in the atmosphere.

  Like evaporation, transpiration is influenced by temperature and wind speed. It is also influenced by water availability to plants. For example, deciduous trees shed their leaves in climates with either dry or cold seasons to reduce moisture loss through transpiration.
• Condensation

  Condensation is the phase change of vapour to liquid water.

  It occurs when air is cooled to its dew point. At this critical temperature air becomes saturated with vapour resulting in condensation. Clouds form through condensation in the atmosphere.
• Types of clouds

  Cumuliform clouds

  With flat bases and considerable vertical development most often form when air is heated locally through contact with the Earth's surface. This causes heated air parcels to rise freely through the atmosphere convection), expand (due to the fall in pressure with altitude) and cool. As cooling reaches the dew point, condensation begins and clouds form.
• Types of clouds

  Stratiform or layer clouds

  Develop where an air mass moves horizontally across a cooler surface (often the ocean). This process, together with some mixing and turbulence, is known as advection.
• Types of clouds

  Wispy, cirrus clouds

  Form at high altitude, consist of tin ice crystals. Unlike cumuliform and stratiform clouds they do not produce precipitation and therefore have little influence on the water cycle.
• Lapse rates

  Environmental lapse rate (ELR)

  The EL is the vertical temperature profile of the lower atmosphere at any given time.

  On average the temperature falls by 6.5°C for every kilometre of height gained.
• Lapse rates

  Lapse rates describe the vertical distribution of temperature in the lower atmosphere, and the temperature changes that occur within an air parcel as it rises vertically away from the ground. There are three types of lapse rate Their interaction explains the formation of clouds.
• Lapse rates

  Dry adiabatic lapse rate (DALR)

  The DALR is the rate at which a parcel of dry air (i.e. less than 100 percent humidity so that condensation is not taking place) cools. Cooling, caused by adiabatic expansion. is approximately 10 °C/km.
• Lapse rates

  Saturated adiabatic lapse rate (SALR)

  The SALR is the rate at which a saturated parcel of air (i.e. one in which condensation is occurring) cools as it rises through the atmosphere. Because condensation releases latent heat, the SALR, at around 7 °C/km, is lower than the DALR.
• Cloud formation and lapse rates

  Clouds are visible aggregates of water or ice or both that float in the free air. We have seen that they form when water vapour is cooled to its dew point. Cooling occurs when:

  Air warmed by contact with the ground or sea surface, rises freely through the atmosphere. As the air rises and pressure falls it cools by expansion (adiabatic expansion). This vertical movement of air is known as convection.

  Air masses move horizontally across a relatively cooler surface - a process known as advection.

  Air masses rise as they cross a mountain barrier or as turbulence forces their ascent.

  A relatively warm air mass mixes with a cooler one.
• How clouds are formed by convection

  The ground heated by the Sun warms the air in contact with the surface to 18 °C. Because the air is warmer than its surroundings (13 °C) it is less dense and therefore buoyant. This situation, known as atmospheric instability, results in air rising freely in a convection current. When its internal temperature reaches the dew point (8°C) condensation occurs and clouds starts to form. The air continues to rise so long as its internal temperature is higher than the surrounding atmosphere. In this example, equilibrium is attained at 4000 m, at a temperature of - 13 °C. This marks the top of the cloud. With its base at 1000 m, the cloud has a vertical height of 3000 m. Above 4000 m the atmosphere is stable. Air cannot rise freely in this zone because it is cooler (and therefore heavier) than its surroundings.
• Catchment hydrology

  Evaporation

  Evaporation is the phase change of liquid water to vapour and is the main pathway by which water enters the atmosphere.

  Heat is needed to bring about evaporation and break the molecular bonds of water. But this energy input does not produce a rise of temperature in the water. Instead the energy is absorbed as latent heat and released later in condensation. This process allows huge quantities of heat to be transferred around the planet: from the oceans to the continents; and from the tropics to the poles.