• Created by: Tooth04
  • Created on: 13-01-22 13:27

The global hydrological cycle

-A closed system driven by solar energy and gpe. There is a fixed amount of water in the Earth (approx. 1385 million km3). This total is constant and finite. 

  • Main global stores;
  • Oceans - 96.9% of total water
  • Icecaps - 1.9% of total water, 68.7% total freshwater. 
  • Groundwater - 1.1% of total water, 30.1% of freshwater. 

Blue water - water stored in rivers, lakes and grounwater etc (visible part of the HGL cycle). 

Green water - water stored in the soil and vegetation. 

-Strong link between pollution and residence time. Stores with greater residence often incurr greater pollution. 

-Fossil water - ancient, deep groundwater from former pluvial (wetter) periods. 

- Only 2.5% of the worlds water is freshwater, with only 1% readily accessible for humans.

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Key terms

-Convectional rainfall - associated with warm air rising and cooling to create precipitation. 

-Cyclonic rainfall - when warm air is forced to rise above cold air, causing it to cool and lose its ability to hold water vapour creating clouds and rain. 

-Orographic rainfall - when air is forced to rise over a barrier, e.g. a mountain, where it cools and condensates creating rain. 

-Rain shadow - an area of dryland on the downwind side of a mountain that receives little rain due to orogrpahic rainfall. 

-Interception loss- water absorbed by vegetation and transpired.

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Drainage basin fluxes

Interception - where water is stored in vegetation.

  • Interception capabilty varies considerably, with coniferous forests dense needs allowing greater accumulation of water.
  • Interception losses contrasts between deciduous forests in summer (40%) and winter (20%). 
  • High wind speeds decrease interception loss as rain is disloged. 
  • Heavy rain saturates leaves and decreases interception loss.

Infiltration - water soaked by the soil.  Influenced by factors; 

  • Infiltration capacity decreases with ftime, as soil becomes saturated. 
  • Overland flow can occur due to saturated soils. 
  • Soil porosity - affects infiltration capacity.
  • Type, amount or seasonal vegetation; forests experience more significant infiltartion. 
  • Steep slopes promote overland flow where shallow slopes promote infiltration. 
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Drainage basin flows and transfers

- Overland flow (surface run-off) the main flow of rainwater into a river channel. Promoted when precipitation rates exceed infiltartion rates or in arid landscapes as the ground has limited infiltartion capacity.

-Throughflow - transfer of water through natural soil pipelines and percolines. Can occur quickly in porous soils, usually slower than overland flow. 

-Saturated overland flow - as the water table rises to land level. 

-Groundwater flow - movement of water through porous rocks, helps maintain steady channel flow against seasonal variability. 

-Channel flow - natural flow of a river.  

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Drainage basin outputs

  • Evapouration - moisture is lost from water surface level into the atmosphere. Factors;
  • Temperature (most important), Hours of sunlight exposure, humidity and wind speed. 

Transpiration - water lost from plants that moves into the atmosphere, rates depend on; time of year, type and amount of vegetation cover, availiability of moisture and length of growing season. 

-EVT - combined effect of transpiration and evapouration, attributed to 100% water loss in semi-arid and arid areas and 75% in humid areas. 

-PEVT - water loss that would occur on an unlimited supply of water in the soil or in the vegetation, greater differnence in humid areas than arid areas. 


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Human factors that influence a drainage basin

-HI on precipitation - cloud seeding; introducing silver iodide pellets or ammonium nitrate to the atmosphere to increase rainfall in drought stricken areas - results vary.

-HI on evapouration and EVT - deforestation affects transpiration, increases evapouration potential behind megadams such as the Aswan Dam and lake Nasser (EGYPT). Channelisation cuts down surface storage and therefore evapouration. 

-HI on interception - deforestation leads to a decrease in interception and EVT and an increase in surface run-off. This increases flooding potential and a decrease in lag time. 

-HI on soil water and infiltartion - largely results from change in land use; infiltration is 5x greater in forests than grasslands. Farmland reduces interception through compacting soil and increasing overland flow. 

-HI on groundwater - Irrigation has led to declining water tables through over-abstraction. The Aral sea (between Kazakhstan and Uzbekistan), where Soviet irrigation reduced amount of water reaching the Aral sea where by 1994 surface area fell by 50% and salinity levels grew by 300%. 

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Deforestation in the Amazonia

-Over 20% of the forest has been destroyed in the last 50 years by a combination of; cattle ranching, commercial farming for biofules and soya beans, urbanisation and illegal and legal logging. 

-The Amazon rainforest is important by acting as 'green lungs' as carbon sinks, so deforestation leads to greater green-house gas emissions. 

-75% of intercepted water is returned to the atmosphere through EVT, which reduces to 25% when the rainforest is cleared. This enforces a drier climate through desiccation (removal of moisture) and further forest degredation. 

-The ENSO cycle can lead to further significant occurences of drought which can excaberate forest fires and further destruction. 

-As interception is decreased, more water runs off nto the drainage basin which can lead to increased flooding and mudslides, also leading to aquifier depletion as less water infiltrates to recharge them. Overland flow also increases soil erosion. 

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Human influences on drought

-Human building on flood plains is the main factor excaberating flood risk. Economic and population growth has caused the movement of activity onto flood plains. 

  • Urbanisation is a key factor in this; 
  • -creation of impearmeable surfaces increases surface run-off as the water can't infiltrate into the soil. 
  • -Speeding up of drainage via artificial conduits e.g. drains leading to greater localised flow in areas.
  • -Straightening channels to increase flow, meaning more water is sent downstream increasing flooding.(Potential mismanagement). 
  • -Changing land use associated with agricultural development. Deforestation, overgrazing, ploughing and compacted soil usually occurs upstream from an urban area, increasing run-off and alluvium (sediment washed into river channels). 
  • -Floods of 2015 in Britain were put down to;
  • extreme weather caused by climate change 
  • poor land management e.g. improving pastures then overgrazing them
  • green priorities of the EU Water Framework Directive put environmental concerns before maintenece and dredging.
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