Edexcel Geography A-Level - HYDROLOGICAL CYCLE

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global water store

closed water system because water cannot enter or leave the Earth and its atmosphere

stores

water stored as either water vapour, ice, saline or freshwater.

* oceans


* lakes
* aquifers (underground lakes)
* cryosphere (glaciers and ice sheets)

local stores

* vegetation storage
* surface storage
* soil moisture
* groundwater storage

flows

how water moves from one store to another

* infiltration
* throughflow
* percolation
* stem flow
* base flow
* channel flow
* surface runoff

processes

what drives the flows between stores

* precipitation
* evaporation
* transpiration
* cryosphere exchanges
* runoff

total global water supply

* 96.5% of Earth's water is in the oceans and seas
* 2.5% of Earth’s water is in freshwater (not salty)
* 0.9% of Earth’s water is in other saline (salty) water sources

freshwater supply

* 68.8% of Earth's freshwater is in the cryosphere
* 30% of Earth's freshwater is groundwater
* 1.2% of Earth's freshwater is surface water

hydrological cycle in a drainage basin

smaller, open systems. The main features of the drainage basin hydrological cycle are:

* inputs
* outputs
* flows

orographic rainfall

when air masses rise over mountains causing it to condense and rain

frontal rainfall

when two air masses meet at an area of low pressure creating rain

convectional rainfall

rainfall caused by water turning to water vapour due to solar radiation

evaporation

when water turns to water vapour and leaves the drainage basin

transpiration

when water leaves plants through holes in their leaves

channel flow (output)

the volume of water that is flowing in a river channel

interception

when plants capture precipitation in their leaves

infiltration

when precipitation enters into the soil

percolation

the vertical flow of water between soil and rock layers

direct runoff

when water from precipitation or snowmelt flows across the ground’s surface because the surface is impermeable i.e. tarmac

saturated overland flow

when the ground is full of water resulting in runoff

throughflow

the horizontal flow of water through soil/rock layers

groundwater flow

the flow of water horizontality once it has reached the water table

physical factors affecting drainage basins

soil type, rock type, climate, relief of the land, vegetation cover

soil type

impermeable soils can stop infiltration and lead to surface saturation resulting in increased surface runoff, where there are permeable soils, infiltration and percolation can occur allowing groundwater to recharge

rock type

some rocks are impermeable and can stop the infiltration of water into the ground, causing the ground to saturate which leads to surface runoff and increase flows in rivers; permeable rocks allow for infiltration and percolation to happen

climate

cold climates allow for precipitation to fall as snow, the water can be stored and held back until it melts, this reduces the channel flow during the winter but leads to an increase during spring and summer due to glacial melt

relief of the land

steeper slopes mean faster surface runoff and shorter times for water storage

vegetation cover

when an area has a high coverage of vegetation, the interception and evapotranspiration increases but the surface runoff decreases

human factors affecting drainage basins

reservoirs, urbanisation, over-abstraction, deforestation

reservoirs

reservoirs - man-made stores that disrupt the natural water flow by delaying the flow and increasing water lost from evaporation

* when vegetation grows on the reservoir’s surface, an increase in evapotranspiration happens and increases the salinity of the water

dams - reduce the water flow further downstream resulting in the loss of vegetation

urbanisation

a change of land use that results in moving away from the natural environment to towns and cities

* leads to a large number of impermeable surfaces i.e. concrete, which reduce infiltration whilst increasing surface runoff

abstraction

when humans remove water from underground water stores e.g. aquifers

over-abstraction

when the volume of water being removed is greater than the volume of groundwater being replenished, leading to rivers drying up during periods of low rainfall

deforestation

* reduces interception and consequently, rainfall strikes soil directly, leading to soil compaction.
* soil compaction and the removal of tree roots reduces infiltration
* causes an increase in surface runoff (soil saturation occurs faster), resulting in more soil erosion and flooding

water budget

balance between water being evaporated from the oceans and precipitated onto the land

global water budget: limited water for human use

* only 2.5% of the Earth's water is freshwater and of that, less than 1% is accessible for direct human use
* access and quality of water differs depending on location in the world - usually geology that plays a part in affecting water availability

global water budget: water store residence time

water found in the oceans is stored there for longer than the short amount of time that water is held in the atmosphere

global water budget: non-renewable stores

* fossil water is the name given to non-renewable, untouched, ancient freshwater stores. We can find fossil water beneath deserts and in polar areas


* new technology is now allowing us to access more of this water, e.g., the extension of oil drilling technology and the use of satellite imagery can increase the use of these fossil aquifers
* cryosphere is another non-renewable store as it continues to melt

cryosphere hydrology

very little vegetation due to low temperature and limited light, so the ability for plants to grow is reduced

* during the winter, the ground, lakes and rivers are frozen in the cryosphere, winter snow and ice mean that 85% of solar radiation is reflected
* in spring and summer, frozen water thaws in the cryosphere, causing rapid surface runoff and increasing evaporation

as ice and soils thaw, biogenic gases (gases produced by or used by organisms e.g. methane) are released into the atmosphere

permafrost

soil that is permanently frozen