An extremely detailed set of notes for the water cycle topic for A Level Edexcel Geography. I managed to achieve an A* in A Level Geography in 2022 using these notes as I included absolutely everything listed on the specification.
Water
The global hydrological cycle
= a closed system made up of a series of processes. There’re no external inputs/outputs so
the volume of water is finite & constant. So no water is new water – only thing that changes
is the state in which water exists in (ice, liquid, vapour) – changes overtime due to climate.
à it’s a system with inputs, stores, flows & outputs
Stores = places where water is held
Processes = way in which water moves between stores
Closed system = where inputs & outputs are balanced
Residence time = the average amount of time a water molecule will spend in
a store
The cryosphere = the frozen water part of the Earth system (e.g. glaciers, ice
sheets, sea ice)
The power that drives the global hydrological cycle comes from 2 sources –
Solar energy = energy from sun – it drives evaporation & transpiration
Gravitational potential = ways in which water accelerates under gravity, thus
transporting it to rivers & then the sea. This is responsible for
energy
surface run off & percolation.
Oceans à vast majority of water is stored in liquid form
Cryosphere à water is largely found in solid state with some in liquid form as melt
water & lakes
Land à stored in rivers / lakes / groundwater in liquid form. Often known as blue
water (visible part of the hydrological cycle). Can also be stored in vegetation or
beneath surface in the soil (where is often known as green water –invisible part of the
cycle)
Atmosphere à water largely exists as vapour but clouds can contain minute droplets
of liquid water or ice crystals (at high altitude)
, Of freshwater stores –
* The cryosphere holds 69% of global
freshwater (the largest)
* Groundwater holds 30%
* Less than 1% is stored in the
biosphere (vegetation & soil
moisture)
Flows & Fluxes
Flows Fluxes
= the transfers of water from 1 = the rates of flow between stores
store to another (greatest fluxes occur over the oceans)
There’s little freshwater available (only 2.5%) & only 1% of all freshwater is ‘easily
accessible surface freshwater’ some is locked up in glaciers & ice sheets.
Non-renewable water sources –
Þ Cryosphere losses
When ice melts it won’t become ice again as it was formed during an ice age
Þ Fossil water (Ancient, deep groundwater from former pluvial periods)
There’s fossil water deep below the Sahara Desert but it isn’t reachable for human
use
Local hydrological cycle
= a system of linked processes (inputs, flows & outputs), on a local scale the water
cycle is an open system
INPUTS (PRECIPITATION)
• Caused by the cooling & condensation of water moisture in atmosphere –
forming clouds that release moisture in form of rain / snow / hail / sleet.
Primary factors affecting volume/condition of precipitation
• Seasonality – likely to result in DBS operating at different flow levels at different
times of year (in some climates (monsoon/Mediterranean) there’re strong
seasonal patterns of rainfall)
• Variability – sudden/long term changes to climate can occur which can affect
precipitation levels
• Latitude – location of drainage basin has a major impact on climate & so the
volume & type of precipitation falling (in most cases – higher the latitude from
the equator = colder the climate (snowfall occurs more often than rainfall)
, FLOWS
• Infiltration – movement of water from surface into the soil (infiltration capacity
refers to maximum rate water can be absorbed by the soil)
• Interception (also storage) – retention of water by plants & soils which is
absorbed by vegetation/evaporated
• Percolation – a deeper transfer of water into permeable rocks
• Throughflow – the lateral transfer of water downslope through the soil
• Groundwater flow – the slow transfer of percolated water through pervious
(permeable) or porous rocks
• Surface runoff – movement of water overland
• Stem flow – down the plant stems/trunks
• Throughflow (also storage) – when precipitation falls onto plants & excess is
dropped/falls off vegetation as its too heavy
STORES
• Soil/soil moisture – water stored
OUTPUTS
in the soil which is utilised by
• Evaporation – the direct loss
plants
of water moisture to the
• Ground water/aquifer – water
atmosphere (rate increases
stored in the pore spaces of
when weather is
rock
warmer/windier/drier
• Interception – water
• Transpiration – water is lost
intercepted by plants on their
from plants & transferred to
branches & leaves before
the atmosphere
reaching the ground
• Discharge – channel flow
• Surface storage – water stored
into another larger drainage
in puddles/ponds/lakes
basin (lake/sea)
• River channel – water stored in
river
The water table is the upper level at which the pore spaces & fractures in the
ground become saturated – it’s used to assess drought conditions, health of wetland
systems etc.
Drainage Basin
= a subsystem within the global hydrological cycle (local hydro. Cycle). It’s an open
system with external inputs & outputs that cause the amount of water in the basin to
vary overtime (hourly/daily/monthly/yearly).
It’s an area of land drained by a river & its tributaries.
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