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The Water Cycle Notes
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In-depth notes for the Edexcel Geography A-level topic The Water Cycle and Insecurities. Includes case studies and theory notes and diagrams.
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The Water CycleEQ1: What are the processes operating within the hydrological cycle from global to
local scale
5.1 The global hydrological cycle is of enormous importance to life on earth
Characteristics of The Global Hydrological Cycle
- Closed system} fixed amount of water in the Earth [1385 million km3]; had no
external inputs or outputs; total volume of water is constant and finite
- Quality of water changes not quantity
o Big issue as can’t use poor quality water
- Driven by solar energy and gravitational potential energy
- Water can exist in different states within the system, proportion of each can
vary due to both physical and human reasons
- Anthropologic causes are key driver for climate change
- During an ice age more water is held as ice and less in the form of water in the
oceans. Leads to sea levels dropping; in the last ice age they dropped 140m
lower than today
- Security of water main issue} hence the building of many water storage
reservoirs
- Total percentage of fresh, potable (drinkable), accessible water 0.07%
- Water is of strategic importance for economic development} no water = no
workforce health
Systems approach – study hydrological phenomena by looking at the balance of
inputs and outputs and how water is moved between stores by flows
Stores – reservoirs where water is held e.g. oceans
Fluxes – rate of flow between stores} driven by key processes such as precipitation,
evaporation, cryospheric exchange and run-off
Process – physical mechanisms that drive the fluxes of water} gravity; solar energy
Cryosphere – areas of the Earth where water is frozen into snow or ice} greatly
impacted by global warming, turning freshwater stores into saltwater, land based ice
contributing to sea level change
Blue Water – water stored in rivers, streams, lakes and groundwater in liquid form
(visible part of hydrological cycle)
Green Water – water stored in the soil and vegetation (hidden part of hydrological
cycle)
- The amount of time water will stay in a particular store is known as the
residence time
- Can slow down the entire cycle, e.g. if there is a lot of water in the cryosphere
then it will take longer to move to the next stage of the cycle
- Strong link between residence time and pollution
o Blue water and areas with longer residence time are more susceptible to
pollution as it is in situ for a longer length of time
, - Eutrophication} surface run off containing pesticides and chemicals enters
water sources, encourages algae growth, leads to destruction of wildlife in water
source
- Fossil water (the result of former wet periods) can be found in ancient
groundwater under the Sahara Desert. It is not renewable or reachable for
human use
- More accessible stores e.g. soil moisture and small lakes, have much smaller
residence times. This is because it is easily lost due to evaporation,
transpiration, groundwater flow or recharge
Precipitation (Input) – movement of water, in any form, from the atmosphere to the
ground
Evaporation (Output) – change in state of water from a liquid to a gas
Residence time – average time a water molecule will spend in a reservoir or a store
Fossil water – ancient, deep groundwater from former pluvial (wetter) periods
Transpiration – diffusion of water from vegetation into the atmosphere, involving a
change from a gas to a liquid
Groundwater flow – slow transfer of percolated (gradually filter through a porous
surface) water underground through pervious or porous (allows water to pass
through) rocks
Differing hydrological processes
Polar Hydrology Tropical Rainforest Hydrology
- Seasonal differences - Few seasonal differences
- Winter snow insulates the ground} - Dense vegetation intercepts and
85% of solar radiation is reflected consumes up to 75% of
- Permafrost creates impermeable precipitation
surfaces - Some returns by
- Lakes and rivers are frozen evapotranspiration
- Limited vegetation = less heat - Rainforests generate their own rain
absorption - Less than 25% of rainfall reaches
- Spring thaw creates rapid run-off rivers or other surface water
- Summer thaw also increases run- - Deforestation reduces evaporation,
off as well as increases radiation in turn reducing vapour and rainfall
(locally)
The cryosphere: Here, seasonal thaws - Constant high temperatures
bring increased surface saturation and
thinning permafrost. If this thaw Tropical rainforests: Here,
becomes continuous, water flows away permanently dense forest produces high
and is lost (Cryospheric loss) rates of evapotranspiration, with water
returning to the surface as precipitation
that feeds large rivers
Polar regions
- Contribute to the circulation of water and the transfer of heat around the world
(drives the hydrological cycle
- Ocean circulation occurs - Thermohaline circulation, sometimes called the
global conveyor belt
, How thermohaline circulation works
1. Ocean water in the polar regions is colder, more saline (salty) and denser
than in the Tropics, so it sinks
2. The cold sinking water draws in warmer water from the ocean surface above,
which in turn draws water across from the Tropics
3. The movement of water from the Tropics draws cold water up from the
bottom of the ocean to be warmed again
5.2 The drainage basin is an open system within the global hydrological cycle
The drainage basin: an area of land drained by a river and its tributaries, and is
frequently referred to as a river catchment
- Subsystem within the global hydrological cycle
- Open system} has external inputs and outputs that cause the amount of water
in the basin to vary over time; changes can occur on different temporal (time)
scales (hourly, seasonally, and annually)
- Boundary of the drainage basin (aka catchment area) is defined by the
watershed, usually a high ridge of land which divides and separates waters
flowing to different rivers
The drainage basin: inputs and outputs
Input Storage Flows Output
Interception:
Infiltration: Water
Precipitation: Temporary storage,
entering the Evaporation:
Release of water water caught by
topsoil. Common conversion of water
through rain and plants and
during slow and to vapour
snow (etc.) buildings before
steady rainfall
reaching the soil
Runoff: Water
Vegetation Percolation: runs into the sea
Storage: any Downward seepage (Overland flow is
moisture taken and of water through where precipitation
held by plants rock under gravity runs directly off the
surface)
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