2.1.1 - concepts of a system and mass balance
Inputs, outputs, stores and flows in the water cycle,
including the concept of mass balance
System - a set of interrelated components that are
linked together to form a functioning whole.
Input - how water is introduced into the drainage basin
system
Example - precipitation (rain, snow, hail)
Output - how water is released back into the sea or
atmosphere
Example - evapotranspiration, evaporation, stream flow
leading to the sea
Flow - water moving from one place to another
Example - infiltration, percolation, overland flow,
throughflow, groundwater flow, stream flow
Store - stores of water in the ocean, ice caps, land and
atmosphere
Example - interception, surface store, soil water,
groundwater
Mass balance - the water cycle is a closed system,
where all inputs and outputs move within the system
but don’t cross the system’s boundaries. The mass
balance wil;l not change as the amount of water in the
system is fixed at 1.38 million km^3. WWater is held in,
on or above the earth and flows are transfers of water
between stores. The water cycle is an interrelationship
between land, ocean and the atmosphere.
Distribution, size and characteristics of major stores of
water including lakes, oceans, atmosphere and
cryosphere, vegetation, soil and groundwater stores
The water cycle:
- The water cycle shapes landscapes, transports
minerals and is essential to life and ecosystems
on the planet.
The ocean contains 97% of earth’s water → heat from
the sun causes evaporation → water vapour rises and
condenses as clouds → advection occurs where the
wind moves clouds through the atmosphere → water
forms from clouds as snow, rain, sleet etc → in high
areas, snow and ice accumulates then melts back into
water or rises as vapour → water flows downwards
above the ground, known as runoff, which creates
rivers and streams → plants take up water from the
ground and transpire it back into the air → water
infiltrates into the ground, absorbs minerals and seeps
back out → water penetrates the earth’s crust and
comes back out as geysers or volcanic streams.
Cryosphere - all areas of the earth where water is
present as snow or ice and holds 2% of earth’s water.
Example - ice sheets/caps, sea ice, permafrost, alpine
,glaciers
Biosphere - all areas of earth with living organisms
and hold more than 0.01% of earth’s water.
Example - plants, animals, birds, fungi, insects,
bacteria
Atmosphere - layer of gas between earth’s surface
and space, held in place by gravity and holding less
than 0.01% of earth’s water.
Hydrosphere - all water on earth in liquid, solid or gas
form, can be saline or fresh and holds 97% of earth’s
water.
Example - liquid (rivers, lakes), solid (ice), gas (water
vapour)
Lithosphere - the outermost part of the earth,
including the crust and upper parts of the mantle and
holds 1% of earth’s water.
Change in size of stores over time including sea level
change and cryospheric processes (ice accumulation
and ablation)
Short term sea level change:
- Heavy rain and storms increase water transfers
in the locality
Annual seasonal variations:
- Short term changes in ice accumulation and
ablation occur annually due to seasonal
changes in temperature during winter and
summer.
- Impact transfer rates like the monsoon season
in Asia.
Spanning over years:
- El Nino Southern Oscillation, impacting
precipitation on a large scale.
- The Aral Sea began to retreat in the 60s. The
water from the two seas feeding the sea were
used for irrigation so it shrank in 1989. This had
problems such as locals being prevented from
fishing, salty grass grew so animals that ate the
grass grew ill and died. However, a sand dam
was built and water returned. When it was
flooded, a permanent dam was built to restore
the water store allowing economic growth in the
area.
Centuries:
- Climate change from human activity, changing
precipitation and evaporation rates. This
influences flows between the land and
atmosphere, affecting ablation rates between
the cryosphere and other parts of the system.
Long term sea level change: eustatic change
- Global change in the volume of oceans, which
has changed significantly over geological time.
, - In Cryogenian ice ages 750-650 million years
ago, more water is frozen, decreasing water in
the hydrosphere (making sea levels drop) and
subsequently increasing water in the
cryosphere.
- In interglacial Paleocene periods 65-35 million
years ago, temperatures are high so ice melts
and sea levels rise, creating a contrasting effect
with more water in the hydrosphere.
- This has exacerbated climate change and
global warming in the 21st century.
Processes which control transfers within and between
land, ocean, atmosphere and cryosphere at a range of
time (minutes to millenia) and space (hillslope to
global) scales
Short term: climatic factors
- Seasonal variations have an impact on water
transfer rates.
- In Asia, it is dry from November to May but
there is a monsoon season from June to
October.
- Wet seasons include heavy rain, flash floods
and storms that increase the transfers of water.
- This alters water stores dramatically, however it
is not long-term as the climate reverts to being
warm and dry in the remainder of the year, so
balances out its effects.
Long term: Milankovitch cycles
Eccentricity - earth encounters more variation in the
energy it receives from the sun when the earth’s orbit is
elongated rather than circular. Energy is used, which
melts ice and warms the earth so a circular orbit is
more likely to lead to water deficit.
Tilt - the tilt of the earth’s axis varies from 22.2 degrees
C to 24.5 degrees C and changes every 40,000 years
so the northern hemisphere is more tilted towards the
sun. Here, it receives more solar radiation so more
water is stored in the hydrosphere. Therefore, the
southern hemisphere, which is tilted away from the
sun, receives more water deficit.
Precession - a gradual change in the orientation of the
earth due to tidal forces, caused by gravitational
forces. This makes seasonal contrasts more extreme.
For example, one having an abundance of precipitation
and the other receiving water deficit.
2.1.2 - catchment hydrology - the drainage basin
as a system Drainage basin - this is a subset of the global hydrologic
cycle. It is defined as a catchment area forming part of
the earth’s surface area, which is drained by a particular
, stream or river. It is an open system, which allows energy
and matter to be transferred across its boundary from
external areas.
Input: precipitation type, amount, duration and intensity
- Precipitation is the only physical input into the
drainage basin.
- Precipitation can be in the form of rain, hail,
sleet or snow.
- The amount is determined by how often it rains
as well as how long it rains for and how strong
the rain falls.
- The duration and intensity affects how a
drainage basin responds to a weather event.
For example, high intensity rainfall can result in
flash flooding.
Flows: throughfall and stemflow, infiltration, overland
(saturation and infiltration excess) flow, throughflow,
percolation, groundwater flow and channel flow
Throughfall - rainwater dripping from leaves and
branches to the ground.
Stemflow - water that flows to the ground via
vegetation, stems and trunks.
Infiltration - the vertical movement of water into the
ground from the surface.
Overland flow - water that runs across the land after
rainfall, either before it enters a watercourse, after it
leaves a watercourse as floodwater or after it rises to
the surface naturally from underground.
Throughflow - the lateral transfer of water that moves
downhill through the soil, via a matrix of pore spaces,
fissures and pipes.
Percolation - the vertical transfer of water from the soil
into the underlying bedrock.
Groundwater flow - the vertical and lateral movement
of water through a drainage basoim’s underlying rock
due to gravity and pressure.
Channel flow - the movement of water within the river
channel.
Saturation excess - rainfall continues for a long time so
the ground becomes saturated and surface runoff
occurs.
Infiltration excess - rainfall intensity is so great that not
all water can infiltrate, even if the soil is dry.
Stores: interception store, vegetation store, surface
store, soil moisture store, channel store, groundwater
store
Interception store - the leaves, stems and trunks of
vegetation can act as a barrier to precipitation reaching
the land’s surface and water can be stored temporarily
