WATER AND CARBON CYCLES
WATER AND CARBON CYCLES AS NATURAL SYSTEMS
Water and carbon cycles are complex systems with multiple interrelationships. A systems approach helps to simplify the
system and see it as a bigger picture.
The global water and carbon cycles are closed systems (no inputs or outputs). Local water and carbon cycles are open
systems (with inputs and outputs).
Term Definition Water cycle example Carbon cycle example
Input Material or energy moving into Precipitation from atmospheric Precipitation from atmospheric
the system from the outside. stores (clouds). stores (clouds) containing dissolved
carbon dioxide.
Flows The relationships between Runoff, evaporation, infiltration, Burning, CO2 absorption,
components of a system. action of precipitation decomposition.
Stores The individual elements of the Lakes, vegetation, soil, river Trees and plants, soils
system. bodies. (decomposing matter), rocks
(geological makeup or containing
water), organisms.
Output Material or energy from the River water leaving the mouth Runoff containing dissolved carbon
system moving to the outside. of the river into the sea. dioxide that leaves the system.
Atmospheric stores leaving the
system.
Energy The power behind flows in a Thermal energy drives state Photosynthesis.
system, a driving force. changes of water. GPE and
kinetic energy causing river
flows down slopes.
Positive A cyclical sequence of flows Increased thermal energy, Increased thermal energy, melting
feedback leads to an increase of growth or thermal expansion + ice sheet permafrost, greenhouse gases
something, promoting melting, rising sea level, further released, enhanced greenhouse
environmental instability. ice sheet melting. effect, further thermal energy.
Negative A cyclical sequence of flows Thermal energy, surface Increased CO2 in atmosphere,
feedback leads to decrease of decline of temperatures rise, evaporation, increased plant growth, increased
something (neutralising effect), increased cloud cover, increase photosynthesis, CO2 removed from
promoting dynamic equilibrium. in solar energy being reflected, the air.
surface temperatures cool.
Dynamic This represents a state of
equilibrium balance within a constantly
changing system.
THE GLOBAL WATER CYCLE
STORES WITHIN THE GLOBAL WATER CYCLE
Hydrospher - Any liquid water.
e - Oceans, which contain saline water, make up 97% of global water stores (only 2.5% of stores
are freshwater).
- Lakes (50-100 years) and rivers (2-6 months).
Lithosphere - Water stored in the crust.
- Groundwater makes up 30% of global freshwater stores. This is water stored in unequally
distributed, vast underground reservoirs called aquifers. Aquifers most commonly form in
, porous and permeable rocks such as chalk and sandstone where water either enters the rocks
directly when they are exposed on the ground, or indirectly as the water drains through the
overlying soil. Shallow groundwater aquifers can store water for up to 200 years, but deeper
fossil aquifers, formed during wetter climatic periods, may last for 10,000 years.
- Rocks and soil. Soil acts as a more temporary store, holding water for 1-2 months.
- Upper mantle.
Cryosphere - Any frozen water.
- Glaciers, ice caps and ice sheets (69% of freshwater stores). From accumulation to ablation,
glaciers may store water for 20-100 years.
- Seasonal snow cover will last for 2-6 months.
Atmosphere - Water vapour.
Water moves through stores at different rates, and remains in stores for different amounts of time. This time is called
residence time.
PROCESSES DRIVING CHANGE IN THE MAGNITUDE OF STORES OVER TIME AND SPACE
Evaporation - Occurs when energy from solar radiation hits the surface of water or land and causes liquid
water to change state from a liquid to a gas (water vapour).
- As water evaporates it uses latent heat energy, so cools its surroundings.
- The rate of evaporation can depend on: amount of solar energy; availability of water;
humidity of air (closer the air is to saturation point, the slower the rate of evaporation);
temperature of the air (warmer air can hold more water vapour than cold air).
- Terrestrial plants lose water through transpiration where water is transported from the roots
of a plant to its leaves and then lost through pores on the leaf surface. Leaves also intercept
rain as it falls, and this water can be evaporated before it reaches the soil.
Condensation - As air cools it is able to hold less water vapour, so if cooled sufficiently it will get to a
temperature where it becomes saturated (dew point temperature).
- Excess water in the air will then be converted to liquid water by condensation.
- Water molecules need condensation nuclei (e.g. smoke, salt, dust) or surfaces (e.g. leaves,
grass, windows) that are below dew point temperature to condense on.
- If the surface is below freezing point then water vapour sublimates, changing directly from
gas to solid in the form of hoar frost.
Precipitation - Condensation is the direct cause of all precipitation.
Cryospheric - It is thought that there have been 5 major glacial periods in the earth’s history. The most
processes recent started 2.58 million years ago, continues today and is called the Quaternary
glaciation.
- During this time there have been glacial periods (interruptions of the hydrological cycle)
when, due to the volume of ice on land, sea level was approximately 120m lower than
present (cryospheric change has a regulatory role in sea levels) and continental glaciers
covered large parts of North America, Europe and Siberia.
- We are currently in an interglacial period where global ablation exceeds accumulation and
the hydrological cycle has returned and sea levels have risen increased volumes of water
and thermal expansion, which triggers further calving of ice (positive feedback).
- Over the last 740,000 years there have been 8 such glacial periods.
- Permafrost forms when air temperatures are so low that they freeze any soil and
groundwater, but rarely occurs under ice because the temperatures are not low enough.
DRAINAGE BASINS AS OPEN SYSTEMS
A drainage basin is the area that supplies a river with its supply of water, separated by high land called a watershed.
The drainage basin hydrological cycle (inputs, flows, stores, outputs):
- Precipitation lands in the bare surface or, more likely, vegetation cover (an interception store).
- A lot of water intercepted by plant surfaces is evaporated back into the atmosphere. Coniferous trees intercept
more than deciduous trees in winter, but this is reversed in summer.
, - Throughfall is the movement of water that reaches the ground directly through gaps in the vegetation
canopy when the canopy-surface rainwater storage exceeds its storage capacity. Stemflow is where water
flows down stems or trunks etc.
- On reaching the ground, water soaks into the soil through infiltration, controlled by gravity, capillary action and
soil porosity.
- Soil storage is the volume of water stored in the soil. Soil porosity is the most important factor. Coarse,
textured soils have larger pores and fissures than fine-grained soils so allow for more water flows. The burrowing
of worms and plant roots can also increase macro and micro channels within the soil.
- Water will move vertically down through the soil and unsaturated bedrock through percolation, and can then
be held in pore spaces in the rocks as groundwater and then passes into the zone of saturated rock where it
can move vertically and laterally by groundwater flow which is slow and can feed into rivers during long period
of drought.
- Porous rocks able to store a lot of water are called aquifers.
- Vegetation storage is when plants remove water from the soil and store it in their structure which may be lost
back to the atmosphere by transpiration, where water is diffused through the stomata of leaves and changes
state from liquid to gas. Broad leaves have relatively high rates of transpiration whilst needle-shaped leaves and
spines have evolved to minimise water loss.
- If rainfall intensity is greater than the infiltration rate then the soil has reached field capacity (dependent on
texture, sandy soils having low capacity and clay like soils higher) and the soil will become saturated. Water will
build up on the surface as surface storage which is usually in the form of puddles which is rare outside of a
man made environment with impermeable surfaces or in periods of long, intense heavy rainfall. More permanent
surface water stores include lakes, wetlands and dams. Much of this surface storage then evaporates back into
the atmosphere, especially under warm, dry windy conditions like that of semi-arid climates. The total volume of
water outputted from the system through transpiration and evaporation is referred to as evapotranspiration.
- When surface stores are fully saturated then overland flow (horizontal) will begin on slopes which are very
fast and will rapidly reach the nearest channel. Any lateral movement of water downslope is called throughflow
and is generally much slower (in a vegetated area may be quicker due to root channels).
- Rivers transfer water by channel flow and the water that leaves the drainage basin this way is called run-off.
FACTORS AFFECTING THE RATE OF INFILTRATION
Duration of As the length of the rainfall event increases, the rate of infiltration decreases and surface runoff
rainfall increases. This is because there is an increasing volume of water in the soil, reducing the potential for
infiltration.
Antecedent If the soil is relatively dry there is more potential for infiltration to occur. However, initially there is little
soil moisture difference between wet or dry soils as there may still be pore spaces near the surface that contain air
as water has moved downwards. Once these pore spaces are filled infiltration increases.
Soil porosity As porosity increases, infiltration increases and surface runoff decreases. There is a positive
relationship between porosity and infiltration and an inverse relationship between infiltration and
surface runoff. This is due to the large size of the pore spaces - large pore spaces allow more water to
infiltrate than small pore spaces.
Vegetation Vegetation has a major impact on infiltration and surface runoff. Infiltration is rapid over bare earth and
cover lower under forest cover, It is high over bare earth initially, but declines rapidly as the pre spaces in the
upper soil layer fill up with water. Under vegetated surfaces infiltration decreases more slowly as there
is a reduced volume of water reaching the ground and the reduced speed with which water reaches the
ground.
Raindrop As raindrop size increases, infiltration decreases - mainly due to the size of the raindrop exceeding the
size pore spaces they are entering. As infiltration decreases, surface runoff increases.
Slope angle As slope angle increases, so does the influence of gradient, infiltration rates decrease and surface
runoff increases. This is because gradient affects the length of time water remains on a slope - it
remains on the slope for longer on lower angle slopes, so there is more potential for infiltration.
The balance between inputs and outputs is known as the water balance.
𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛 = 𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 + 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑣𝑎𝑝𝑜𝑡𝑟𝑎𝑛𝑠𝑝𝑖𝑟𝑎𝑡𝑖𝑜𝑛 ± 𝑐ℎ𝑎𝑛𝑔𝑒𝑠 𝑖𝑛 𝑠𝑡𝑜𝑟𝑎𝑔𝑒
