Water and carbon

Terms: inputs, outputs, flows, stores

• inputs → where matter or energy is added to the system.

• outputs → where matter or energy leaves the system.

• flows → where matter or energy moves in the system.

• stores → where matter or energy builds up in the system.

Terms: boundaries, open systems, closed systems

• boundaries → limits to the systems e.g., watershed.

• open systems → when systems receive inputs and transfer outputs of energy or matter with other systems.

  • on a local scale, the carbon and water cycles are both open systems.

• closed systems → when energy inputs equal outputs.

  • on a global scale, they are both closed systems.

Terms: positive feedback, negative feedback, dynamic equilibrium

• positive feedback → a chain of events that nullifies the impacts of the original event, leading to dynamic equilibrium.

• negative feedback → when a chain of events amplifies the impacts of the original event.

• dynamic equilibrium → when inputs equal outputs, despite changing conditions.

Global distribution of water

• freshwater = 2.5% of all Earth's water.

• approximately 1.6% of the freshwater is locked away:

  • 68.7% as ice within the cryosphere.

  • the remaining 30.1% is groundwater.

• in total, just 0.9% of the Earth's total freshwater, is accessible to humans.

Global distribution and size of major stores of water: lithosphere

Lithosphere → water stored as groundwater = 30.1%

• whilst the level of storage capacity is low, this store captures water for the longest periods of time.

• water can flow through the lithosphere into underground aquifers but this transfer may be relatively slow, often taking many years.

• some water is stored within bedding planes, joints and pores in rocks and can remain there for hundreds of years.

Global distribution and size of major stores of water: hydrosphere

Hydrosphere → water stored as liquid = 1%

• processes impacting the hydrosphere are runoff and precipitation – inputs water to the store and evaporation moves water from the ocean into the atmosphere.

• these changes have minimal impact on the storage capacity – however long-term climatic change events e.g., ice ages, have the potential to lower the storage capacity significantly.

Global distribution and size of major stores of water: cryosphere

Cryosphere → water stored as ice = 68.7%

• major stores include the Antarctic and Greenland ice sheets, polar sea ice and mountain glaciers.

• annual changes to ice coverage have minimal impact upon storage capacity.

• but during ice ages, cryospheric storage increases and during warmer inter-glacial periods, it reduces.

Global distribution and size of major stores of water: atmosphere

Atmosphere → water stored as water vapour = 0.2%

• water is removed from water surfaces through evaporation and then stored temporarily as water vapour – then condenses before releasing back to earth as precipitation.

• also, transpiration from plants releases water vapour into the atmosphere.

Processes driving change in the magnitude of these stores over time and space, including flows and transfers: evaporation

Evaporation → the process of turning liquid into gas.

• occurs when energy from the sun hits the surface of water, increasing the amount of water stored in the atmosphere.

Temporal variation:

• Evaporation rates vary by season – warmer seasons i.e., summer, with warm, dry air and lots of solar radiation means evaporation will be high.

Spatial variation:

• Temperature of the air – warmer air can hold more water than cold air.

• Amount of solar energy – more solar radiation = more evaporation.

• Availability of water – more water available = more potential evaporation.

Processes driving change in the magnitude of these stores over time and space, including flows and transfers: condensation

Condensation → the conversion of water vapour/gas into liquid.

• Occurs when air cool to its dew point, forming clouds or fog.

Temporal variation:

• Often occurs at night when temperatures drop and there’s a lack of solar radiation.

• Frequent in cooler months.

Spatial variation:

• Common over cooler regions or high altitudes e.g., mountainous areas.

• Frequent in coastal areas, where moist air cools quickly.

Processes driving change in the magnitude of these stores over time and space, including flows and transfers: cloud formation

Cloud formation → when water vapour condenses around tiny particles.

• This can happen due to convection where warm air rises, it cools, forming cumulus clouds.

Temporal variation:

• Seasonal patterns e.g., monsoon seasons in Asia – cloud formation increases this time.

• In winter, frontal clouds more common in mid-latitudes due to the interaction of cold and warm air masses.

Spatial variation:

• High level of convection in the tropics due to intense heat – frequent/tall cumulonimbus clouds.

• Moist air from oceans in coastal areas cool quickly – forms clouds near shorelines.

Processes driving change in the magnitude of these stores over time and space, including flows and transfers: causes of precipitation

Precipitation → when water falls from the clouds towards the ground as rain, snow or hail etc.

• Frontal rainfall → where warm and cold air meet, forcing the warm air to rise above the cool air, cooling as it rises. Eventually leads to cloud formation and rainfall.

• Convectional rainfall → the sun heats the planet’s surface and the air above, providing the air with extra energy to rise upwards in thermals.

• Relief rainfall → when air masses are pushed up over mountainous/upland areas, cooling the air to form clouds and rainfall (major method of precipitation in UK).

• Orographic rainfall → rain associated with topography, when warm air meets mountains and is forced to rise and cool.

Temporal variation:

• Precipitation increases during monsoon seasons e.g., Pakistan.

• In temperate area, rainfall peaks in winter due to frequent frontal systems e.g., frontal uplift.

• Climate change leads to shifting wind patterns, changing precipitation patterns.

Spatial variation:

• Convectional rainfall dominates in the tropic due to strong solar heating and rapid cloud formation e.g., Amazon.

• Frontal rainfall in mid-latitudes where warm and cold air masses meet e.g., Europe or North America.

• Limited precipitation in polar regions due to low temperatures and reduced atmospheric moisture.

Processes driving change in the magnitude of these stores over time and space, including flows and transfers: cryospheric processes at hillslope

Cryospheric processes at hillslope → involve the movement, storage and melting of snow and ice in localised areas, such as slopes and valleys.

Key processes include:

• Freeze-thaw weathering

• Snow accumulation during colder months

• Snowmelt – in warmer periods, leading to surface runoff and even slope instability.

Temporal variation:

• Snow accumulation occurs in winter as snow build up, whilst snowmelt dominates in summer months.

• Climate change – warmer climates may reduce snow cover and cause permafrost to thaw.

Spatial variation:

• Polar or high-altitude areas see permanent or seasonal permafrost, while temperate regions experience freeze-thaw cycles.

• South-facing slopes i.e., the Northern Hemisphere melt faster due to more sunlight, while north-facing slopes retain snow longer.

Drainage basins as open systems: inputs

Precipitation:

• Precipitation → any water that falls to the surface of the earth from the atmosphere including rain, snow and hail.

• Some of the water may be intercepted by plants and trees – later evaporates.

• Some of the water travels as stemflow and either stores as puddles, flows over the ground, or infiltrates the soil.

Drainage basins as open systems: outputs

Evapotranspiration:

• Evapotranspiration → comprised of evaporation and transpiration:

• Evaporation → occurs when water is heated by the sun, causing it to become gas and rise into the atmosphere.

• Transpiration → occurs in plants when they respire through their leaves, releasing water they absorb through their roots, which then evaporates due to heating by the sun.

• ET accounts for nearly 100% of annual precipitation in arid areas & 75% in humid areas.

• Transpiration varies with vegetation e.g.; broad leaves have higher rates than needle-shaped leaves as there is more surface area.

• Transpiration varies with crop type e.g., grass-like crops like wheat have low rates.

Runoff:

• Runoff → when water flows over the land as surface water.

• The type of precipitation affects the amount of runoff – snow delays runoff, but increases once it melts.

• The intensity of precipitation affects the amount of runoff – large amounts of rainfall in a short time (aka flash flooding) increases the amount of runoff.

• Human activities, such as impermeable surfaces from infrastructure, increase runoff.

Drainage basins as open systems: stores

Interception:

• Interception → water landing on plants on their branches and leaves before reaching the ground.

• Type of precipitation affects the drainage basin – snowfall can store on plant surfaces until it melts, delaying water input to the system.

• Areas with higher temperatures reduce interception storage – evaporation moves the position of water within the system.

• It’s more significant during light rain or small showers – most of precipitation never reaches the soil and doesn’t exceed the capacity.

Surface storage:

• Surface storage → the total volume of water stored in puddles, ponds and lakes.

• Can be natural e.g., lakes, floodplains or man-made e.g., reservoirs, urban drainage systems.

• Plays key role in regulating river discharge – stores excess precipitation, reducing flood risk.

• Climate change is reducing ice caps and glaciers – decreases long-term water storage.

Soil water:

• Soil water → the amount of water stored in the soil which is utilised for plant growth.

• Capillary action → involves water being transmitted upwards towards the soil surface and atmosphere.

• Or the water can be absorbed or held.

• In fine-textured soils, there is a high proportion of small pores, holding the water at high suctions.

• In coarse-textured soils, much of the water is held in fairly large pores at low suctions – very little is held in small pores.

Drainage basins as open systems: stores

Groundwater:

• Groundwater → water that is stored in the pore spaces of permeable rock underground.

• Groundwater accounts for 30.1% of all freshwater on earth.

• Groundwater recharge → refilling of water in pores where water has dried up or been extracted by human activity.

• Human activities have been depleting major groundwater stores e.g., the Ogallala aquifer in Texas has experienced a fall in the water table of 50m in under 50 years.

Channel storage:

• Channel storage → water that is temporarily stored in a river channel.

• Can be controlled by human factors such as dam construction – increased storage.

• During dry seasons, channel storage decreases – some river experience intermittent flow or dry up completely.

• Channel storage is influenced by land use – urbanisation increases runoff into rivers, increasing storage.

Drainage basins as open systems: flows

Stemflow:

• Stemflow → the flow of intercepted water down the trunk or stem of a plant.

• More significant in dense forest, where tree canopies intercept high rainfall.

• Helps direct water to the base of trees – promotes deep infiltration & reduces surface runoff.

• Varies depending on tree species – smooth-barked trees encourage stemflow e.g., beech.

• Varies on the season – in deciduous forest, stemflow decreases in winter when leaves are absent.

Infiltration:

• Infiltration → downward movement of water from the surface into the soil.

• Water soaks into the soil through gravity as well as capillary action (the attraction of water molecules to soil particles).

• Soil type affects the rate of infiltration – thin, frozen or already saturated soils have a lower infiltration capacity.

• Trees can promote infiltration – the roots form pathways for water to percolate underground.

Drainage basins as open systems: flows

Overland flow (surface runoff):

• Overland flow → the flow of water that occurs when excess stormwater, meltwater, or other sources flows over the earth's surface.

• Water can flow over a large surface area (aka sheetflow) or into small channels (aka rills).

• Not common in the UK – much of the land is covered by vegetation, so is absorbed.

• Common in urban areas e.g., roads, where there are less permeable surfaces.

Channel flow (river discharge):

• Channel flow → the movement of water within the river channel.

• Low channel flow during a drought may result in river drying e.g., the Colorado River experiences reduced discharge due to overuse and climate change.

• Geology affects flow – permeable rocks e.g., limestone allow infiltration, reducing direct channel flow.

• Human activities can aid the flow – man-made dams and reservoirs control discharge levels.

Concept of water balance

Precipitation = total runoff + evapotranspiration +/- storage (change in)

• The water balance → expresses the process of water storage and transfer in a drainage basin system.

• The change in storage value could be positive or negative:

  • Negative → there are more outflows (runoff and evapotranspiration) than inflows (precipitation).

  • Positive → there are more inflows (precipitation) than outflows (runoff and evapotranspiration).

Runoff variation and the flood hydrograph

• Flood hydrograph → represents rainfall for the drainage basin of a river and the discharge of the same river on a graph.

• Discharge → the volume of water passing through a cross-sectional point of the river at any one point in time (measured in cumecs), made up of baseflow and stormflow.

• Rising limb → the line on the graph that represents the discharge increasing.

• Falling limb → the line of the graph that represents the discharge decreasing.

• Lag time → the time between peak rainfall and peak discharge.

• Baseflow → the level of groundwater flow.

• Stormflow → comprised of overland flow and throughflow.

• Bankfull discharge → the maximum capacity of the river – if discharge exceeds this then the river will burst its banks and be in flood.

Flashy and subdued hydrographs

Flashy:

• Short lag time

• Steep rising and falling limb

• Higher flood risk

• High peak discharge

Subdued:

• Long lag time

• Gradually rising and falling limb

• Lower flood risk

• Low peak discharge

Changes in the water cycle over time to include natural variation including storm events, seasonal changes

Storm events:

• Cause sudden increases in rainfall, leading to flooding and replenishment of some water stores – unlikely to cause long-term change.

• Can overwhelm river channels → temporary increases in channel flow and discharge.

• Increased surface runoff → reduced infiltration due to waterlogging.

Seasonal changes:

• Less precipitation, more evapotranspiration in summer due to higher temperatures.

• Reduced flows in the water cycle in winter as water is stored as ice.

• Reduced interception in winter, when deciduous trees lose their leaves.

• Increased evapotranspiration in summer, as deciduous trees have their leaves and there are higher temperatures.

• Soil moisture and infiltration – soil may become impermeable if frozen.

Cryospheric processes:

• In the past, glaciers and icecaps stored significant proportions of freshwater through the process of accumulation.

• Currently, almost all of the world’s glaciers are shrinking, causing sea levels to rise.

• If all the world’s glaciers and icecaps melted, sea levels would rise by around 60m.

Droughts:

• Cause major stores to be depleted and the activity of flows within the water cycle to decrease – may cause long-term damage as they become more common due to climate change.

Changes in the water cycle over time to include human impact: farming practices

Farming practices:

• Ploughing breaks up the surface, increasing infiltration – but exposes soil to erosion.

• Arable farming (crops) can increase interception and evapotranspiration.

• Pastoral farming (animals) compacts soil, reducing infiltration and increasing runoff.

• Irrigation removes water from local rivers, decreasing their channel flow.

• Use of fertilisers and pesticides can pollute water sources via runoff.

Changes in the water cycle over time to include human impact: land use change

Land use change:

• Deforestation e.g., for farming, reduces interception and evapotranspiration. But infiltration increase as dead plant material in forests usually prevents infiltration.

• Construction reduces infiltration due to more impermeable surfaces, but increases runoff.

• Urbanisation causes storm drains accelerate flow to rivers – shorter lag times and higher flood peaks.

• Green roofs and Sustainable Urban Drainage Systems (SUDS) → use grass and soil to reduce the amount of impermeable surfaces, helping to tackle urban flooding problems in some cities.

Changes in the water cycle over time to include human impact: water abstraction

Water abstraction → water removed from stores for human use e.g., agriculture:

• Reduces the volume of water in surface stores e.g., lakes.

• It can lower water tables and reduce base flow in rivers.

• Water abstraction increases in dry seasons e.g., water needed for irrigation.

• Human abstraction from aquifers as an output to meet water demands is often greater than inputs into the aquifer – decline in global long-term water stores.

Global distribution, and size of major stores of carbon: lithosphere

• Amount → 99.983% of total carbon

• Forms of carbon → the largest of the carbon stores, as sedimentary rocks contain carbon such as limestone (calcium carbonate), hydrocarbons (fossil fuels) and marine sediments from shells and marine skeletons.

• Carbon is this store can be released by volcanic eruptions and combustion of fossil fuels.

• Residence time → 240-300 million years.

Global distribution, and size of major stores of carbon: hydrosphere

• Amount → 0.0076% of total carbon

• Forms of carbon → 90% of oceanic carbon is dissolved as bicarbonate, with carbonate ions and dissolved CO2 also found in oceans, rivers and lakes.

• Majorly found in the Pacific Ocean as it has the largest capacity.

• Enters through diffusion from the atmosphere.

• Residence time on the surface → 25 years.

• Residence time in the deep → 1250 years.

Global distribution, and size of major stores of carbon: cryosphere

• Amount → 0.0018% of total carbon

• Forms of carbon → frozen ground (permafrost) of tundra and arctic regions contains plant material, as well as ice caps.

• Residence time → 1000’s of years, but ice cores show millions of years.

• But global warming is threatening the release of carbon from melting ice caps and glaciers around the world.

Global distribution, and size of major stores of carbon: biosphere

• Amount → 0.0012% of total carbon

• Forms of carbon → living plants and animals, including marine and aquatic life, as well as soils and dead organic matter.

• Deforestation and land-use changes affect this store.

• Residence time → 18 years – carbon constantly cycled via photosynthesis.

Global distribution, and size of major stores of carbon: atmosphere

• Amount → 0.0015% of total carbon

• Forms of carbon → mainly as carbon dioxide CO2 and methane CH4.

• Concentrations vary due to seasons, emissions, vegetation cover.

• Links to greenhouse effect – more atmospheric carbon → more trapped infrared radiation → warming.

• Residence time → 6 years.

Photosynthesis

• Photosynthesis → the process by which plants, algae, and some bacteria convert carbon dioxide (CO₂), water (H₂O), and sunlight into glucose (C₆H₁₂O₆) and oxygen (O₂).Its role is to remove CO₂ from the atmosphere, storing carbon in biomass.

• In addition, it drives primary productivity, influencing carbon sequestration in ecosystems.

Photosynthesis at a seral scale (the stages of vegetation succession and can relate to specific environments)

Temporal variation:

• Early seral (pioneer) stage → photosynthesis is low due to limited vegetation e.g., lichens, mosses.

• Mid-seral stage → photosynthesis increases as grasses, shrubs and small trees establish.

• Climax community → high and stable photosynthesis due to dense plant cover.

Spatial variation:

• Hydrosere (water-based succession) → higher initial photosynthesis due to aquatic plants – increases and wetland forests develop.

• Psammosere (sand dune succession) → low photosynthesis initially as sand is nutrient-poor – increases at organic matter accumulates.

• Lithosere (rock succession) → very slow photosynthesis initially – increases as soil forms eventually.

Photosynthesis at a continental scale (the scale of an entire continent, including biomes, climates and ecosystems)

Temporal variation:

• Temperate regions (Europe) → high photosynthesis in summer, low in winter – trees lose leaves in winter from deciduous trees, lowering rates.

• Climate change → large scale deforestation e.g., Amazon, reduces continental scale photosynthesis.

• Urbanisation → increased droughts in some areas e.g., the Sahel, reduce photosynthesis due to water stress.

• Global warming → temperate regions are increasing in warmth, extending growing seasons and increasing photosynthesis.

Spatial variation:

• Tropical biomes → highest photosynthesis rates – due to constant sunlight, warm temperatures and high precipitation.

• Tundra and polar regions → extremely low photosynthesis due to cold temperatures and short growing seasons.

• Altitude → higher elevations have limited photosynthesis – due to lower temperatures and reduces atmospheric CO2 available.

Respiration

• Respiration → where organisms break down glucose (C₆H₁₂O₆) to release energy, producing carbon dioxide (CO₂) and water (H₂O) as by-products.

• Respiration can occur aerobically which involves the requirement of oxygen and so releases more energy.

• But, in low-oxygen environments anaerobic respiration occurs, producing less energy and sometimes methane.

• Its role is to return CO₂ to the atmosphere and balance photosynthesis.

Respiration at a seral scale

Temporal variation:

• Early seral stage → respiration is low due to limited biomass (few organisms present).

• Mid-seral stage → respiration increases from plant growth – increases in organic matter lead to greater decomposition rates, increasing microbial respiration.

• Climax community → respiration is high and stable from balanced carbon inputs from photosynthesis and outputs from respiration – as animals become present at this stage, animal respiration contributes significantly.

Spatial variation:

• Hydrosere → low oxygen levels means dominant anaerobic respiration – shifts to aerobic as soil forms.

• Psammosere → harsh conditions mean initial low respiration – but, increases as soil fertility improves.

• Lithosere → slow biomass development limits respiration.

• Microclimate effects → warmer, wetter areas within an ecosystem tend to have higher respiration rates.

Respiration at a continental scale

Temporal variation:

• Occurrence → respiration occurs continuously as it’s a vital process for organisms – unlike photosynthesis which stops at night.

• Daily cycles → respiration is often higher at night in plants – lack of photosynthesis doesn’t replace carbon levels.

• Seasonal cycles → higher rates in summer from warmer temperatures and greater biological activity – low in winter as cold temperatures slow rate, especially in temperate and boreal regions.

• Climate change → rising global temperatures increases respiration rates, leading to more CO2 release.

Spatial variation:

• Tropical biomes → high respiration due to warm, moist conditions and rapid decomposition.

• Polar and tundra regions → low respiration due to permafrost and limited biological activity.

• Altitude → higher elevations have lower respiration due to cooler temperatures and thinner air.

• Topography → valleys with more moisture and organic matter have higher rates.

Decomposition

• Decomposition → a process by which dead organic material (plants, animals, and microorganisms) is broken down, releasing CO₂, methane, nutrients, and organic matter into the ecosystem.

• Micro-organisms (bacteria & fungi) and detritivores (worms, insects) are key decomposers.

• Its role is to return carbon stored in organic matter to the atmosphere – also releases nutrients into soils, supporting plant growth.