1.c. The carbon and water cycles have distinctive processes and pathways

What are the processes of the water cycle?

• Evaporation and transpiration → condensation → precipitation → interception (throughfall and stemflow) → infiltration, percolation, throughflow, groundwater flow and surface runoff

What is the water balance equation?

• Precipitation (P) = Evapotranspiration (E) + Streamflow (Q) ± Storage

(1) What is evaporation?

• The change of liquid water to vapour

• The thermal energy is absorbed as latent heat and released later in condensation

What is latent heat?

• The energy absorbed or released during a change of state without changes in temperature

(1) What is transpiration?

• The diffusion of water vapour to the atmosphere from the stomata of plants

What are evaporation and transpiration influenced by?

• Temperature

• Wind speed

• Water availability

(2) What is condensation?

• The change of vapour to liquid water

• Air is cooled to its dew point

• Clouds form through condensation in the atmosphere

What is dew point?

• The critical temperature at which air cools to become saturated

What are clouds?

• Visible aggregates of water or ice or both that float in the free air

What are the 3 main types of clouds?

• Cumuliform

• Stratiform

• Cirrus

What are the characteristics of cumuliform clouds?

• Clouds with flat bases with vertical development

• Cooling by: convection, advection or orographic uplift

What are the characteristics of stratiform clouds?

• Layer clouds

• Cooling by: advection, orographic uplift or weather front

What are the characteristics of cirrus clouds?

• Clouds with wispy and tiny ice crystals

• Cooling by: convection, frontal uplift

What are the cooling mechanisms of cloud formation?

• Convection

• Advection

• Orographic (and frontal) uplift

• Weather front

What is convection?

• Surface heating causes air to rise freely through the atmosphere

• As the air rises and pressure falls, it cools by adiabatic expansion

What is advection?

• Warm air masses move horizontally across a cooler surface

What is orographic uplift?

• Air masses are forced to rise as they cross a mountain barrier or as turbulence forces their ascent

What is frontal uplift?

• Warm air is forced to rise over cold air along warm or cold fronts

What are weather fronts?

• Warm air masses are mixed with cooler air masses

What are lapse rates?

• The vertical distribution of temperature in the lower atmosphere

• The temperature changes that occur within an air parcel as it rises vertically away from the ground

What are the 3 types of lapse rates?

• Environmental (ELR)

• Dry adiabatic (DALR)

• Saturated adiabatic (SALR)

What is environmental lapse rate?

• The vertical temperature profile of the lower atmosphere at any given time

• The temperature falls by 6.5°C for every km of height gained

What is dry adiabatic lapse rate?

• The rate at which a parcel of dry air (less than 100% humidity so that condensation is not taking place) cools

• Cooling (adiabatic expansion) is ~10°C every km

What is saturated adiabatic lapse rate?

• The rate at which a saturated parcel of air (100% humidity) cools

• Condensation releases latent heat, ~7°C every km

What is atmospheric instability?

• Air is warmer than its surroundings - less dense and buoyant

• Air rises freely in a convection current

(3) What is precipitation?

• Water and ice that falls from clouds

• Forms: rain, snow, hail, sleet and drizzle

How does precipitation form?

• Vapour in the atmosphere cools and condenses into tiny water droplets to form clouds

• These droplets aggregate, reach a critical size and leave the cloud as precipitation

What are the 3 main factors affecting precipitation?

• Latitude

• Intensity

• Duration

How does latitude affect precipitation?

• In high latitudes and mountainous catchments, precipitation falls as snow and remain on the ground for several months

• A time lag between snowfall and run-off

How does intensity affect precipitation?

• Low-intensity precipitation (less than 2.5mm/hour) moves slowly

• High-intensity precipitation (10-15mm/hour) moves rapidly (into streams and rivers)

How does duration affect precipitation?

• Prolonged events, linked to depressions and frontal systems, may deposit exceptional amounts of precipitation and cause river flooding

(4) What is interception?

• Rainwater which is caught by vegetation, storing it temporarily on branches, leaves and stems

• Reduces the chances of flooding

What can happen to moisture that has been intercepted?

• Interception loss

• Throughfall

• Stemflow

What is interception loss?

• Rainwater that evaporates or falls to the ground

What are the main factors affecting interception loss?

• Interception storage facilities

• Wind speed

• Vegetation type

• Tree species

How do interception storage facilities affect interception loss?

• Before rainfall: vegetation are dry - their ability to retain water is at a maximum

• After rainfall: vegetation are saturated - stemflow and throughfall increases

How does wind speed affect interception loss?

• Evaporation rates increases with wind speed

• Turbulence also increases with wind speed, causing additional throughfall

How does vegetation type affect interception loss?

• Interception losses are greater from grasses than agricultural crops

• Trees have a large surface area and aerodynamic roughness, so they have higher interception losses than grasses

How do tree species affect interception loss?

• Interception losses are greater from evergreen conifers than from broad-leaved and deciduous trees

• Conifers have leaves all year round, and water adheres to the spaces between conifer needles

What is throughfall?

• Rainwater that is briefly intercepted before dripping to the ground

What is stemflow?

• Rainwater that flows to the ground along branches and stems (prolonged or intense rainfall)

(5) What is infiltration?

• The vertical, downwards movement of water into the soil

(5) What is percolation?

• The vertical, downwards movement of water through the soil and rock layers

What are the differences between infiltration and percolation?

• Infiltration occurs closer to the surface of the soil

• Infiltration transports water from the surface. Percolation moves water through the soil profile

What is infiltration capacity?

• The maximum rate at which soil can absorb rainwater

When does overland flow occur?

• When rainfall intensity exceeds infiltration capacity

When does saturated overland flow occur?

• Rainfall always infiltrates the soil

• Overland flow only occurs when soil becomes saturated and the water table rises to the surface

(5) What is throughflow?

• The lateral, sideways movement of water through the soil into water bodies

(5) What is groundwater flow?

• The vertical and lateral movement of water through rock pores and joints into water bodies

What is ablation?

• The loss of ice from snow, ice sheets and glaciers due to melting, evaporation and sublimation

• Flooding

What are the fluxes of the carbon cycle?

• The carbon cycle doesn’t have a ‘set order’ - it occurs simultaneously

• Precipitation, photosynthesis, decomposition, weathering, respiration and combustion

How does precipitation flux carbon?

• Atmospheric CO₂ dissolves in rainwater to form weak carbonic acid

How does anthropogenic emission affect rainfall?

• Increased acidity of rainfall in ocean surface waters with harmful effects on marine life

How does photosynthesis flux carbon?

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

• The volume of carbon exchanged by photosynthesis and respiration each year is 1000x greater than that moving through the slow carbon cycle

How can decomposition flux carbon?

• Bacteria and fungi breakdown dead organic matter, extracting energy such as CO₂ to the atmosphere and mineral nutrients to the soil

How do climatic conditions affect decomposition?

• The fastest rates occur in warm, humid environments such as the tropical rainforest

• The slowest rates occur in cold environments such as the tundra or drylands

How does respiration flux carbon?

• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

• The volume of carbon exchanged by respiration and photosynthesis each year is 1000x greater than that moving through the slow carbon cycle

How does combustion flux carbon?

• Organic material reacts or burns in the presence of oxygen

• The combustion process releases CO₂ as well as other gases such as sulphur dioxide and nitrogen oxides

How can combustion produce natural fuels?

• Long, cold winters slow the decomposition of forest litter on the forest floor

• Wildfires free carbon and nutrients previously inaccessible to forest trees

• It opens up the forest canopy, creating new habitats and increasing biodiversity

How does human activity affect combustion?

• Firing of forest and grassland to clear land for cultivation or to improve the quality of grazing

• The combustion of fossil fuels

What is natural sequestration?

• The process by which natural carbon sinks absorb and store CO₂ from the atmosphere

What are the 2 types of natural sequestration?

• Physical (inorganic) pump

• Biological (organic) pump

What is natural sequestration - physical pump?

• The mixing of surface and deep ocean waters by vertical currents, creating an even distribution of dissolved carbon

How does natural sequestration - physical pump - store carbon?

• Surface ocean currents transport the dissolved CO₂ polewards where it cools, becomes denser and sinks

• Downwelling

• Cold, carbon-rich water rises and CO₂ diffuses into the atmosphere

What does downwelling do in natural sequestration - physical pump?

• Downwelling occurs in certain places

• Dissolved carbon is carried to the ocean depths where individual carbon molecules transport the carbon to areas of upwelling

What is natural sequestration - biological pump?

• Carbon is exchanged between the oceans and atmosphere through the actions of marine organisms

How does natural sequestration - biological pump - store carbon?

• Phytoplankton photosynthesises to produce organic material

• Carbon locked in the phytoplankton either accumulates in sediments on the ocean floor or is decomposed and released into the ocean as CO₂

How do marine organisms benefit from natural sequestration - biological pump?

• Coccolithophores, molluscs and crustaceans extract carbonate and calcium ions to manufacture plates, shells and skeletons of calcium carbonate

• Most of this carbon rich material is lithified to form chalk and limestone

How does vegetation store carbon?

• Land plants in rainforests and boreal forests contain huge stores of carbon

• Most of this carbon is extracted from atmospheric CO₂ through photosynthesis

How does (chemical) weathering flux carbon?

• Rainwater is a weak carbonic acid which dissolves limestone and chalk

• Carbonation releases carbon from limestone

• The process is effective beneath a soil cover as the higher concentration of CO₂ is acidic

How does (physical) weathering flux carbon?

• Freeze-thaw breaks rocks down into smaller particles and increases the surface area exposed to chemical attack

How does (biological) weathering flux carbon?

• Chelation is rainwater mixed with decaying organic material in the soil which forms acids attacking rock minerals

• In tropical environments when decomposition is rapid and forest trees provide leaf litter