Water and carbon cycle

• Closed system


  The global hydrological cycle is the circulation of water around the earth. It is a closed system of linked processes so there are no external inputs or outputs. For this reason, the amount of global water is finite and constant.
• Global hydrological cycle


  • The only thing that does change is the state in which the water exists (liquid, vapour, ice). The proportions of global water held in each state vary over time with changes in climate.
• Driven by Solar Energy and Gravitational Potential Energy


  The power that drives the global hydrological cycle comes from two sources: 1. Solar energy: in the form of heat 2. Gravitational potential energy: causes rivers to flow downhill and precipitation to fall to the ground.
• Water stores


  • Oceans
  • Glaciers and ice sheets (cryosphere)
  • Surface runoff
  • Atmosphere
• Freshwater stores


  • The cryosphere is the largest, holding 69% of global freshwater
  • Groundwater holds 30%
  • Less than 1% is stored in the biosphere (vegetation and soil moisture)
• Flows and fluxes


  • Evaporation 400,000 (oceans to atmosphere)
  • Precipitation 370,000 (atmosphere to oceans)
  • Evaporation 60,000 (atmosphere to landmasses)
  • Precipitation 90,000 (atmosphere to landmasses)
  • Surface runoff 30,000 (landmasses to oceans)
• The global water budget takes into account all the water that is held in stores and flows of the global hydrological cycle. The most significant feature of the budget is that only 2.5% of it is freshwater; the rest is in oceans. Even more remarkable is that only 1% of all freshwater is 'easily accessible surface freshwater'. Nearly 70% is locked up in glaciers and ice sheets.
• Residence time


  The average time a molecule of water will spend in one of the stores. Residence times vary from 10 days in the atmosphere to 3,600 years in the oceans and 15,000 years in an ice cap.
• Fossil water


  Ancient, deep groundwater made from pluvial (wetter) periods in the geological past
• Cryosphere


  Made up of those areas of the world where water is frozen into snow or ice
• From a human viewpoint, the most critical feature of the global water budget is that accessible surface water is a mere 1% of all the world's freshwater, and this is the major source of water for human use.
• Proportions of global water

  • 97.5% in oceans
  • 2.5% freshwater
  • 69% in ice caps and glaciers
  • 30% in groundwater
  • 1% as easily accessible surface water
  • 52% in lakes
  • 38% as soil moisture
  • 8% as atmospheric water vapour
  • 1% in rivers
  • 1% as accessible water in plants
• Inputs


  Precipitation patterns and types
• Characteristics of precipitation


  • Form: rain, snow or hail
  • Amount
  • Intensity
  • Seasonality
  • Distribution
• Flows


  • Interception
  • Infiltration
  • Percolation
  • Throughflow
  • Groundwater flow
  • Surface runoff
  • River or channel flow
• Drainage basin


  The area of land drained by a river and its tributaries, sometimes referred to as a river catchment. The boundary of a drainage basin is defined by the watershed.
• Drainage basin


  • It is an open system with external inputs and outputs. Since those inputs vary over time, so does the amount of water in the drainage basin.
• Climate


  Mainly impacts on the inputs and outputs. Climate has a role in influencing the type and amount of precipitation overall and the amount of evaporation (i.e. the major inputs and outputs). Climate also has an impact on vegetation type.
• Soils


  Largely affect the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff). Soils determine the amount of infiltration and throughflow, and indirectly, the type of vegetation.
• Geology


  Largely affects the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff). Geology can impact on subsurface processes such as percolation and groundwater flow (and, therefore, on aquifers). Indirectly, geology affects soil formation.
• Relief


  Largely affects the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff). Relief can impact on the amount of precipitation. Slopes can affect the amount of runoff.
• Vegetation


  Largely affects the relative importance of the different flows within the system (of these flows perhaps the most important is surface runoff). The presence or absence of vegetation has a major impact on the amount of interception, infiltration and occurrence of overland flow, as well as on transpiration rates.
• Humans disrupt the drainage basin cycle by accelerating processes, (deforestation, changing land use) and creating new water storage reservoirs or by abstracting water.
• Human changes that disrupt the drainage basin system


  • Changes to rivers and drainage
  • Changes to the character of the ground surface (its shape, texture and covering)
• River management


  • Construction of storage reservoirs holds back river flows
  • Abstraction of water for domestic flow and industrial use reduces river flows
  • Abstraction of groundwater for irrigation lowers water tables
• Deforestation


  • Clearance of trees reduces evapotranspiration, but increases infiltration and surface runoff
• Changing land use - agriculture

  • Arable to pastoral: compaction of soil by livestock increases overland flow
  • Pastoral to arable: ploughing increases infiltration by loosening and aerating the soil
• Changing land use - urbanisation

  • Urban surfaces (tarmac, tiles, concrete) speed surface runoff by reducing percolation and infiltration
  • Drains deliver rainfall more quickly to streams and rivers, increasing chances of flooding
• Components of the drainage basin most affected by humans

  • Evaporation and evapotranspiration
  • Interception
  • Infiltration
  • Groundwater
  • Surface runoff
• River regime


  The annual variation in the discharge or flow of a river at a particular point, and is usually measured in cumecs.
• Factors influencing river regimes

  • The size of the river and where discharge measurements are taken along its course
  • The amount, seasonality and intensity of the precipitation
  • The temperatures, with possible meltwater and high rates of evaporation in the summer
  • The geology and soils particularly their permeability and porosity; groundwater noted in permeable rocks is gradually released into the river as base flow
  • The type of vegetation cover: wetlands can hold water and release it slowly into the river
  • Human activities aimed at regulating a river's discharge
• Storm hydrograph


  Plots the occurrence of a short period of rain (maybe a heavy shower or storm) over a drainage basin and the subsequent discharge of a river.
• Main features of a hydrograph

  • Rising limb
  • Peak discharge
  • Lag time
  • Falling or recessional limb
  • Base flow
• Variation in storm hydrograph shape

  • Closely linked to the nature of the rainfall event

  A result of the particular physical characteristics of individual drainage basins
• 'Flashy' hydrograph


  Very steep limbs, especially rising limbs, a high peak discharge and a short lag time
• 'Delayed', 'flat' or 'subdued' hydrograph


  Gently inclined limbs, a low peak discharge and a long lag time
• Urbanisation


  Changes the characteristics of the land surface, leading to increased surface runoff and reduced infiltration and evapotranspiration
• Impacts of urbanisation


  • Construction work leads to the removal of the vegetation cover

  Bare soil is eventually replaced by a covering of concrete and tarmac, both of which are impermeable and increase surface runoff

  The high density of buildings means that rain falls on roofs and is then swiftly fed into drains by gutters and pipes

  Drains and sewers reduce the distance and time rainwater travels before reaching a stream or river channel

  Urban rivers are often channelised with embankments to guard against flooding. When floods occur, they can be more devastating

  Bridges can restrain the discharge of floodwaters and act as local dams, thus prompting upstream floods
• Planners' role in managing flood risk


  Strengthening the embankments of streams and rivers

  Putting in place flood emergency procedures

  Steering urban development away from high-risk areas such as floodplains
• Factors affecting the shape of storm hydrographs

  • Weather/climate

  Rock type

  Vegetation

  Land use