PMT- Water & Carbon Cycles

• Systems


  Composed of: Inputs - Where matter or energy is added to the system, Outputs - Where matter or energy leaves the system, Stores - Where matter or energy builds up in the system, Flows - Where matter or energy moves in the system, Boundaries - Limits to the system (e.g. watershed)
• Open systems


  Systems that receive inputs and transfer outputs of energy or matter with other systems
• Closed systems


  Systems where energy inputs equal outputs
• Dynamic equilibrium


  When inputs equal outputs despite changing conditions
• Positive feedback


  A chain of events that amplifies the impacts of the original event
• Negative feedback


  A chain of events that nullifies the impacts of the original event, leading to dynamic equilibrium
• On a local scale the carbon and water cycles are both open systems, but on a global scale, they are closed systems
• Each of these systems contains flows/transfers, inputs, outputs and stores/components
• Local drainage basin system


  An open system where water may be lost as an output through evapotranspiration and runoff, but more water may be gained as an input through precipitation

  
• Inputs, outputs, flows and stores that drive and cause changes in the water cycle over time


  • Precipitation
  • Evapotranspiration
  • Streamflow

  
• Precipitation


  Any water that falls to the surface of the earth from the atmosphere including rain, snow and hail

  
• Types of rainfall


  • Convectional
  • Relief
  • Frontal
• Evapotranspiration


  Compromised of evaporation and transpiration. Evaporation occurs when water is heated by the sun, causing it to become a 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
• Streamflow


  All water that enters a drainage basin will either leave through the atmosphere, or through streams which drain the basin

  
• Infiltration


  The process of water moving from above ground into the soil. The infiltration capacity refers to how quickly infiltration occurs
• Overland flow


  When precipitation falls at a greater rate than the infiltration capacity

  
• Percolation


  Water moves from the ground or soil into porous rock or rock fractures. The percolation rate is dependent on the fractures that may be present in the rock and the permeability of the rock
• Throughflow


  Water moves through the soil and into streams or rivers. Speed of flow is dependent on the type of soil

  
• Surface runoff (Overland flow)


  Water flows above the ground, as sheetflow (lots of water flowing over a large area), or in rills (small channels similar to streams, that are unlikely to carry water during periods where there is not any rainfall)

  
• Groundwater flow


  Water moves through the rocks. Ensures that there is water in rivers, even after long period of dry weather

  
• Streamflow


  Water that moves through established channels
• Stemflow


  Flow of water that has been intercepted by plants or trees, down a stem, leaf, branch or other part of a plant
• Stores


  • Soil water - Water stored in the soil which is utilised by plants
  • Groundwater - Water that is stored in the pore spaces of rock
  • River channel - Water that is stored in a river
  • Interception - Water intercepted by plants on their branches and leaves before reaching the ground
  • Surface storage - Water stored in puddles, ponds, lakes etc.

  
• Water table


  The upper level at which the pore spaces and fractures in the ground become saturated
• Water balance


  Used to express the process of water storage and transfer in a drainage basin system and uses the formula: Precipitation = Total Runoff + Evapotranspiration +/- (change in) Storage

  
• The water balance of an area will change dependent on physical factors, especially during seasonal variations of temperature and precipitation

  
• Impacts on the water cycle on a local scale


  • Deforestation
  • Storm events
  • Seasonal changes
  • Agriculture
  • Urbanisation

  
• Soil water budget


  Shows the annual balance between inputs and outputs in the water cycle and their impact on soil water storage/availability
• The maximum possible level of storage of water in the soil is field capacity. Once the field capacity is reached, any rainfall after this will not infiltrate the soil and is likely to cause flooding
• Seasonal variation of the soil water budget


  • Autumn - Greater input from precipitation than output from evapotranspiration, soil moisture levels increase, water surplus occurs
  • Winter - Potential evapotranspiration from plants reaches a minimum, precipitation continues to refill the soil water stores
  • Spring - Potential evapotranspiration increases as plants start growing again, still a water surplus
  • Summer - Evapotranspiration peaks and rainfall is at a minimum, soil water stores are depleting, water deficit may occur

  
• The global water cycle is comprised of many stores, the largest being oceans, which contain 97% of global water. Only 2.5% of stores are freshwater of which 69% is glaciers, ice caps and ice sheets and 30% is groundwater. Surface and other freshwater only accounts for around 1%
• In summer, the hotter weather leads to


  Utilisation of soil water as evapotranspiration peaks and rainfall is at a minimum
• The output from evapotranspiration is greater than the input from precipitation


  The soil water stores are depleting
• If there is a long hot summer and spring, a lack of winter rainfall, or a drought the year before

  A water deficit may occur

  
• Global water stores


  • Oceans (97%)
  • Freshwater (2.5%)
  • Glaciers, ice caps and ice sheets (69% of freshwater)
  • Groundwater (30% of freshwater)
  • Surface and other freshwater (1% of global stores)
• Areas where water can be stored


  • Hydrosphere (any liquid water)
  • Lithosphere (water stored in the crust and upper mantle)
  • Cryosphere (any water that is frozen)
  • Atmosphere (water vapour)
• Aquifers


  Underground water stores that are unevenly distributed globally
• Water storage times


  • Shallow groundwater aquifers (up to 200 years)
  • Deeper fossil aquifers (10,000 years)
  • Glaciers (20-100 years)
  • Lakes (50-100 years)
  • Seasonal snow cover and rivers (2-6 months)
  • Soil water (1-2 months)
• Inter-Tropical Convergence Zone (ITCZ)


  Low pressure zone on the equator with very heavy rainfall, partly responsible for monsoons
• The global atmospheric circulation model is the main factor that determines


  Cloud formation and rainfall