Water

• Flux = transfer of water between stores
• Residence times = the estimated storage times of the world's water resources e.g. ice sheets being "locked up"
• Total water:

  Ocean = 97.5%

  Fresh water = 2.5%

  • Groundwater = 35%
  • Glaciers = 68.7%
• Groundwater residence time = 2weeks - 10,000 years whilst glaciers = 1,000 years
• Water cycle = the continued circulation of the water within the Earth's hydrosphere.
• ICTZ (Inter tropical convectional currents) = rising warm air resulting in it cooling and causing clouds + rain. It is the biggest flux
• Physical factor affecting drainage basin:

  • Relief - gentle slopes promote percolation. Steep = more runoff due to gravity, reducing time for infiltration. Increases flood risk (high gradient)
• Physical factor affecting drainage basin:

  • Climate - dry climates may reduce percolation as less water is available for infiltration into the ground. Increased percolation = increased surface runoff.
• Percolation = the process of water moving downwards through the soil due to gravity.
• Physical factor affecting drainage basin:

  • Soil - sandy soil has higher percolation rates. Compact soils have less permeability so higher surface runoff.
• Physical factor affecting drainage basin:

  • Geology - permeable rocks increases percolation. Impermeble will increase surface runoff
• Physical factor affecting drainage basin:

  • Vegetation - dense vegetation can reduce runoff by interception. PLants helps to maintain percolation rates.
• Human factors affecting drainage basin:

  • Over abstraction - Aral Sea case study + Thames Basin (690mm/year of rainfall (low), 40% of London water from chalk aquifers which are replenished by Malborough downs - low rain)
• Human factors affecting drainage basin

  • Deforestation - trees important for global hydrological system but are fragile. Positive feedback loop. (Amazon - 20% of forest been destroyed in last 50 years due to business. Before deforestation, 75% of water absored returned to atmosphere, now only 25% does)
• Orographic rainfall = (seen in west UK) warmer, moist air is foreced to rise. As it reaches upland areas, it cooles over high ground making heavy rain.
• Convectional rainfall = when an area of low pressure which's warm and moist meets colder (polar) air. The warmer air rises over the colder air, forming heavy rain.
• Convectional rainfall = (typical of SE England in summer) where the sun warms the air which rises. This air cools and causes intense rainfall and electrical storms.
• Water budget = the balance between water input and output in a given area.
• Percipation = runoff/ river discharge + potential evapotranspiration +/- soil moisture or groundwater storage
• Potential evapotranspiration = an estimate on the amount of water lost through evaporation and transpiration from plants. Affected by air temps and air quality.
• Groundwater recharge = hydrological process which replenishes groundwater in aquifers
• Flood hydrograph

  
  

  • Lag time = time between peak rainfall and peak discharge
• Flood hydrograph

  • Peak discharge = highest river flow
• Flood hydrograph

  • Rising limb = increase discharge in discharge /flow

  
• Flood hydrograph

  • Urban = steep rising limb as impermeable surface so high surface runoff. Steep falling limb as limited underground water storage
  • Rural = shallower rising limb as lots of infiltration. Lower peak flow and fatter as can manage more water. Long lag time due to permeable surface and high infiltration.
• Factors affecting storm hydrographs:

  
  

  • River size (bigger take longer to peak)
  • River shape (long narrow rivers mean water takes longer to get downstream so longer lag time and lower peak)
  • Drainage density (drains carry water efficiently so low lag time and high peak)
  • Rock permeability (permeable rock = increase infiltration = less surface runoff)
  • Relief
  • Vegetation
  • Land use + urbanisation
  • Water management
• Global atmospheric circulation:

  
  

  (0 to 90) Low, high low high
• Tipping point = when a system changes from one state to another
• Positive feedback = when a change leads to a decrease with a system and creates instability
• Negative feedback = when a change leads to stability through reinforcing a system
• Normal year:

  • Trade winds East > West. Takes warm surface water with it
  • Cold water upwelling
  • Warm air rises over Indonesia, creating an area of low pressure.
  • Air circulates back into upper atmosphere bringing high pressure over South America.
• El Nino year:

  • Trade winds in Western Pacific weaken and die resulting in reverse direction of flow
  • High pressure over Australia with descending air. Droughts
  • Rising air moves to the East, resulting in higher sea temperatures (6-8 degs warmer)
  • Calmer conditions in Pacific
• La Nina year (exagerated normal year):

  • Extremerly strong trade winds westwoods causing 1m increase in sea levels in Indonesia
  • Low pressure develops with strong convectional uplift as warm waters heat atmosphere - high rainfall
  • Increased upwelling in South America - cold + high pressure = droughts
• Polar cell = 60 - 90 degs

  Ferrel cell = 30 -60 degs

  Hadley cell = 0 - 30 degs
• Ground water flooding = flooding that occurs after the ground has become saturated from prolonged heavy rainfall
• Surface water flooding = occurs after intense rainfall has insufficient time to infiltrate into soil so flows overland.
• Flash flooding = flood with a very short lag time
• Extreme monsoonal rain:

  • 70% of rain falls in 100 day period (July - Sept)
  • 80% of Bangladeshis at risk
• Snowmelt:

  • Resultant water cannot cannot infiltriate into ground
  • 2013 Norfolk - flooding caused by melting snow and ice after mild and wet weather caused rapid thawing. This + rain caused flooding.
• Other:

  • 12% of world's population consumers 85% of its water