GGP101 Hydrology 2

Global water cycle functions and stores
1. Key Functions
  1. Supplies freshwater
  2. Drives primary biological production
  3. Regulates climate
    1. Greenhouse effect
    2. Transports heat
2. Stores (4 main types)
  1. Ocean
  2. Terrestrial waters
  3. Ice
  4. Atmosphere
  5. Storage measured in 10^3 km^3
  6. Flux within stores
    1. Flux from ocean to land is most important for terrestrial water resources
    2. Flux from land to ocean is theoretically equal to difference in Precipitation and Evapotranspiration
    3. Flux measured in 10^3 km^3yr^-1
  7. Turnover within stores
    1. Main control on turnover within stores is surface area : volume ratio
  8. Exchange between stores
    1. Surace to volume ratio
    2. Energy available
      1. Incoming energy
      2. Radioactive and thermal properties of surface
    3. Efficiency of energy use
      1. Liquid to gas more efficient than solid to liquid/gas
    4. Ease of exchange
      1. Most exchange where least resistance to flow
      2. 'Water' and environmental controls
3. Freshwater resources
  1. We rely on the smallest stores in the hydrological cycle
    1. Fastest rates of turnover
    2. Greatest short-term fluctuations in supply
  2. Human impacts
    1. Irrigated soils and reservoirs each hold more freshwater than all the world's rivers
      1. However turnover rate in rivers is higher
Global water cycle processes and patterns
1. Precipitation
  1. Key controls on global precipitation
    1. Atmospheric circulation
      1. Driven by solar heating
      2. Hadley Cells and Intertropical Convergence Zone with return flow to equator off-set by Earth's rotation (trade winds)
      3. Jet streams between cells: west to east
      4. Energy (and moisture) transfer
        Between 30deg N and 30deg S - Hadley Cells
        Higher latitudes - cyclones and anti-cyclones, frontal systems
      5. Two main peaks in precipitation at zones of convergence
      6. Atmospheric circulation also exerts an influence on oceanic circulation
    2. Oceanic circulation
      1. Largest source of water
      2. Main reclamation and recycling plant
      3. Major heat store
      4. Major convyor belt for heat (energy)
      5. Oceanic controls on precipitation
        Surface oceanic currents
        Surface currents mirror atmospheric circulation leading to poleward transfer of heat from equator (wind drag)
        Affect regional evaporation and precipitation patterns (e.g. west-coast deserts)
        Note role of landmasses
        Wind can also influence upwelling dynamics
        Deep ocean currents
        Thermohaline circulation
        Transfers heat from Indian ocean to North Atlantic
        Driven by evaporation in Northern Atlantic
        Increased density linked to cooling and increased salinity
        Water sinks and returns south
        Main effect is heat transfer
        Oceanic upwelling events
        Driven by surface drag and/or density currents
        Noteable effect on sea surface temperatures
        Impacts on regional precipitation patterns
        El Nino Southern Oscillation
        Warm waters from western equatorial Pacific invade eastern part due to weakening of Trade Winds
        Disrupts upwelling of cold waters along the western coast of South America
        Heavy precipitation coastal Ecuador and Peru
        Dought in eastern Australia, southern India and southern Africa
        Wider global impacts
        Avg 5 yr circulation
    3. Land mass
      1. Orographic Precipitation
        Air forced to rise and cool over areas of high relief
        Mostly causes repeated precipitation in same location
        Mechanisms
        Trigger instability by upwards motion or differential heating
        Increase cyclonic precipitation by stalling it (esp mid-late winter)
        Force convergence through valleys
      2. Global patterns
    4. Seasonal wind patterns
      1. Monsoons: seasonal reversals in wind patterns
      2. Linked to migration of Intertropical Convergence
      3. Wet and dry conditions
    5. Storm systems
      1. Hurricanes/typhoons
        Individuals convections cells form over warm seas
        200-500 mm rain/day
      2. Typically western ocean zones away from equator where:
        1. Free convection release enough latent heat by condensation
        2. Sufficient Coriolis force to cause rotation
        3. Warm ocean currents move system to higher latitude and greater coriolis force
      3. Lower latitudes: coriolis force weaker, trade winds break up convective cloud growth
      4. Mid-latitude
        Frontal storms: Main source of rain in mid-latitudes
        Convective cells: local air 10% warmer than surrounding air
  2. Change in precipitation
    1. Global Only 1% change - not significant
    2. NW Africa > -40%
2. Evaporation
  1. Described by Dalton's Law
    1. E = u (es - ea)
      1. u = function of windspeed
      2. es = saturation pressure at temp
      3. ea = current vapour pressure
  2. Global patterns
    1. Ocean
      1. Generally greatest at low latitudes and where sea temperature is warm
        Equatorial cloud cover leads to slight reduction
        Eastern margins of many oceans reduced due to upwelling
        Notable exceptions e.g. El Nino Southern Oscillation
    2. Land
      1. More complex
3. Runoff
  1. Estimating global runoff
    1. Estimates of global runoff ideally based on well-established gauging stations on major rivers
    2. Record coverage, length and incompleteness are major hurdles
    3. Data not available for up to 1/3 of river basins
    4. Some tropical areas not covered i.e. where precipitation and runoff are high
    5. Alternative is the climatological method
      1. RO = P - E
        RO = Runoff
        P = Precipitation
        E = Evaporation
  2. Change in flow
    1. 1951-2000 - Global 0.4% change - not significant
    2. 1951 - 2000 regional
      1. Semi-arid - > -30%
      2. No significant change in Europe
      3. Change in some areas highly variable in rivers
Climate and human factors
1. R = P - (ET + S + C)
  1. Change in runoff over the study period would be expected to match change in precipitation
    1. If it doesn't, then here must be a change in evapotranspiration, storage or consumption
2. Authors found that study rivers fitted one of three classes
  1. Normal
  2. Deficit
  3. Excess

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