water cycle

• Global hydrological cycle


  The circulation of water around the earth
• The global hydrological cycle is a closed system of linked processes so there are no external inputs or outputs
• The amount of global water is finite and constant, the only thing that changes is the state in which the water exists
• Sources of power that drive the global hydrological cycle

  • Solar energy (in the form of heat)
  • Gravitational potential energy (causes rivers to flow downhill and precipitation to fall to the ground)
• Main water stores


  • Oceans
  • Atmosphere
  • Biosphere
  • Cryosphere (e.g. glaciers)
  • Groundwater
  • Surface water
• Stores


  Reservoirs where water is held
• Flows


  The transfers of water from one store to another
• Fluxes


  The rates of flow between stores
• Only 2.5% of all water is freshwater, the largest store of fresh water is the cryosphere
• 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 and cryosphere are considered non-renewable water stores
• Characteristics of precipitation that can affect the drainage cycle

  • Form (rain, snow or hail)
  • Amount
  • Intensity
  • Seasonality
  • Distribution
• Types of precipitation


  • Convectional
  • Cyclonic
  • Orographic
• Flows that transfer precipitation into the drainage network

  • Interception
  • Infiltration
  • Percolation
  • Throughflow
  • Groundwater flow
  • Surface runoff
  • River or channel flow
• Outputs from the drainage basin

  • Evaporation
  • Transpiration
  • Discharge (channel flow)
• Drainage basin


  The area of land drained by a river and its tributaries, sometimes referred to as a river catchment
• Physical factors that can impact the inputs, flows and outputs of a drainage basin

  • Climate
  • Soil
  • Geology
  • Relief
  • Vegetation
• Ways humans disrupt the drainage basin cycle

  • River management
  • Deforestation
  • Changing land use (agriculture)
  • Changing land use (urbanisation)
• Deforestation in the Amazon basin has disrupted the drainage basin cycle in a number of ways
• Water budget


  The annual balance between precipitation, evapotranspiration and runoff (inputs and outputs)
• Precipitation = runoff + evaporation +- changes in storage
• Stages of the water budget graph

  • Soil moisture surplus
  • Soil moisture utilisation
  • Peak evaporation
  • Soil moisture deficit
  • Soil moisture recharge
• River regime


  The annual variation in the discharge or flow of a river at a particular point
• Factors affecting river regime

  • Channel capacity of the river
  • Area and relief of the drainage basin
  • Volume, pattern and intensity of precipitation
  • Climate
  • Geology of the soil (affecting the input of groundwater)
  • Anthropogenic activities
• Components of a storm hydrograph

  • Rising limb
  • Peak flow
  • Lag time
  • Falling limb
  • Base flow
• Characteristics of a flashy storm hydrograph vs a subdued storm hydrograph


  • Short lag time, high peak, steep rising limb (flashy)
  • Long lag time, low peak, gently sloping rising limb (subdued)
• Factors affecting storm hydrograph characteristics

  • Weather/climate
  • Rock type
  • Soil
  • Relief
  • Basin size
  • Vegetation
  • Antecedent conditions
  • Human activity
• Drought


  An imbalance where input is less than output in the hydrological cycle
• Types of drought


  • Meteorological (rainfall deficit)
  • Hydrological (stream flow deficit, lower groundwater levels)
  • Agricultural
  • Socio-economic (food deficit)
• Features of meteorological drought

  • Low precipitation
  • High temperatures
  • Strong winds
  • Increased solar radiation
  • Reduced snow cover
• Impacts of meteorological drought

  • Loss of soil moisture
  • Irrigation supply drops
  • Reduction in water available for consumption
• Features of hydrological drought

  • Reduced infiltration
  • Low soil moisture
  • Little percolation and groundwater recharge
• Impacts of hydrological drought

  • Reduced storage in lakes and reservoirs
  • Less water for urban supply
  • Poorer water quality
  • Threats to habitats
• El Niño Southern Oscillation (ENSO)


  The change in water body patterns within the Southern hemisphere, leading to unusual weather conditions
• La Niña


  The build-up of cooler-than-usual subsurface water in the tropical part of the Pacific, which can lead to severe drought conditions
• Desertification


  The process by which once-productive land gradually changes into a desert-like landscape
• People are not the cause of droughts, however they can make droughts more severe
• Causes of desertification in the Sahel region

  • Changing rainfall patterns
  • Vegetation stress and die-off
  • Soil erosion
  • Rapid population growth
  • Overgrazing
  • Overcultivation
  • Deforestation
  • Civil wars
• Careful management of scarce water resources has stopped desertification from happening in Australia
• Ecological resilience


  The capacity of an ecosystem to withstand and recover from a natural event (e.g. drought or flooding) or some form of human disturbance