The water cycle and water security

• Hydrological cycle


  The cycle of water circulation between the earth's surface and atmosphere, involving precipitation, runoff, and groundwater flow
• Water and the hydrological cycle are paramount in supporting life on earth
• The hydrological cycle operates on a variety of spatial scales where physical processes control the circulation of water between stores on land, oceans, the cryosphere and atmosphere
• The global hydrological cycle works as a closed system with inputs (rainwater), outputs, stores and flows within, driven by solar energy and gravitational potential energy
• The hydrological cycle


  1. Evaporation

  2. Condensation

  3. Cloud formation

  4. Moisture transport

  5. Precipitation

  6. Evaporation

  7. Percolation

  8. Groundwater

  9. Runoff

  10. Oceans
• The majority of earth's water is in saline oceans and freshwater is largely locked-up within icecaps and glaciers
• The global water budget limits water availability for human use and water stores have different residence times; some stores, such as those storing fossil water, are non-renewable
• Drainage basin


  The area of land drained by a river
• Flows within a drainage basin

  1. Interception

  2. Infiltration

  3. Throughflow

  4. Percolation

  5. Evaporation

  6. Transpiration

  7. Channel flow
• Physical factors that determine flows, inputs and outputs in a drainage basin

  • Climate
  • River systems
  • Geology
• Humans disrupt the drainage basin by accelerating processes such as deforestation and changing land use
• Urbanisation increases the proportion of impermeable surfaces which prevents precipitation penetrating the ground, meaning less groundwater is stored and thus water availability declines
• Water budget


  Shows the annual balance between inputs (precipitation) and outputs (evapotranspiration) and their impact on soil water availability
• Climate types that influence water budgets

  • Temperate
  • Tropical
  • Polar
• River regime


  Indicates the annual variation of discharge of a river and gives an indication of the climate, geology and soils
• Factors that influence the shape of a storm hydrograph

  • Shape of drainage basin
  • Size of drainage basin
  • Drainage density
  • Rock type
  • Soil and vegetation
  • Relief
• As global temperatures increase, due to climate change, river discharge is likely to increase as glacier meltwater increases river input
• Drought


  A long period of time with below average precipitation
• Meteorological causes of drought

  • Short-term precipitation deficit
  • ENSO cycles
  • Anticyclones
  • Changes in the ITCZ
• Human activity increases drought risks due to the over-abstraction of surface water resources and ground water aquifers, reducing water supply
• Deforestation reduces the soil's ability to store water, causing the land to dry out
• Construction of dams on a large river can cause drought by reducing water flow downstream
• Droughts have impacts on ecosystems functioning as wetlands and the natural environment are unable to get the materials they require for adequate growth
• Flooding


  Surpluses within the hydrological cycle that can have disastrous impacts for people, destroying social wellbeing and economic security
• Meteorological causes of flooding

  • Intense storms
  • Heavy and prolonged rainfall
  • Series of depressions
  • Extreme monsoonal rainfall
  • Snowmelt
• Human actions such as urbanisation and deforestation increase flood risk
• Mismanagement of rivers and inadequate hard engineering systems also exacerbate flood risk
• Flood damage strains economic activity whilst ruining infrastructure and settlements, and causes environmental impacts involving damage to soils and ecosystems
• Climate change effects the inputs and outputs of the hydrological cycle by altering precipitation and evaporation rates
• Climate change's impacts are uncertain, causing concern over water supplies and efficiency of management
• Water stress


  When each person has below 1700 m3 of water
• Water scarcity


  When freshwater shortages threaten food production and ecosystem wellbeing; there being below 1000 m3 of water per person
• Physical causes of water insecurity

  • Climate variability
  • Saltwater encroachment at the coast
• Human causes of water insecurity

  • Over-abstraction from rivers, lakes and groundwater
  • Water contamination from agriculture
  • Industrial water pollution
• Finite water sources face pressure from rising demands, due to increased population, improving standards of living, industrialisation and agriculture
• The price of water varies globally according to wealth, with the process of cleaning water being expensive
• Water supply is vital for economic development, needed for activities in industry, energy supply and agriculture, and is also important for human wellbeing such as sanitation, health and food preparation
• Water insecurity problems are likely to cause transboundary and international conflicts
• Hard engineering schemes to manage water supply

  • Water transfers
  • Desalination plants
  • Mega dams
• Sustainable water management approaches

  • Water conservation (e.g. rainwater catchment)
  • Water recycling