M6 Earth science

Freshwater Distribution on Earth

  • Freshwater on Earth ≈ 2.5\% of total planetary water.
  • Of this freshwater:
    • Glaciers/Ice: ≈ 68.7\% of freshwater.
    • Groundwater: ≈ 30.1\% of freshwater.
    • Surface water (lakes/rivers): ≈ 1.2\% of freshwater.
  • Implication: Readily accessible freshwater (surface water) is a small portion of total water and most is stored in ice or deep underground.

Water Cycle and Key Processes

  • Evaporation: liquid to gas driven by heat.
  • Condensation: gas to liquid.
  • Precipitation: rain/snow/hail to the surface.
  • Infiltration: water entering soil.
  • Runoff: surface water flow to bodies of water.
  • Percolation: movement through soil/rock.
  • Transpiration: water loss from plants.
  • Evapotranspiration: combination of evaporation and plant transpiration.
  • Groundwater discharge: groundwater feeding surface water.
  • Leaching/Volatilization/Wastewater discharge: other hydrologic processes affecting water quality and fate.
  • Groundwater: Renewable if recharge equals/exceeds extraction; can be non-renewable if extraction outpaces recharge.

Water Resources: Types and Concepts

  • Blue water: surface water and groundwater available for use.
  • Green water: rainfall stored in soil and used by vegetation.
  • Freshwater distribution emphasizes that most freshwater is locked in ice or deep groundwater; surface water is limited.
  • Groundwater can be shared across borders; sustainable management is essential.

Water Quality and Treatment Concepts

  • Grey water: relatively clean wastewater from baths, sinks, and laundry; can be treated and reused for irrigation.
  • Black water: wastewater from toilets; requires proper treatment and disposal.
  • Desalination: converting seawater to freshwater; energy-intensive and costly, limits widespread use.
  • Water pollution effects: lowers dissolved oxygen, reduces aquatic productivity, impacts human water quality.

Virtual Water and Water Footprint

  • Virtual water: water embedded in goods/services to produce them.
  • Water footprint: total freshwater use or pollution associated with a product, person, organization, or nation.
  • Relevance: helps manage water resources, especially in water-scarce regions; links agriculture, industry, and trade to water availability.

Human Impacts on Water Resources

  • Major drivers: population growth, urbanization, agricultural expansion (irrigation), industrial activity, pollution, and improper waste disposal.
  • Agricultural runoff: nutrients (N, P) can cause eutrophication; may lead to bioaccumulation of contaminants.
  • Industrial discharges: can reduce dissolved oxygen and harm aquatic ecosystems.
  • Infrastructure changes: reduced infiltration, increased runoff, higher flood risk, and groundwater recharge impacts.

Conservation, Management, and Simple Actions

  • Water conservation campaigns aim to reduce waste and preserve both blue and green water.
  • Desalination as a potential solution, yet limited by cost and energy needs.
  • Practical actions: reduce pollution, minimize runoff, protect natural recharge areas, reuse grey water where safe, support sustainable water policies.

Quick Recall: Key Terms to Remember

  • Water cycle components: \text{evaporation}, \ \text{condensation}, \ \text{precipitation}, \ \text{infiltration}, \ \text{runoff}, \ \text{percolation}, \ \text{transpiration}, \ \text{evapotranspiration}.
  • Freshwater distribution references: 68.7\% in glaciers/ice, 30.1\% groundwater, 1.2\% surface water (of freshwater).
  • Water types: blue water vs green water; grey water vs black water.
  • Virtual water and water footprint concepts explained above.

Optional Practice Prompts (Brief Answers)

  • Why is surface water so limited compared to total freshwater?
    • Most freshwater is trapped in glaciers/ice and deep groundwater; only a small fraction is readily accessible as surface water.
  • What makes groundwater renewable or non-renewable?
    • Renewable when recharge matches/exceeds extraction; non-renewable when withdrawals exceed recharge over long timescales.
  • How do gray and black water differ in reuse potential?
    • Gray water can be treated and reused (e.g., irrigation); black water requires more intensive treatment.
  • What is a water footprint?
    • A measure of total freshwater used and/or polluted to produce goods/services or support activities.
  • What is the role of desalination in water security?
    • Desalination can augment supply but is energy-intensive and costly, limiting widespread use.

Formulae and Quick Facts

  • Water chemical formula: \mathrm{H_2O}
  • Example of a simple distribution fact: freshwater share ≈ 2.5\% of total Earth water, with most freshwater locked in ice and deep groundwater.