4.1 Introduction to water systems

Significant ideas:

  • The hydrological cycle is a system of water flows and storages that may be disrupted by human activity.
  • The ocean circulatory system (ocean conveyor belt) influences the climate and global distribution of water (matter and energy).

Knowledge and understanding:

  1. Solar radiation drives the hydrological cycle.
  2. Fresh water makes up only a small fraction (approximately 2.6% by volume) of the Earth’s water storages.
  3. Storages in the hydrological cycle include organisms, soil and various water bodies, including oceans, groundwater (aquifers), lakes, rivers, atmosphere, glaciers and ice caps.
  4. Flows in the hydrological cycle include evapotranspiration, sublimation, evaporation, condensation, advection (wind-blown movement), precipitation, melting, freezing, flooding, surface runoff, infiltration, percolation, and stream- flow or currents.
  5. Human activities such as agriculture, deforestation and urbanization have a significant impact on surface runoff and infiltration.
  6. Ocean circulation systems are driven by differences in temperature and salinity. The resulting difference in water density drives the ocean conveyor belt, which distributes heat around the world, and thus affects climate.

Applications and skills:

  • Discuss human impact on the hydrological cycle.
  • Construct and analyse a hydrological cycle diagram.


  • Many hydrological cycles are shared by various nations. This can lead to international disputes.

Theory of knowledge:

  • The hydrological cycle is represented as a systems model—to what extent can systems diagrams effectively model reality, given that they are only based on limited observable features?


  • Climate change—causes and impacts (7.2)
  • Terrestrial food production systems and food choices (5.2)
  • Aquatic food production systems (4.3)
  • Resource use in society (8.2)
  • Sustainability (1.4)