HKDSE Geography/M2a/Water Cycle

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The water cycle is an important system within the hydrosphere of the earth.


The major processes of the water cycle are:

  • Evaporation: Water on the earth's surface evaporates, becomes water vapour and rises in the air.
  • Transpiration: Water vapour from plants (not the earth's surface).
  • Evapotranspiration: Evaporation and transpiration.
  • Condensation: It occurs when water vapour from evapotranspiration becomes water droplets and forms clouds.
  • Precipitation: It occurs when clouds are too heavy to support the water droplets and the water falls down as rain, snow or hail.
    • Interception: Trees shield and retain raindrops as they fall.
      • Dripflow: Water drips from the leaves. (less important)
      • Throughfall: Water falls through the trees.(less important)
      • Stem flow: Water flows down tree stems. (less important)
  • Infiltration: Water infiltrates into the soil.
    • Throughflow: Water moves horizontally in the soil after infiltration. (less important)
    • Percolation: Water moves down the soil and reaches the water table (i.e. ground water level)
      • Base flow/Ground water flow: Water moves horizontally in the soil after percolation. (less important)
  • Plant uptake/Absorotion: Plants uptake water from the soil.
  • Overland flow: Water flows on the earth's surface.
    • Surface run-off: Water flows on the earth's surface, not in a channel.
    • Channel flow: Water flows in river channels (more below).
      • Channel outflow: Water flows into the ocean.

A diagram of the whole flow can be found in AL97PIQ4.

Related concepts include relative humidity (amount of moisture in the air) and underground storage of water (amount of water stored underground).

Precipitation vs. Rainfall

Rainfall is a type of precipitation in the form of liquid water droplets called raindrops. Precipitation also includes snow and hail (not included in the compulsory part). The water cycle process is always precipitation, not rainfall.

Overland flow vs. Surface run-off

Surface run-off is a type of overland flow, which includes both run-off and channel flow. In the examination, the two terms are generally interchangeable.

Important Relationships[edit]

Relationships between water cycle processes are important. Infiltration and surface run-off are negatively related. This is important for later concepts as well as frequently tested in MCs and DBQs. They depend on the following factors:

  • Slope gradient ↑ → Infiltration ↓, Surface run-off ↑
  • Vegetation cover ↑ → Interception ↑ → Infiltration ↑, Surface run-off ↑
  • Rock/soil permeability ↑ → Interception ↑ → Infiltration ↑, Surface run-off ↑

Also note that infiltration and underground storage are positively related.

Evapotranspiration and condensation are positively related.

The following factors affect evapotranspiration (CE95Q15):

  • Temperature/Insolation ↑ → Evaporation ↑ → Condensation ↑
  • Relative humidity → Evaporation ↑ → Condensation ↑
  • Wind speed ↑ → Evaporation ↑ → Condensation ↑

Also note that evapotranspiration and relative humidity are positively related. (DSE12Q11)