Skip to main content

About this free course

Become an OU student

Download this course

Share this free course

Potable water treatment
Potable water treatment

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

2.12 Summary

The hydrological cycle is a complex process involving evaporation, transpiration, condensation, air circulation, precipitation, infiltration, surface run-off and percolation. Aquifers are an important source of fresh water supplying about one-third of the UK's potable water demand. Most of the world's water is present as saline water in the oceans.

Self-assessment question

What fraction of the total volume of water that circulates in the Earth's hydrological cycle does river water represent?


i.e. 1.2 millionths of the total available water.

Self-assessment question

What are the components of the hydrological cycle which can modify the volume of groundwater resources?


The components are: rainfall, evapotranspiration, run-off, abstraction of surface water, infiltration to groundwater storage, inflow and outflow of groundwater between the aquifer and other areas or the sea, and the change in groundwater storage due to abstraction.

Self-assessment question

Which of the following is the best description of evapotranspiration?

A The transfer of water from the oceans, seas and land surface to the air.

B The accelerated process of transfer of moisture to the air at a water-air interface caused by an external heat source.

C The transfer of moisture from soil to the air through roots and leaves of plants.

D Items B and C together.

E The total removal of moisture to the atmosphere from land surfaces.


Option D is best, but option E is also valid.

Self-assessment question

How do the following affect infiltration?

(a) Dense vegetation.

(b) Steeply sloping land surface.

(c) Cultivated land.

(d) Roads and buildings.


(a) Dense vegetation reduces the rate of surface run-off and thus increases infiltration. However, the effect will be offset to some extent by interception, whereby a large proportion of the precipitation stays on the leaves of the vegetation and evaporates.

(b) Water runs rapidly off steeply sloping land surfaces so there is little time for significant infiltration to occur.

(c) Cultivated land is in general subject to greater amounts of infiltration, unless the land is sloping, when the effect will be as in (b).

(d) Tarmac, concrete and roofing surfaces are relatively impermeable, so that roads and buildings promote run-off and reduce infiltration.