We now need to look at the physical background of water supply. The basic water cycle is shown on the figure below. Precipitation is the major sources of water, although most return to the atmosphere by evaporation and transpiration. The amount that runs off the surface into the rivers, lakes and reservoirs depends upon temperature, soil type and vegetation.
The usefulness of precipitation depends upon the amount, its distribution throughout the year and it’s reliability. Some countries receive all the water they require from this continual cycle of water as precipitation which falls into the ground. Some countries in the North of Africa and the Middle East have insufficient precipitation and must rely on the underground sources or rivers which flow from other countries.
Sources of freshwater
Rivers, lakes and reservoirs
Most of total river flow is flood run-off, which is not always available for human use. The remaining river flow results mainly from inflow from ground water. Where flood water is captured and stored in reservoirs behind dams it may add to the base flow and provide more water for our use. The storage of water behind dams adds 15% to the natural base flow of the world’s rivers. The easiest way of extracting water from a river is the use of pipes and canals. For domestic use and industrial use of water, we use pipes to get water and for irrigation we build canals.
Extracting from groundwater
This is usually achieved by drilling wells or boreholes and pumping water to the surface or by diverting water from springs fed by groundwater. Underground water bearing rocks are called aquifers.
Diversion of water
Water may be diverted from one part of drainage basin to another area.
This method of providing fresh water from salty or saline water, usually from the sea, is only used where there are no other options. It is a costly process. Much of the water is obtained by distillation. In desalination plants, salty water is heated to produce vapour, which is then condensed to produce freshwater. Desalination plants require great amounts of heat for evaporation to take place.
Recycling used water
While it is not strictly a source of freshwater, recycling may reduce the demand from other sources and so indirectly contribute to the water supply.