The water cycle
The design of a system must take into account:
- the water demand, estimated by statistical or analytical methods
- the water resources, determined from the appropriate hydrogeological and hydrological data for each region.
Assessment of water demands
The volume of water needed to supply a community depends on:
- the size and types of localities being served,
- the municipal, agricultural and industrial demands,
- the practices of the population.
In general, the following mean daily consumptions are assumed per head of population:
- rural communities: 130 to 180 litres (excluding agricultural requirements),
- medium size communities: 200 to 250 litres (including municipal requirements),
- towns: 300 to 450 litres (including municipal requirements), possibly more in urban areas.
It is advisable in all cases to design the water trunk and distribution systems taking into account the prospects for long term urban development in the area.
The presence of communal buildings or industrial type units must be taken into account. The average requirements for common examples are:
- schools: 100 litres per pupil per day,
- slaughter houses: 500 litres per head of livestock,
- dairies, butter and cheese making: 5 litres per litre of processed milk,
- hospitals: 400 litres per bed per day,
- firefighting: a minimum reserve of 120 m3, capable of supplying a DN 100 hydrant for 2 hours (French Standard). Some firefighting departments may have additional requirements.
- industry: each case has to be studied separately.
It is essential to provide a safety margin, to take into account (a) the oversights and the inaccuracies which may affect the estimates and (b) the effective yield of the