Design Example

Water Quantity Estimation
Water Consumption Rate
Fluctuations in Rate of Demand
Design Periods & Population Forecast
Population Forecasting Methods



Water Quantity Estimation

The quantity of water required for municipal uses for which the water supply scheme has to be designed requires following data:

  1. Water consumption rate (Per Capita Demand in litres per day per head)
  2. Population to be served.

     Quantity= Per capita demand x Population

Water Consumption Rate

It is very difficult to precisely assess the quantity of water demanded by the public, since there are many variable factors affecting water consumption. The various types of water demands, which a city may have, may be broken into following classes:

Water Consumption for Various Purposes:


Types of Consumption

Normal Range (lit/capita/day)




Domestic Consumption





Industrial and Commercial Demand





Public Uses including Fire Demand





Losses and Waste




Fire Fighting Demand:

The per capita fire demand is very less on an average basis but the rate at which the water is required is very large. The rate of fire demand is sometimes traeted as a function of population and is worked out from following empirical formulae:


Formulae (P in thousand)

Q for 1 lakh Population)


American Insurance Association

Q (L/min)=4637 ÖP (1-0.01 ÖP)



Kuchling's Formula

Q (L/min)=3182 ÖP



Freeman's Formula

Q (L/min)= 1136.5(P/5+10)



Ministry of Urban Development Manual Formula

Q (kilo liters/d)=100 ÖP for P>50000


Factors affecting per capita demand:

  1. Size of the city: Per capita demand for big cities is generally large as compared to that for smaller towns as big cities have sewered houses.
  2. Presence of industries.
  3. Climatic conditions.
  4. Habits of people and their economic status.
  5. Quality of water: If water is aesthetically $ medically safe, the consumption will increase as people will not resort to private wells, etc.
  6. Pressure in the distribution system.
  7. Efficiency of water works administration: Leaks in water mains and services; and unauthorised use of water can be kept to a minimum by surveys.
  8. Cost of water.
  9. Policy of metering and charging method: Water tax is charged in two different ways: on the basis of meter reading and on the basis of certain fixed monthly rate.

Fluctuations in Rate of Demand

Average Daily Per Capita Demand
= Quantity Required in 12 Months/ (365 x Population)

If this average demand is supplied at all the times, it will not be sufficient to meet the fluctuations.

  • Seasonal variation: The demand peaks during summer. Firebreak outs are generally more in summer, increasing demand. So, there is seasonal variation .
  • Daily variation depends on the activity. People draw out more water on Sundays and Festival days, thus increasing demand on these days.
  • Hourly variations are very important as they have a wide range. During active household working hours i.e. from six to ten in the morning and four to eight in the evening, the bulk of the daily requirement is taken. During other hours the requirement is negligible. Moreover, if a fire breaks out, a huge quantity of water is required to be supplied during short duration, necessitating the need for a maximum rate of hourly supply.

So, an adequate quantity of water must be available to meet the peak demand. To meet all the fluctuations, the supply pipes, service reservoirs and distribution pipes must be properly proportioned. The water is supplied by pumping directly and the pumps and distribution system must be designed to meet the peak demand. The effect of monthly variation influences the design of storage reservoirs and the hourly variations influences the design of pumps and service reservoirs. As the population decreases, the fluctuation rate increases.

Maximum daily demand = 1.8 x average daily demand
Maximum hourly demand of maximum day i.e. Peak demand
               = 1.5 x average hourly demand
               = 1.5 x Maximum daily demand/24
               = 1.5 x (1.8 x average daily demand)/24
               = 2.7 x average daily demand/24
               = 2.7 x annual average hourly demand

Design Periods & Population Forecast

This quantity should be worked out with due provision for the estimated requirements of the future . The future period for which a provision is made in the water supply scheme is known as the design period.

Design period is estimated based on the following:

  • Useful life of the component, considering obsolescence, wear, tear, etc.
  • Expandability aspect.
  • Anticipated rate of growth of population, including industrial, commercial developments & migration-immigration.
  • Available resources.
  • Performance of the system during initial period.

Population Forecasting Methods

The various methods adopted for estimating future populations are given below. The particular method to be adopted for a particular case or for a particular city depends largely on the factors discussed in the methods, and the selection is left to the discrection and intelligence of the designer.

  1. Arithmetic Increase Method
  2. Geometric Increase Method

Worked-out Example