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Civil - Environmental Engineering

Explain about Population Forecasting Methods

   Posted On :  09.07.2016 03:48 pm

1. Arithmetic growth method 2. Geometric growth method 3. Curvilinear method 4. Logistic method 5. Decline growth method 6. Ratio growth

Explain about Population Forecasting Methods ?


Population is one of the most important factors for design of the water systems, so it should be estimated, so as to know the increasing demand and ensure continuous supply to them.


Population data is obtained by previous records and the rate of increase is found out and this used for further analysis, which may be by using the methods described below


1.     Arithmetic growth method

2.     Geometric growth method

3.     Curvilinear method

4.     Logistic method

5.     Decline growth method

6.     Ratio growth


Arithmetic growth method:

It is based on the assumption that the rate of growth of population is constant. It means that the each year population increase by the same increment.




dp / dt = Ka




dp / dt is the rate of change of population


Ka = the constant arithmetic increment


Ka can be determined by finding the slop of the graph of population against time. The population in the future is thus estimated.


Geometric method:

It is based on the hypothesis that rate of change of population is proportional to the population. According to this, method it is assumed that the rate of increase of population growth in a community is proportional to the present population.




dP /dt P => dp / dt = Kg where Kg = Geometric Growth constant.


If P0 is the population at any time t0 and Pf is

the population at time tf then


?Pf P0 dp/p = Kg ? tf t0 dt = Ln (Pf/P0 = Kg (tf/t0)


=> Ln (Pf/P0 = Kg ?t


=> (Pf/P0 = (e) Kg ?t and Pf = P0 (e) Kg ?t

This method gives somewhat larger value as compared to arithmetic method and can be used for new cities with rapid growth. In normal practice, arithmetic and geometric growth average is taken.


Curvilinear method:

In this it is assumed that the population of a city will grow, in the same manner as in other cities in the past. This similarity between the cities includes geographical proximity, similarity of economic base, access to similar transportation system etc. In practice it is difficult to find similar cities.


Logistic method:

When the growth rate of population due to birth, death and migration are under normal situation and not subjected to extraordinary changes due to unusual situation like war, epidemics earth quakes and refugees etc. Then this method is used:


According to this method


P = P sat / (1+ ea+ b?t), where P sat is the saturation population, of the community and a, b are constants. P sat, a and b can be determined from three successive census populations and the equations are


Psat = 2 P0 P1P2 - P12 (P0 + P2) / (P0 P2 - P12)


Decline growth method:


This method like, logistic, assumes that the city has some limiting saturation population and that its rate of growth is a function of population deficit;


Ratio method:

Ratio method of fore casting is based on the assumption that the population of a certain area or a city will increase in the same manner to a larger entity like a province, or a country. It requires calculation of ratio of locals to required population in a series of census years.


Projection of the trend line using any of the technique and application of projected ratio to the estimated required population of projected ratio to the estimated required population in the year of interest. This method of forecasting does not take into account some special calculations in certain area but have the following advantages.


Consumption of water Uses


          1.       Domestic use

          2.       Commercial use

          3.       Public use

          4.       Loss and waste

Domestic use of water:


Domestic uses of water include the consumption of water for drinking, washing, cooking, toilets, livestock etc. the domestic average use per capita per day is 50 - 90 gallons (70 - 380 liters per capita per day). This use is increasing by 0.5% - 1.0% per year and at this time comprises 50% of all the uses of water.

Water uses are for drinking, cooking, meeting of sanitary needs in houses and hotels, irrigating lawns etc. Residential water use rates fluctuate regularly.

Average daily winter consumption is less than annual daily average, whereas summer consumption averages are greater. Similarly, peak hourly demand, is higher than maximum. No universally applied rule for prediction


Commercial and industrial:


This is the amount of water used by the shops, markets, industries, factories etc. It contributes 15 - 24% of total use of water.


It includes factories, offices and commercial places demand. It is based on either having a separate or combined water supply system. Demand of water based on unit production: No. of persons working and floor area





Public use:


The public use of water is that one which is used by city halls, jails, hospitals, offices, schools etc. This consumes 9% of total use of water. Its water demand is 50 - 75 liters per capita per day. Fire protection's need of water is also fulfilled by this sector. The fire demand does not greatly affect the average consumption but has a considerable effect on peak rates. Schools, hospitals, fire fighting etc


Loss and wastes:


: Unauthorized, connections; leakage in distribution system, Hydrant flushing, major line breakage and cleaning of streets, irrigating parks. Total consumption is sum of the above demands. The water which is not intended for specific purpose or use is also called "Un-accounted for". Loss and wastage of water is due to:


1.     Errors in measurements

2.     Leakages, evaporation or overflow

3.     Un-metered uses e.g. fire fighting, main flushing

4.     Un-authorized connections


Factors affecting the use of water


     Size of the city

     Industry and commerce



     Time of the day

     Day of the week or month




Tags : Civil - Environmental Engineering
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