When the design period is fixed, the next step is to determine population in various periods because the population of the towns generally goes on increasing. The population is increased by births, decreased by deaths, increased or decreased migration. The correct present and past population can be obtained from census office. The WSS are not designed for the present population the future population expected by the end of the design period may be estimated by various methods. The method to be adopted to a particular town or city depends on the factors discussed in these methods.
The various methods of forecasting the population are
1. Arithmetical increase method
2. Biometrical increase method
3. Incremental increase method
4. Decreasing rate of increase method or decreasing rate method
5. Simple graphical method
6. Comparative graphical method
7. Master plan method or Zoning method
8. Ration method or Apportionment method
9. Logistics curve method.
ARITHMETICAL INCREASE METHOD
This method is based upon the assumption that the population is increasing at a constant rate, ie. The rate of change of population with time is constant.
If the present population of a particular town is „P? and the average increase in population for past decade „Ia? the future population „Pn? at the end of „n? decades will be
Pn = P+ nIa
This method gives low results for developing areas, which develop faster than the post this method of limited value may be useful for smaller design periods or for old and very large cities
with no industries and which have practically reached their maximum development or approaching saturation.
GEOMETRICAL INCREASE METHOD
This method assures that the percentage increase in population from decade to decade is constant. This method gives high results for young cities expanding at faster rates and useful for old developed cities. If the present population of the city is „P? and the Average percentage increase/ decade „Ig? then the population „Pn? at the end of 'n? future decades will be
Pn P [I + Ig/100 ]n
INCREMENTAL INCREASE METHOD
This method is a combination of the above two methods and therefore gives the advantages of both arithmetic and Geometric increase methods and hence gives satisfactory results. In this method the Average increase is first determined by the arithmetical increase method and to this added the average of the net incremental increase once for every future decade.
pn = po + nIa + [n( n+1)Li]/2
In this method a graph is plotted from the available data, between time and population the curve is then smoothly extended up to the desired year. The method, however, gives very approximate results, as the extension of the curve is done by the intelligence of the curve is done by the intelligence of the designer.
Comparative graphical method
In this method the cities having conditions and characteristics similar to the city under consideration are selected. It is assumed that the city under consideration will also develop in similar fashion of the selected cities. The population growth curve of the city under consideration is drawn using the available data as shown in the figure.
Decrease in sale of increases method or decreasing sale method
Simi rate of increases in poplin goes on reducing as a city reaches towards etc. saturation value this method which makes use of decreases in the % increases is wed & gives a rational result in this method the average decreases in the increases is worked out & then subtracted from the latest increases for each successive decade.
Master plan or zoning method
By & metropolitan cities are generally not allowed to develop in a haphazard & natural way but are allowed to develop only in planned ways The master plan prepared for such cities, divides the city into zones & thus to separate the residential, commend & industrial area?s from each at
The poplin densities are also fixed. It is very easy to calculationdesign poplin using master plan because it will give us as to when & where the given no of houses, industries etc..Would be developed.
Ratio method & apportion method
In this method the cities poplin record is ex as %age of poplin of the whole country.
At the feat, the rations of local to national poplin are worked out for past 4-5 decades. A graph in plotted l/w tone & there ratios to design period to calculation future population.