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.

**POPULATION FORECASTING**

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.

p_{n}
= p_{o} + nI_{a} + [n( n+1)L_{i}]/2

**SIMPLE GRAPHICALMETHOD**

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.

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Environmental Engineering : Demand For Water : Demand For Water: Population Forecasting |

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