The document discusses various methods for population forecasting to estimate future population for designing water supply and sanitation schemes. It describes the Arithmetic Increase Method, Geometric Increase Method, Incremental Increase Method, Decrease Rate of Increase Method, Simple Graphical Method, Comparative Graphical Method, Master Plan/Zoning Method, and Logistic Curve Method. Selection of the appropriate method depends on factors like the city size, growth pattern, and development stage. The objective is to forecast population accurately to ensure water systems are adequately sized but also cost-effective.

1. Population forecasting
Mr. M. H. Mota
Assistant professor
SITCOE

2. Outcome:
To estimate the future population using
various methods of population forecasting

3. Some facts…
• Water supply and sanitation is treated as a State subject as per the
federal Constitution of India and, therefore, the States are provided
with the constitutional right on the planning, implementation and
cost recovery of water supply and sanitation projects.
• The Public Health Engineering Department (PHED) is the principal
agency at the State level for planning and implementation of water
supply and sanitation programmes.

4. Some facts…
• In a number of States, statutory Water Supply & Sanitation Boards
(WSSBs) have taken over the functions of the PHEDs. The basic
objectives for creation of WSSBs have been to bring in the concept of
commercialization in the water supply and sanitation sector
management and more accountability.
• Such Boards have been set up in Assam, Bihar, Gujarat, Karnataka,
Kerala, Maharashtra, Orissa, Punjab, Uttar Pradesh and Tamil Nadu.
The metropolitan cities of Bangalore, Hyderabad and Chennai have
separate statutory Boards.

5. • The objective of a public protected water supply is to supply safe and
clean water in adequate quantity, conveniently and economically as
possible.
• The water to be handled may vary both in quantity and quality and the
degree of treatment required, seasonally, monthly, daily and sometimes
even hourly.
• Water supply management aims to improve the supply by minimizing
losses and wastage and unaccounted for water in the transmission
mains and distribution system.
• Water supply projects may be designed normally to meet the
requirements over a thirty year project after their completion. the time
lag between design and completion of the Project should also be taken
in to account which should not exceed two to five years.
Some facts…

6. • The design population will have to be estimated with due regard
to all the factors governing the future growth and development of
the area in the industrial, commercial, educational, social and
administrative spheres, special factors causing sudden emigration
or influx of population should also be foreseen to extent possible.
• The judgment based on these factors would help in selecting the
most suitable method of deriving the probable trend of the
population growth in the area of the project.

7. Population forecasting…
Design of water supply and sanitation scheme is based on the projected
population of a particular city, estimated for the design period. Any
underestimated value will make system inadequate for the purpose
intended; similarly overestimated value will make it costly.
Factors affecting changes in population are:
 increase due to births.
 decrease due to deaths.
 increase/ decrease due to migration.
The present and past population record for the city can be obtained from
the census population records. After collecting these population figures,
the population at the end of design period is predicted using various
methods as suitable for that city considering the growth pattern
followed by the city.

8. Methods of population forecasting…
 Arithmetic Increase method
 Geometric Increase Method
 Incremental Increase Method
 Decrease Rate of Increase Method
 Simple Graphical Method
 Comparative Graphical Method
 The Master Plan Method or Zoning Method
 Logistic Curve method

9. Arithmetic increase method…
Average rate of increase in population is assumed to be constant
from decade to decade in this method. Average increase per
decade is found out from the previously available census data.
 Mathematically…….. Pņ = P+nd
Where…..
Pņ = future population after n decades
P = present population
n = number of decades
d = average increase per decade

10. Geometric increase method …
Average percentage increase in population is assumed to be
constant from decade to decade in this method.
Averagepercentage increase per decade is found out from the
previously available census data. By using the formula given below
the future population is worked out.
Mathematically…….. Pņ= P(1+r/100) 𝑛
Where,
• Pņ= future population after n decades
• P= present population
• n= number of decades
• r= average percentage increase per decade

11. Incremental increase method…
The advantages of both arithmetic increase method and geometrical
increase method are included in this method. Average increase per decade
is found out first of all and average percentage increase per decade is
worked out as in arithmetic increase method and geometric increase
method respectively.
 Mathematically…….. Pņ=𝑃+𝑛𝑑+[(𝑛 /𝑛+1) /2]𝑡
Where,
Pņ = future population after n decades
P = present population
t = average incremental increase per decade
d = average increase in population per decade

12. Decrease Rate of Growth Method …
It has been seen that all life grow within limited space. If the complete
growth of a very old city is plotted, it will be seen that the curve has S-
shape, which indicates that early growth takes place at an increasing
rate, latter growth is at a decreasing rate which indicates that saturation
limit is reached.
In this method, the average decrease in the percentage increase is worked
out and is then subtracted from the latest percentage increase for each
successive decade.

13. Graphical Method…
In this method, the populations of last few decades are correctly plotted
to a suitable scale on graph. The population curve is smoothly extended
for getting future population. This extension should be done carefully
and it requires proper experience and judgment.

14. Comparative Graphical Method…
• In this method the population data of project is plotted along with
past population data of number of towns which have grown under the
similar conditions. The curve of the city under consideration is
extended carefully after studying the pattern of other cities.

15. The Master Plan Method or Zoning Method …
 The development of towns and cities is not allowed in haphazard way. For
the development of the towns and cities, their master plans are prepared.
 The city is divided into various zones such as commercial centres,
industrial areas, residential areas, the schools, colleges, parks etc.
 The future expansion of the cities is strictly regulated by various bylaws of
corporations and other local bodies, according to the master plan.
 The master plans are prepared for the development of the cities for 25-30
years. The population densities for various zones of the towns to be
developed are also fixed. Now when the population of a particular zone is
fixed, it is very easy to design the water supply schemes for the particular
zones. The future development of the water works is also designed on the
basis of the master plan.

16. Population density…

17. Logistic Curve method…
• It is a modern method of the population forecasting where the
population data of the past is analyzed critically by the software and
the forecasting is done . Mathematically the solution is obtained
using the relation as stated below…
Formula:
where
 Ps = Upper limit/saturation population
 Py=Population at starting
 P=Population at any time ‘t’
 K= constant

18. Selection of the methods…
 Generally ‘Arithmetic Increase method’ is suitable for old and large city
where the population growth is linear.
 Geometric Increase Method or ‘Incremental Increase Method’ is suitable
for newly developing cities where the growth rate is more because of its
growth pattern and large amount of increase is expected because of
possible migration due to development.
 ‘Decrease Rate of Increase Method’ is suitable for a matured and well
developed city which is approaching the saturation in terms of growth
potential.
 Simple Graphical Method is suitable for developing towns where the
development has a good potential to attract the people.

19.  Comparative Graphical Method is suitable for the towns or cities which
are showing the similar pattern in which the development of few towns
has been observed.
 Logistic Curve method can be used for large cities where the
generalization in any terms is difficult to state and all possible patterns
has to be considered for forecasting of population.

20. Please remember…..
 Suitability of each method is to be determined critically.
 Care has to be taken while selecting any method for the
forecasting in any given case.
 The trend of increase in the population shown by the city under
consideration can be the most suitable way to take the decision
to implement any method for forecasting the population.

22. That’s it for the session!!!!
Lets meet in next session
to learn more….