This presentation goes into brief about how the population of a given locality can be measured using population measuring tools also it gives a brief about the different methods of population forecasting. It explains the types and terminologies of the methods of population forecasting.

1. Population Forecasting
By 075BAR038
Rajay Bajracharya

2. POPULATION
• It is the total number of organisms of the same species living
together at a certain place during a certain time interval
• Measured every 10 years in a general population Census
• Population density is the number of people per unit area
• Usually measured in sq. km. or sq. mile

3. POPULATION FORECASTING
• It is the method of predicting the future population size on
the basis of current day population and data.
• Design of water supply and sanitation scheme is based on the
projected population of a particular city, estimated for the
design period.

4. Factors affecting population forecasting
• Increase due to births
• Decrease due to deaths
• Increase/ decrease due to migration
• Increase due to annexation

5. Population Forecasting Methods
• Arithmetical Increase Method
• Geometrical Increase Method (Or Geometrical Progression Method
• Incremental Increase Method
• Graphical Method
• Comparative Graphical Method
• Master Plan Method
• Logistic Curve Method

6. Arithmetical Increase Method
• Suitable for large and old city with considerable development.
• This method assumes that population increases at a constant rate.
• Hence,
dP/dt = x
• i.e., rate of change of population with respect to time is constant.
• Therefore, Population after nth decade will be
Pn= Po + n.x
• Where, Pn is the population after ‘n’ decades and ‘P’ is present
population

7. Geometrical increase method
• The percentage increase in population from one decade to another is assumed to
remain constant
• It gives a higher value, so it is suitable for a new city at the beginning of its
development
• Pn = P (1 + Ig/100)n
• Where,
Ig = geometric mean (%)
P = present population
N = no. of decades

8. • Modification of arithmetical increase method.
• Suitable for an average size town under normal condition where the growth rate is
found to be in increasing order.
• Per decade growth rate is not assumed to be constant, but progressively increasing and
decreasing
• Depends the average incremental increase in past is positive or negative.
• Hence, population after nth decade is
Pn = Po+ n.X + {n (n+1)/2}.Y
• Where, Pn = Population after nth decade X = Average increase Y = average
Incremental increase
Incremental increase method

9. • 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.
• Needs proper experience and judgement
• It is done best when compared to a city with
similar population and growth
GRAPHICAL METHOD

10. MASTER PLAN METHOD
• Newer, bigger cities are planned by authorities
• It is planned for the coming 25-30 years
• It is divided into residential, commercial and other required subzones
• The population density for each zone is fixed and estimated
• From that, the required water supply and waste/sewage generated is
calculated
• Easy method to precisely design facilities

11. LOGISTIC CURVE METHOD
• Used when growth rate due to births, deaths and migrations is normal, not
affected by epidemics, wars, disasters
• Population follows growth curve characteristics of living things within limited
space and economy
• Logistic curve is the S shaped curve obtained when the population of the city is
plotted with respect to time

14. QUESTIONS

15. Forecast of population for 25 years in a settlement with a current
population of 50,000 people
For this we take the population of Gulariya of Bardiya with current population of 53,107 (2011)
With average population increase of 6600
𝑝𝑛 = 𝑝0 + 𝑛 ∗ 𝑥
Where Pn is population in N decades
Po is initial population and X is the population increase Average
Then
Pn in 25 years,
𝑝25 = 𝑝0 + 2.5 × 6600
=53107 + 2.5*6600
= 69607

16. Calculation of demand of water for the settlement making appropriate
assumptions
To calculate the water supply demand of settlement, we take the average of 100L per person according to
WHO standards.
Thus,
Water Supply Demand,
= Pn * 100
= 69607 * 100
= 69607100 Litres
Using Arithmetic Progression Method, The water demand of the district of Gulariya is 69607100 Litres.