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

**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.

**Mathematically;**

dp
/ dt = Ka

**Where,**

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.

*Mathematically:*_{?}

dP /dt P => dp / dt = K_{g} **where** K_{g}
= Geometric Growth constant.

If P_{0} is the population at any time t_{0} and P_{f} is

the
population at time t_{f} then

**?**P_{f}**
**P_{0}**
**dp/p
= K_{g}** **? t_{f}**
**t_{0}**
**dt
= Ln (P_{f}/P_{0}** **= K_{g}**
**(t_{f}/t_{0})

=> Ln
(P_{f}/P_{0} = K_{g} ?t

=> (P_{f}/P_{0}
= (e) K_{g} ?t and P_{f} = P_{0}
(e) K_{g} ?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+ e^{a}+
b?t),
** where** P

P_{sat}
= 2 P_{0} P_{1}P_{2} - P_{1}^{2} (P_{0}
+ P_{2}) / (P_{0} P_{2} - P_{1}^{2})

**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.

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