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

**Explain
about 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.

**Consumption of water
Uses**

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1. Domestic use

2. Commercial
use

3. Public use

4. Loss and
waste

Domestic use of water:

Domestic uses of water
include the consumption of water for drinking, washing, cooking, toilets,
livestock etc. the domestic average use per capita per day is 50 – 90 gallons
(70 – 380 liters per capita per day). This use is increasing by 0.5% - 1.0% per
year and at this time comprises 50% of all the uses of water.

Water uses are for
drinking, cooking, meeting of sanitary needs in houses and hotels, irrigating
lawns etc. Residential water use rates fluctuate regularly.

Average daily winter
consumption is less than annual daily average, whereas summer consumption
averages are greater. Similarly, peak hourly demand, is higher than maximum. No
universally applied rule for prediction

**Commercial and industrial:**

This is the amount of
water used by the shops, markets, industries, factories etc. It contributes 15 –
24% of total use of water.

It includes factories,
offices and commercial places demand. It is based on either having a separate
or combined water supply system. *Demand of water* based on unit
production: No. of persons working and floor *area*

**Public use:**

The public use of water
is that one which is used by city halls, jails, hospitals, offices, schools
etc. This consumes 9% of total use of water. Its water demand is 50 – 75 liters
per capita per day. Fire protection's need of water is also fulfilled by this
sector. The fire demand does not greatly affect the average consumption but has
a considerable effect on peak rates. Schools, hospitals, fire fighting etc

**Loss and wastes:**

: Unauthorized,
connections; leakage in distribution system, Hydrant flushing, major line
breakage and cleaning of streets, irrigating parks. Total consumption is sum of
the above demands. The water which is not intended for specific purpose or use
is also called "Un-accounted for". Loss and wastage of water is due
to:

1. Errors
in measurements

2. Leakages,
evaporation or overflow

3. Un-metered
uses e.g. fire fighting, main flushing

4. Un-authorized
connections

**Factors affecting the use of water**

§ Size
of the city

§ Industry
and commerce

§ Climate

§ Time
of the day

§ Day
of the week or month

Tags : Civil - Environmental Engineering

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