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WATER DEMAND
AND
POPULATION
FORECASTING
Amity Institute of Environmental Sciences

WATER DEMAND
While planning a water supply scheme, it is
necessary to find out not only the total yearly
water demand but also to assess the required
average rates of flow and the variations in
these rates. The following quantities are therefore
generally assessed and recorded.
1. Total annual volume in Litres or million litres.
(1MLD = 106L/d)
Annual Average rate of flow in litres per day , i.e.
V/365
Annual average rate of flow in litre per day per
person (l/c/d), called per capita demand.

CLASSES OF WATER DEMAND

WATER REQUIREMENT FOR
DIFFERENT USES

PER CAPITA DEMAND
It is the annual average amount of daily water
required by one person and includes the
domestic use, industrial and commercial uses,
public use, wastes etc. It may therefore be
expressed as:
Per capita demand (q) in l/d/h or l/c/d =
Total yearly water requirement of the city in
litres (V) / 365 X Design population

FACTORS AFFECTING PER
CAPITA DEMAND
Size of thecity
Climaticconditions
Types of habitat ofpeople
Industrial and commercial activities
Quality of watersupplies
Pressure in the distributionsystem
Development of sewage facilities

DESIGN PERIOD
Design period may be defined as:
“It is the number of years in future for which the
given facility is available to meet the demand.”
Or
“The number of years in future for
which supply will be more than demand.”

Why Design period is provided ?
Design period is provided because
It is very difficult or impossible to provide
frequent extension.
It is cheaper to provide a single large unit
rather to construct a number of small units.

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. Changes in
the population of the city over the years occur, and the system should
be designed taking into account of the population at the end of the
design period. Factors affecting changes in population are:
increase due to births
decrease due to deaths
increase/ decrease due to migration
increase due to annexation.
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.

POPULATION
FORECASTING
Arithmetic Increasemethod
Geometric IncreaseMethod
Incremental IncreaseMethod
Decrease Rate of IncreaseMethod
Simple GraphicalMethod
Comparitive GraphicalMethod

ARITHMETIC INCREASE
METHOD
This method is suitable for large and old city with
considerable development. If it is used for small,
average or comparatively new cities, it will give lower
population estimate than actual value. In this method
the average increase in population per decade is
calculated from the past census reports. This increase
is added to the present population to find out the
population of the next decade. Thus, it is assumed that
the population is increasing at constant rate.
Hence, dP/dt = C i.e., rate of change of population with
respect to time is constant.

INCREMENTAL INCREASE
METHOD

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. The best
way of applying this method is to extend the curve by comparing with
population curve of some other similar cities having the similar
growth condition.

COMPARATIVE GRAPHICAL METHOD

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EXTRAPOLATION
TECHNIQUES
Real Estate Analysts - faced with a difficulttask
long-term projections for small areas such as
Counties
Citiesand/or
Neighborhoods
Reliable short-term projections for small areas
Reliable long-term projections for regions countries
Forecasting task complicatedby:
Reliable, Timely and Consistent information

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SOURCES OF FORECASTS
Public and Private SectorForecasts
Forecasts may be based on large quantities
of current and historical data

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PROJECTIONS ARE
IMPORTANT
Comprehensive plans for thefuture
Community General Plans for
Residential LandUses
Commercial LandUses
Related LandUses
Transportation Systems
Sewage Systems
Schools

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PROJECTIONS VS.
FORECASTS
The distinction between projections and
forecasts are important because:
Analysts often use projections when they
should be using forecasts.
Projections are mislabeled as forecasts
Analysts prepare projections that they know will
be accepted as forecasts without evaluating the
assumptions implicit in their analytic results.

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PROCEDURE
Using Aggregate data from the past to project the
future.
Data Aggregated in two ways:
total populations or employment without identifying the
subcomponents of local populations or the economy
I.e.: age or occupational makeup
deals only with aggregate trends from the past without
attempting to account for the underlying demographic
and economic processes that caused the trends.
Less appealing than the cohort-component techniques or
economic analysis techniques that consider the
underlying components of change.

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WHY USE AGGREGATE DATA?
Easier to obtain andanalyze
Conserves time andcosts
Disaggregated population or employment data
often is unavailable for small areas

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EXTRAPOLATION: A TWO STAGE PROCESS
Curve Fitting-
Analyzes past data to identify overall trends of
growth or decline
Curve Extrapolation -
Extends the identified trend to project the future

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ASSUMPTIONS AND
CONVENTIONS
Graphic conventionsAssume:
Independent variable:
Dependent variable:
x axis
y axis
This suggests that population change (y axis)
is dependent on (caused by) the passage of
time!
Is this true orfalse?

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Assumptions and Conventions
Population change reflects the change in aggregate
of three factors:
births
deaths
migration
These factors are time related and are caused by
other time related factors:
health levels
economic conditions
Time is a proxy that reflects the net effect of alarge
number of unmeasured events.

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Caveats
The extrapolation technique should never be used to
blindly assume that past trends of growth or decline
will continue into the future.
Past trends observed, not because they will always
continue, but because they generally provide the best
available information about the future.
Must carefullyanalyze:
Determine whether past trends can be expected to
continue, or
If continuation seems unlikely, alternatives must be
considered

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Alternative Extrapolation
Curves
Linear
Geometric
Parabolic
ModifiedExponential
Gompertz
Logistic

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Linear Curve
Formula: Yc = a + bx
a = constant or intercept
b = slope
Substituting values of x yields Yc
Conventions of the formula:
curve increases without limit if the b value > 0
curve is flat if the b value = 0
curve decreases without limit if the b value < 0

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Linear Curve

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Geometric Curve
Formula: Yc = abx
a = constant (intercept)
b = 1 plus growth rate (slope)
Difference between linear and geometric curves:
Linear:
Geometric:
constant incremental growth
constant growth rate
Conventions of theformula:
if b value > 1 curve increases without limit
b value = 1, then the curve is equal toa
if b value < 1 curve approaches 0 as x increases

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Geometric Curve

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Parabolic Curve
Formula: Yc = a + bx + cx2
a = constant (intercept)
b = equal to the slope
c = when positive: curve is concave upward
when = 0, curve is linear
when negative, curve is concave downward
growth increments increase or decrease as the x variable
increases
Caution should be exercised when using for long
range projections.
Assumes growth or decline has nolimits

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Parabolic Curve

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Modified Exponential Curve
Formula: Yc = c + abx
c = Upper limit
b = ratio of successive growth
a = constant
This curve recognizes that growthwill
approach a limit
Most municipal areas have defined areas
i.e.: boundaries of cities or counties

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Modified Exponential Curve

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Gompertz Curve
Formula: Log Yc = log c + log a(bx)
c = Upper limit
a = constant
Very similar to the Modified Exponential Curve
Curvedescribes:
initially quite slow growth
increases for a period, then
growth tapers off
very similar to neighborhood and/or city growth
patterns over the long term

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Gompertz Curve

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Logistic Curve
Formula: Yc = 1 / Yc-1 whereYc-1 = c + abX
c = Upper limit
a = constant
Identical to the Modified Exponential and Gompertz
curves, except:
observed values of the modified exponential curve and the
logarithms of observed values of the Gompertz curve are
replaced by the reciprocals of the observed values.
Result: the ratio of successive growth increments of the
reciprocals of the Yc values are equal to a constant
Appeal: Same as the Gompertz Curve

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Logistic Curve

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