POPULATION AND WATER DEMAND FORECASTS

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POPULATION AND WATER DEMAND FORECASTS

• Design of a water supply or sewerage system
  requires an estimate of the water demand.
• Water demand is usually estimated through
  population forecasting and applying a per person
  estimate of water demand or wastewater
  generation.

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POPULATION FORECASTING

• Short-term projections of water demand are needed
  to prepare for operational decisions such as
  chemical purchases.
• Long-term predictions are required for major
  design projects.
• The following information is useful for making a
  reasonable estimate of population growth
• Census data
• Industrial growth projections
• Birth and death rates
• Governmental activities
• Maps
• USGS topographic
• Plat maps
• Planning maps

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POPULATION AND WATER DEMAND FORECASTS

• Population forecasting in a community first
  requires a plot of the previous population as a
  function of time to see what trends exist.
• Population growth in a given community can be
  typically broken down into several phases as
  shown on the following figure.

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Decreasing rate of increase
Stationary phase
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3
5
Population, Y
Linear or arithmetic increase
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1
Exponential or geometric increase
Time, years
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METHODS OF POPULATION FORECASTING

• Exponential, geometric, or constant-percentage
  increase
• In the early growth of a community, the
  population rate increase is usually directly
  related to the current population as

in which, KP is the constant percentage growth
coefficient.
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METHODS OF POPULATION FORECASTING

• Exponential, geometric, or constant-percentage
  increase

To determine KP, plot ln Y versus time and draw a
best fit line through the data points. The
slope of the line is KP.
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METHODS OF POPULATION FORECASTING

• Exponential, geometric, or constant-percentage
  increase
• To estimate Y at some time, t, in the future

in which Y2 and t2 are the present population at
time t2.
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METHODS OF POPULATION FORECASTING

• Arithmetic, linear, or constant growth

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METHODS OF POPULATION FORECASTING

• Arithmetic, linear, or constant growth
• To determine Ku, plot population versus time and
  draw the best fit line through the data. Ku is
  the slope of the line. To estimate future
  population

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METHODS OF POPULATION FORECASTING

• Decreasing Rate of Increase
• As a population continues to grow, it becomes
  limited by availability of resource in the
  community and growth slows down.

in which, Z is the limiting population perhaps
based on a maximum population density, zoning
restrictions, technology, and so on.
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METHODS OF POPULATION FORECASTING

• Graphical Extension
• A simpler technique is to extend the best-fit
  line into the future. However, this technique is
  quite subjective and the prediction of future
  population will vary from one analyst to another.

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METHODS OF POPULATION FORECASTING

• Graphical Comparison to Similar but Larger Cities
• Another graphical procedure deduces population
  from the growth of similar communities. Find
  similar communities in the region that have
  larger populations and plot their populations as
  a function of time.
• Select a reference population value and assume
  that the community will grow from the reference
  value at a rate similar to growth from the
  reference value of the larger communities. That
  is, the plots are shifted along the time scale
  until the reference values co-inside as shown in
  the figure below.

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METHODS OF POPULATION FORECASTING

• Graphical Comparison to Similar but Larger Cities

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METHODS OF POPULATION FORECASTING

• Growth Based on Birth, Death and Migration Rates
• Most communities keep records on the births and
  deaths that occur within the community so, if
  something is known about migration in and out of
  the community, a population balance can be used
  to determine the rate of population growth.

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METHODS OF POPULATION FORECASTING

• Growth Based on Birth, Death and Migration Rates
• The birth rate can be estimated from a normalized
  birth rate (birth per year per 1000 population)
  or from age specific fertility rates (births per
  year per 1000 women in a specific age group) if
  the age and sex distribution of the population is
  known. Likewise, death rates can be estimated
  from a normalized death rate (deaths per year per
  1000 population) or from age specific death rates
  if the age distribution of the population is
  known. The migration rate is probably the
  hardest to determine but estimates may be
  available from local planning agencies.

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METHODS OF POPULATION FORECASTING

• Growth Based on Birth, Death and Migration Rates
• If a negligible rate of migration is assumed,
  population growth can be estimated by

Or, more accurately as
The second equation is used to forecast the
natural rate of increase one year at a time by
reviving the age distribution for successive
years.
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METHODS OF POPULATION FORECASTING

• Summary
• Sometimes it is not easy to decide what method of
  population forecasting to use or where on a
  typical growth curve a given community is. The
  engineer needs to temper his judgement by making
  conservative assumptions and by providing
  flexible designs that allow for expansion or
  contraction of facilities as necessary. The
  longer the population extrapolation, the less
  reliable the estimate.

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WATER DEMAND

• Analyzing existing water usage rate records is
  the best way to estimate the water demand

This includes residential, commercial,
industrial, and public usage the amount of
water required to support the average citizen.
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WATER DEMAND

• Another approach is to look at metered water
  usage but often metered usage is less and
  sometimes much less than pumpage because of leaks
  in the system. It is not unusual for 10 to 20
  of the pumped water to be lost from the system.
• Several data summaries indicate the average water
  demand in the U.S. is 150 to 160 gpcd (46
  domestic use, 13 public use, 18 commercial,
  and 23 industrial), ranging from 90 gpcd
  (Arkansas) to 230 gpcd (Nevada). This wide range
  of water demand is due to a number of reasons.

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FACTORS AFFECTING THE USE OF WATER

• 1) Climate Average summer usage is about 125
  of average day while winter use is about 80 of
  average day. Maximum day and peak hour will
  normally occur during the summer.
• 2) Economic Conditions Water use is related to
  economic status.
• 3) Composition of the Community the mix of
  domestic, public, commercial and industrial use
  affects the overall demand.
• 4) Water Pressure Water use increases with
  increased pressure due to increases in leakage.
  Normal water pressures range from 60 120 psi.
• 5) Cost of Water Use decreases as cost
  increases.

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FACTORS AFFECTING THE USE OF WATER

• In general, maximum day demand is about 1.8 times
  average day demand or is about 270 gpcd, ranging
  from 1.2 to 4.0 times average day.
• The peak hourly demand ranges from 1.5 to 12
  times average day. Typically, the design of a
  water distribution is based on maximum daily
  demand plus fire flow.
• Average day demand is used to estimate average
  operational costs such as energy needs and
  chemical purchases.
• Peak hour is used to design storage facilities
  while maximum day is used to design the water
  treatment plant.

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INDUSTRIAL WATER DEMAND

• Industrial water demand is usually expressed as
  demand per unit of production.
• For example, a cannery uses from 2000 to 10,000
  gallons per ton of produce processed.
• The production of gasoline requires 7 to 34
  gallons of water per gallon of gas.
• Fifteen gallons of water are required to produce
  a gallon of beer and 82,000 to 230,000 gallons of
  water are required to process a ton of wood
  during paper production.
• The trends in industry is toward water
  conservation to reduce operating costs. It is
  best to check with each local industry to
  determine their current and anticipated needs.

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FIRE FLOWS

• Fire fighting must be considered in the design of
  any municipal water supply. Fire insurance rates
  are based in part, on the ability of the
  distribution system to maintain sustained high
  flows. Recommended quantities of water have been
  established by the National Board of Fire
  Underwriters

in which, F is the fire flow in gpm, A is the
total floor area in square feet, and C is a
coefficient based on the type of construction
(1.5 for wood frame 0.6 for fire-resistant
construction)
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FIRE FLOWS

• 500 gpm (residential) lt F lt 8000 gpm (commercial
  industrial)
• The duration of a fire flow ranges from 4 to 10
  hours. More detailed information can be obtained
  from fire insurance companies.