Water Demand and Supply

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Water Demand and Supply

• CE 370 - Lecture 2

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• Before designing any water project, the amount of
  water that is required must be determined. To
  obtain such information we have to know the
  following
• number of people that will be served
• Water consumption (per capita)
• Factors affecting consumption

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Forecasting Population

• Before a water project is constructed, a decision
  on the design period of the project has to be
  taken. This may depend on the amount of fund
  available for that certain project.
• Since most of Saudi cities are growing in
  population, the design period depends mainly upon
  the rate of population growth. The real problem
  here is how to forecast, as accurately as
  possible, the population 10, 20 or 30 years in
  the future.

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• Present population can be obtained through
• Bureau of census
• City directories
• Planning commissions
• Use the ratio of population to the number of
  children in schools from previous enumeration.
• The most difficult part is the estimation of
  population in the future. There are two types of
  population estimates
• short term (1-10 years)
• long term (10-50 years)

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• Different mathematical and graphical approaches
  are used in order to project population, but
  there are no exact solutions since many factors
  are included
• city trade territory
• industrial expansion
• rate of development in the surroundings
• location with regard to airport, railroads or
  ports.
• sudden events such as discovery of an oil field,
  development of new industries will upset all
  estimates.

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• The most widely employed mathematical or
  graphical methods for forecasting population are
  
• Arithmetical method by adding to the existing
  population the same number of people for each
  future period (of limited value and can be used
  for old and very large cities).
• Constant percentage growth rate (must be used
  with caution because it may produce too large
  results, especially if the city is young. The
  method can be applied to old cities not
  undergoing great expansion).
• Logistic method (This method depends upon the
  fact that population will grow until they reach a
  saturation population which is established be
  limit of economic opportunity).
• The ratio method which based on upon the belief
  that populations of cities will have a
  relationship to the population in the whole
  country.
• Graphical interpolation-extension (this method
  may be considered the most generally applicable
  one).

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• Knowledge of the population of a region permits
  estimates to be made of the total quantities of
  water needed. To design water distribution
  networks, additional information regarding the
  spatial distribution of the population to be
  served must also be obtained. Population
  densities may be estimated from data collected on
  existing areas. If local data are not available,
  Table 1 can be used as a guide.

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Table 1 Guide to Population Density.
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WATER CONSUMPTION

• Available water to a city can be classified based
  on its ultimate use. Water is used for
• Domestic. Which includes water supplied to
  houses, hotels, etcSuch water is used for
  sanitary, washing, bathing, drinking and other
  purposes such as air conditioning of residences,
  irrigation and sparkling of privately owned
  gardens and lawns. The practice of irrigation
  will have a considerable effect upon total
  consumption. Domestic consumption may be
  expected to be about 30 per cent of the total.

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• Industrial. Which includes water supplied to
  industrial. The importance of industrial
  consumption depends on
• Size of the industry
• Whether or not the industry uses the public water
  works.
• The industrial use could range between 15 to 60
  per cent of the total, averaging at 32 percent.

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• Commercial. Which includes water supplied to
  commercial areas. The quantity of water required
  for commercial use is expected to mount at about
  20 per cent of the total. In some cases, water
  consumption for industrial and commercial
  purposes was related to the floor area of the
  building served. 300 gallon per day per 1000 sq.
  feet was used.

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• Public Use. Which includes water served to
  public buildings such as city halls, jails and
  schools as well as public service such as
  sprinkling and flushing streets and fire
  protection. Such services may consume water at
  about 10 to 15 gallons per capita. Extinguishing
  very large fires will cause the rate of use to be
  high for short periods.

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• Loss and Waste. This the water which is
  uncounted for although some of the loss and waste
  may be approximated in the sense of cause and
  quantity. Unaccounted-for-waste is due to meter
  and pump leakage, unauthorized water connection
  and leaks in the mains. Unaccounted-for water,
  and also water wasted by consumers, can be
  reduced by careful maintenance of the water
  distribution system and metering of the water
  services. In metered and moderately well
  maintained water system, water loss may mount to
  about 15 per cent of the total.

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Total water consumption is the sum of the
afore-mentioned uses and the loss and waste.
Table 1 shows the consumption of water for
various uses.
The figures, given in Table 1, are not fixed
but may vary from one city to another. Each city
has to be studied carefully especially the
industrial and commercial uses as well as the
actual or probable loss and waste.
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FACTORS AFFECTING WATER CONSUMPTION

• The determination of water demand of an area
  requires knowledge and experience of the social,
  economic and regional development. Review of
  water consumption records showed a wide range of
  values. This is due to the following factors
• Climate conditions. Warm dry regions have higher
  consumption rates than cooler regions. In
  addition, water usage is affected by the
  precipitation levels in the region. Where summer
  is hot and dry, much water will be used for
  watering lawns. Domestic use will increase by
  more bathing, while public use will be affected
  by much street sprinkling and use in parks and
  recreation fields for watering grass and
  supplying fountains. Higher temperature will
  also lead to high water use for air conditioning.

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• Size of the city. In small cities, it was found
  that the per capita per day water consumption was
  small due to the fact that there are only limited
  uses of water in those cities. Small cities have
  larger area that is inadequately served by both
  water and sewer systems than larger cities. In
  the unsewered home, water consumption will rarely
  exceed 10 g/c/d, while in sewered home, it will
  equal or exceed 45 g/c/d on the average.

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• Characteristics of the population. Domestic use
  of water was found to vary widely. This is
  largely dependent upon the economic status of the
  consumers, which will differ greatly in various
  sections of a city. In high-value residential
  areas of a city the water consumption per capita
  will be high. In low-value areas where sewerage
  is not available or where a sigle faucet serves
  one or more homes, water consumption will be very
  low (15 g/c/d), while it is about 60 g/c/d in
  apartment houses located in high-value
  residential areas.

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• Industries and commerce. Presence of industrial
  activities has a great effect on water
  consumption. Since industrial use has no direct
  relation to the population, great care must be
  taken when estimating present or future water
  consumption of a city. Information should be
  collected on existing industries, their actual
  water consumption and the probability of
  establishing new industries in the future.
  Commercial consumption is that of the retail and
  wholesale trade houses and office buildings.
  Figures on commercial consumption are few and
  widely divergent, and if the consumption is
  desired for any district, a special investigation
  should be made.

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• Metering. Communities that are metered usually
  show a lower and more stable water use pattern.
  Metering of services consists of placing a
  recording meter in the line leading from the
  water main to the building served. Consumers are
  then billed for the water they use. Charging
  flat rates has no relation to the actual amount
  of water used or wasted. It is almost impossible
  to construct a good system of water charges
  unless they are based upon actual water
  consumption.

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• Water quality. Consumer perception of bad water
  quality can decrease the water usage rate.
• Cost of water. A tendency toward water
  conservation occur when cost of water is high.
• Water pressure. Rates of water usage increase
  with increases in water pressure.
• Water conservation. Public awareness and
  implementation of water conservation programs by
  utilities tend to have an impact on the water
  usage rate.

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• Wastewater reuse. Wastewater reuse offers
  attractive alternatives to developing new
  supplies.
• Municipal reuse
• Industrial reuse
• Irrigation reuse
• Recreational reuse
• Environmental protection.
• Thermal water discharge
• The use of scrubbers to remove sulfur dioxide

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Example Find the population of City A in 50
years from 1970.
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Example The following Table shows the water
consumption for City A from 1949 to 1969.
Estimate the water annual and daily consumption
rates for 1970 and 1990.
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• Solution
• Q1 Water consumption in 1949 59962638 cubic
  meter
• Q2 Water consumption in 1969 161182948 cubic
  meter
• Annual increase rate
• but this rate was not stable or constant during
  the 20-year period. Based on that the 20-year
  period must be divided into smaller time segments
  each of 5 years (as an example) to come up with a
  figure that is closer to the real one.

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• 1950 - 1955
• Water consumption in 1950 64023813 cubic meter
• Water consumption in 1955 80018378 cubic meter
• Annual increase rate
• 1955 - 1960
• Water consumption in 1955 80018378 cubic meter
• Water consumption in 1960 90614914 cubic meter
• Annual increase rate

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• 1960 - 1965
• Water consumption in 1960 90614914 cubic meter
• Water consumption in 1965 121411634 cubic meter
• Annual consumption rate
• 1965 - 1969
• Water consumption in 1965 121411634 cubic meter
• Water consumption in 1969 161181948 cubic meter
• Annual consumption rate
• Finding the average of 3 closer annual
  consumption rates, it will be 6. This value
  will be used to predict future water consumptions.

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• In 1975
• Water consumption in 1969 161182948 cubic meter
• Prediction period from 1969 to 1975 6 years
• Water consumption in 1975 161182948 ? (1.06)6
  228641000 cubic meter
• Average daily consumption 228641000/365
  626400 cubic meter
• In 1980
• Prediction period from 1969 to 1980 11 years
• Water consumption in 1980 161182948 ? (1.06)11
  305973000 cubic meter
• Average daily consumption 305973000/365
  838300 cubic meter

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• In 1985
• Prediction period from 1969 to 1985 16 years
• Water consumption in 1985 161182948 ? (1.06)16
  409461000 cubic meter
• Average daily consumption 409461000/365
  1121800 cubic meter
• In 1990
• Prediction period from 1969 to 1990 21 years
• Water consumption in 1990 161182948 ? (1.06)21
  547952000 cubic meter
• Average daily consumption 547952000/365
  1501300 cubic meter

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• In 1995
• Prediction period from 1969 to 1995 26 years
• Water consumption in 1995 161182948 ? (1.06)26
  733283000 cubic meter
• Average daily consumption 547952000/365
  2009000 cubic meter
• In 2000
• Prediction period from 1969 to 2000 31 years
• Water consumption in 2000 161182948 ? (1.06)31
  981298000 cubic meter
• Average daily consumption 547952000/365
  2688500 cubic meter

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Assuming the maximum daily consumption is at
175, the results are tabulated as follows
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Solution
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WATER RESOURCES

• Rain Water
• Surface Water
• Ground Water
• Desalinated Sea-water
• Treated Wastewater

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VARIATIONS IN WATER CONSUMPTION RATES

• Seasonal Variations
• In summer, daily water consumption rate may reach
  120 to 160 of average daily consumption rate
  throughout the year. In winter, daily water
  consumption may reach only 70 of average daily
  use throughout the year.
• Daily Variations
• Water consumption varies from one to another.
  Daily variation could reach maximum of 130 to
  170 of average daily consumption during the year
  or may reach a minimum value of 60 of average
  daily water consumption during the same year.
• Hourly Variations
• Maximum rate may reach up to 150 of average daily
  rate, of the same day, at the peak, or may reach
  225 of average daily consumption during one year.

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POPULATION FORCASTING

• Arithmetical Method
• P P0 IT
• Population in 1980 Population in 1970
  Increase
• 142325 12000
  154325
• Population in 1990 142325 12000 2 166325
• Population in 2000 142325 12000 3 178325
• Population in 2010 142325 12000 4 190325
• Population in 2020 142325 12000 5 202325

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• Incremental Increase
• P P0 IT IG(T) (T-1) (T-2) .1
• Population in 1980 Population in 1970
  Increase Increase Change
• 142325 12000?1 443 ? 1
  154770
• Population in 1990 142325 12000?2 443 ?
  21 167660
• Population in 2000 142325 12000?3 443 ?
  321 180995
• Population in 2010 142325 12000?4 443 ?
  4321 194775
• Population in 2020 142325 12000?5 443 ?
  54321 209000

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• Geometric Increase
• P P0 (1IP)n
• Population in 1980 Population in 1970
  (1INCREASE)
• 142325 (10.138)1 161966
• Population in 1990 142325 (10.138)2 184301
• Population in 2000 142325 (10.138)3 209787
• Population in 2010 142325 (10.138)4 238679
• Population in 2020 142325 (10.138)5 271841