Water Demand and Supply

1. Water Demand and Supply CE 370 - Lecture 2

2. 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

3. 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.

4. 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)

5. 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.

6. 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 by 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).

7. 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.

8. Table 1: Guide to Population Density.

9. 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, etc…Such 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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

22. Example: Find the population of City A in 50 years from 1970.

23. 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.

24. 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.

25. 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 =

26. 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.

27. 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

28. 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

29. 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

30. Assuming the maximum daily consumption is at 175%, the results are tabulated as follows:

31. Solution

32. WATER RESOURCES • Rain Water • Surface Water • Ground Water • Desalinated Sea-water • Treated Wastewater

33. 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 day 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.

34. 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

35. 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  [2+1] = 167660 Population in 2000 = 142325 + 120003 + 443  [3+2+1] = 180995 Population in 2010 = 142325 + 120004 + 443  [4+3+2+1] = 194775 Population in 2020 = 142325 + 120005 + 443  [5+4+3+2+1] = 209000

36. Geometric Increase P = P0 + (1+IP)n Population in 1980 = Population in 1970 (1+%INCREASE) = 142325 (1+0.138)1 = 161966 Population in 1990 = 142325 (1+0.138)2 = 184301 Population in 2000 = 142325 (1+0.138)3 = 209787 Population in 2010 = 142325 (1+0.138)4 = 238679 Population in 2020 = 142325 (1+0.138)5 = 271841