Planning for water use is very important in any society because the enormous need of water in different activities of man. The world has about 70% of its space occupied by water but majority of these are oceans and seas which are salt water. The treatment of these water is usually very costly. A group of ancient time sailors who sailed through endless seas once cried, ‘water water everywhere, but no water to drink’. The fresh water bodies suitable for human use are scarce. Thus, there is very great need to plan water use. This planning is necessary for the design of water works system such as tanks, reservoirs etc.
Accurate determination of water need for a society or community depends on the estimate of water use while considering factors affecting water requirements and population forecasting. The total water consumption can be obtained as the sum of domestic consumption, water requirement for public use, fire demand, industrial demand and losses due to leaks, evaporation etc. The later is difficult to account for and usually taken as 10% of the total water demand. Table 1 shows the usual normal water consumption in litres per capita per day (lpcd) in the USA.
Table 1; Normal water consumption (lpcd)
| Class of consumption | Normal Range | Average |
| Public | 20 – 80 | 40 |
| Domestic | 70 – 340 | 210 |
| Commercial | 40 – 120 | 70 |
| Industrial | 70 – 300 | 190 |
| Miscellaneous | 20 – 110 | 60 |
| Total | 220 – 950 | 570 |
These water consumption rate can be affected by many factors which varies based on hourly, daily, weekly, monthly and yearly basis. For instance, in a given day, water use may be high from the morning hours till close to noon and low at night. It then becomes important to determine the peak values per hour or day.
Peak value/hour = 2 to 3 times average/hour
Peak value/day = 1.2 to 2 times average/day
The factors on which water consumption depend are: climatic factors, habit and lifestyle, industry and commerce, plumbing facilities, sewerage system, water tax, size of city and population characteristics.
Population forecasting
Population forecasting is required after determining the per capita requirement of water in other to determine the average total use. Population rate usually depends on migration, urbanization, industrialization, new scientific discoveries, birth control, birth and death rates etc.
Methods of population forecasting
There are many methods of population forecasting that include:
Arithmetic methods
In this method, the rate of population change is assumed constant. If i and f represent initial and future population and time respectively,
Then, pf – pi = Ka (tf – ti)
Where pf = future population
pi = initial population
tf = future time
ti = initial time
Ka = constant growth rate
If pe and te represent population and time in some earlier year,
Then, Ka = (pf – pi)/ (tf – ti) = (pi – pe)/ (ti – te)
Thus, when pi, ti, pe and te are known, pf can be estimated for a particular time, tf
Comparative method
In this method, the future population can be prepared by plotting the population of several cities having a similar growth pattern. It is assumed that the population of the city under study would grow in similar manner to the older and larger cities. The forecast is made by extrapolating the population curve of the city under study into the future according to the trend of other cities.
Component method
In this method, the information about the deaths, births and migration of the community, when they are available can be used to make the population forecast.
Compound method
In this method, the future population may also be estimated using the compound formula:
pf = pi (1 + K)n
Where pf and piare while K and n are the growth rate and design period respectively.
Geometrical method
In this method, the logarithmic function, ln is introduced and the growth rate becomes Kg.
The method is similar to arithmetic method, but
Kg = (lnpf – lnpi)/ (tf – ti) = (lnpi – lnpe)/ (ti – te)
To know whether to use arithmetic method or geometric method, first plot the past population values in an ordinary graph paper. If the outcome is linear, use arithmetic method, but if the outcome appears more quadratic or shows upward concavity, use geometrical method instead
Graphical method
In this method, the population in the past years is plotted against time and the graph extended into the future following the past population trend.
Ration and correlation method
In this method, it is assumed that the growth of a smaller area is closely related to the population growth of the region in which the smaller area is located. Thus, the future population of the smaller area can be estimated by using the forecast of the future population of the region.
Note: Even though these methods are often used, it is more ideal to estimate the population of a community by considering general and unique local conditions.
Example
The population count of a community over forty (40) years gives the following information below. Estimate the population of the community in 2070.
| Year | Population (x103) |
| 1966 | 15 |
| 1976 | 22 |
| 1986 | 35 |
| 1996 | 43 |
| 2006 | 51 |
From the linear trend of the graph, arithmetic method is assumed the most suitable.
The first step is to determine Ka. Ka is calculated based on the interval of years used. In this example, the interval is 10 years.
Ka1 = ((22 – 15) x 103)/10 = 700
Ka2 = ((35 – 22) x 103)/10 = 1300
Ka3 = ((43 – 35) x 103)/10 = 800
Ka4 = ((51 – 43) x 103)/10 = 800
Average Ka = (Ka1 + Ka2 + Ka3 + Ka4)/4 = (700 + 1300 + 800 + 800)/4 = 900
pf = pi + Ka (tf – ti) = 51, 000 + 900 (2070 – 2006) = 108, 600
Note that more accurate estimates may be obtained using larger spread of years with closer intervals even though this comes with more stress. Excel template can be used to make this easier.
References
Agunwamba, J.C. (2008): Water Engineering Systems. De-Adroit Innovation, Enugu, Nigeria.
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