Population Forecasting & Water Demands

Total Quantity of water required for a town depends on:

1. Rate of demand
2. Design period
3. Population (at the end of design period)

Rate of demand: Expressed as lphd, lpcd etc

Varius type of demand. (As per IS:1172_ 1993)

a) For an average Indian town without full flushing system (for LPG)

• Domestic ( 50 to 60%) —135 l.p.c.d
• lndustnal & commercial (20to25%) — 70
• Public use (5to 10%) — 10
• Losses & thefts (15 to2O %) —55
• Total demand: 270 l.p.c.d

b) With full flushing system (HIG)

• Domestic —200 l.p.c.d
• Industrial & commercial — 70
• Public use - 10
• Losses & thefts — 55
• Total demand: 335 l.p.c.d

>>The per capita demand for  nursing homes, Boarding Schools and Hostels: 135 lpcd.

Domestic demand: bathing, cooking etc...

Industrial and commercial  demand: Offices,hostels, hospitals,factories, industries etc.

1. Offices-45 to 90 lpcd
2. Day Schools —45 to 90 lpcd
3. Residential Schools -135 to 225 lpcd
4. Hostels— 180 lit/bed
5. Restrorent -70 lit/seat
6. Hospitals (<100 beds) —340 lit/bed
7. Hospitals (>100 beds) —450 lit/bed
8. Bus & Railway Stations, Airport -70 lpcd

Public use: Lawn sprinkling, public parks etc.

Losses: Leakage due to bad Plumbing, etc.

Fire demand: (Generally 5 to 10% of total demand of the city.

• Generally 3 streams for each fire @ 1100 l/min.
• Minimum water Pressure. 1 to 1.5 kg/cm2 at the fire hydrant.
• Quantity is less but rate of supply is Very high. Therefore it affects the distribution system rather than storage tanks.

Empirical Formulae for fire demand:

1. Kuichling’s formula:Q =3182 (P)1/2   Where Q=Demand in lt/min, P = population in thousands
2. Freeman's formula: Q = 1136 [(P/5)+10]
3. National board of fire underwriters formula:                                           Q=4637(P)1/2 [1 – 0.01] (P)1/2
4. Buston’s Formula: Q = 5663(P)1/2

Factors affecting the rate of demand :

1. Size of city
2. climatic conditions
3. Types of gentry and habits
4. Industrial and commercial activity.
5. Quality of water supply
6. Pressure in the distribution system
7. Development of sewage facilities
8. System of supply
9. Cost of water
10. Policy of metering

System of supply:

a) Intermittent system : Generally consumption is less.

b) Continuous system: Generally Consumption is more.

Variation in demand and their effects on the design of various components of a water supply scheme.

Maximum daily demand = 1.8 x Average daily demand

Maximum hourly demand = 1.5 X Average hourly demand of a Maximum daily

Demand = 2.7 X Average hourly demand

Total Draft: Greater of the following

j) Max. daily demand + Fire demand, it is called coincident draft.

ii) Max. hourly demand.

Components design demand)

1. Sources of supply : Maximum daily consumption. 
2. Pipe mains: (Source to service reservoir) Maximum daily consumption.
3. Filters and other units: Twice the average daily demand (sometimes Max. daily demand).             To take care of break down.
4. Pumps Twice the average demand
5. Distribution system: Total draft.

Design period: A reasonable future period for which provision is made in water supply scheme.

• It shall not be too Large to become a burden on the present users or too short to be uneconomical.
• A design period of 20 to 30 years is generally adopted.
• It depends on useful life of component. difficulty in future expansion, funds availability, anticipated rate of population growth ,interest rate etc

Component	Design period
Dams	50 years
Conveying main pipes	30 years
Distribution system	30 years
Water treatment units	15 years
Pumps, Service Reservoir	15 years

Population Forecasting Method:

Based on law of probability and therefore give approximate estimate.

Arithmetic increase method:

• Rate of change of population with time is assumed to be constant.
• Applicable to old and large cities with no industrial growth and reached a saturation or maximum development.
• This method yields lower results for rapidly growing cities.

P_n = ( P₀ + nx )

Where, P₀= latest known population.

P_n = Prospective population after ‘n’ decades.

X bar  = average increase in population per decade.

Geometrical increase method:

• Percentage increase in population from decade to decade is assumed to be constant

It gives good result for young and rapidly expanding cities

P = P₀ [1 + r/100]ⁿ

Where r is geometric mean % increase

Incremental increase method: Growth rate is assumed to be progressively increasing or decreasing, depending upon whether the average of the incremental increases in the past is positive or negative. The population for a future decade is worked out by adding the mean arithmetic increase to the last known population as in the arithmetic increase method, and to this is added the average of incremental increases, once for first decade, twice for second and so on.

Decreasing rate method:In this method, the average decrease in the percentage increase is worked out, and is then subtracted from the latest percentage increase to get the percentage increase of next decade.

Simple graphical method:In this method, a graph is plotted from the available data, between time and population. The curve is then smoothly extended upto the desired year. This method gives very approximate results and should be used along with other forecasting methods.

Comparative graphical method :In this method, the cities having conditions and characteristics similar to the city whose future population is to be estimated are selected. It is then assumed that the city under consideration will develop, as the selected similar cities have developed in the past.

Ratio method In this method: the local population and the country's population for the last four to five decades is obtained from the census records. The ratios of the local population to national population are then worked out for these decades. A graph is then plotted between time and these ratios, and extended upto the design period to extrapolate the ratio corresponding to future design year. This ratio is then multiplied by the expected national population at the end of the design period, so as to obtain the required city's future population.

Logistic Curve Method:The three factors responsible for changes in population are : 

1. Births,
2. Deaths 
3. Migrations.

Logistic curve method is based on the hypothesis that when these varying influences do not produce extraordinary changes, the population would probably follow the growth curve characteristics of living things within limited space and with limited economic opportunity. The curve is S-shaped and is known as logistic curve.