Water Demand & Population Forecasting Notes

Last reviewed 16 Sept 2026 · 7 min read

Planning a water supply scheme

A public water supply scheme must supply enough water of acceptable quality at adequate pressure to the population at the end of the design period. The first steps are to estimate the per capita demand, the future population and the variations in demand.

Types of water demand

Demand	Includes
Domestic	Drinking, cooking, bathing, washing clothes and utensils, flushing toilets, gardening — usually the largest share in towns
Industrial	Factories, processing units (varies greatly with type of industry)
Institutional and commercial	Schools, hospitals, hotels, offices, restaurants, railway and bus stations
Public (civic) use	Street washing, sewer flushing, public parks, fountains
Fire demand	Fire fighting (small annual quantity but high rate)
Losses and wastage	Leakage from mains and fittings, unauthorised connections, meter errors — part of non-revenue water

Per capita demand norms (CPHEEO)

Classification of town/city	Recommended maximum water supply (litres per capita per day)
Towns with piped water supply but without sewerage	70
Cities with piped water supply where sewerage exists or is planned	135
Metropolitan and mega cities with piped supply and existing/planned sewerage	150
Towns supplied through public stand posts	40

These figures exclude unaccounted-for water; an allowance (commonly up to about 15%) is added for losses.
Rural household tap connections under the Jal Jeevan Mission are planned for 55 lpcd.

Factors affecting per capita demand

Size of the city (larger cities — higher demand); climate (hot, dry — higher); living standards and habits of people; industrial and commercial activity; pressure in the distribution system (higher pressure — more wastage); quality of water; metering and cost (metering reduces demand); sewerage system (flush toilets increase demand); policy on continuous or intermittent supply; efficiency of the water works administration (leak control).

Fire demand

FormulaFire demand formulas (

P

= population in thousands)

Formula	Fire demand
Kuichling	$Q = 3182 P$ (litres/min)
Freeman	$Q = 1136 (\frac{P}{10} + 10)$ (litres/min)
National Board of Fire Underwriters (central congested areas)	$Q = 4637 P (1 - 0.01 P)$ (litres/min)
Buston	$Q = 5663 P$ (litres/min)
Indian practice (CPHEEO)	$Q = 100 P$ (kilolitres/day)

Fire hydrants are provided at intervals along mains, and the distribution system (especially storage) is designed to meet fire flow at adequate pressure.

Variations in demand

Water demand varies:

Seasonally — higher in summer.
Daily — with days of the week and festivals.
Hourly — two peaks in most towns (morning and evening), minimum at night.

FormulaPeak demands (common rule-of-thumb values)

Maximum daily demand ≈ 1.8 × average daily demand
Maximum hourly demand ≈ 1.5 × maximum daily demand = 2.7 × average daily demand
Goodrich's formula: $p = 180 t^{- 0.10}$ — percentage of the annual average demand for a period of $t$ days (e.g. about 180% for a day, 148% for a week)

Peak factors for distribution systems (CPHEEO):

Population	Peak factor
Up to 50 000	3.0
50 000 to 2 lakh	2.5
Above 2 lakh	2.0
Rural water supply schemes	3.0

Design flows for components

Component	Designed for
Source, intake, raw water pumps and mains	Maximum daily demand (often average daily for sources with storage)
Treatment plant	Maximum daily demand
Service reservoirs	Hourly fluctuations (balancing), fire and emergency storage
Distribution system	Maximum hourly demand (or maximum daily + fire demand, whichever is greater)

Design period

The design period is the number of years for which a component is designed to be adequate. It depends on the useful life of the component, ease of future expansion, rate of population growth, cost and interest rates, and the performance of the component in early years (under-loading).

Design periods recommended by CPHEEO (typical):

Component	Design period (years)
Storage by dams	50
Intake structures, conveyance mains, distribution system, trunk mains	30
Pumping machinery	15
Water treatment units	15
Service (clear water) reservoirs	15
Land acquisition for future extensions	30

The design period is counted from the expected year of completion of the project, not from the start of design.

Population forecasting

The future population is estimated from past census data (India's census is decennial).

FormulaMathematical methods (

P_{0}

= latest population,

n

= number of decades)

1. Arithmetic increase method — constant increase per decade (old, large, saturated cities):

P_{n} = P_{0} + n \overset{x}{ˉ}

$\overset{x}{ˉ}$ = average increase per decade.

2. Geometric increase method — constant percentage growth (young, rapidly growing cities):

P_{n} = P_{0} (1 + r)^{n}

$r$ = geometric mean of the decadal growth rates $= (r_{1} r_{2} \dots r_{t})^{1/ t}$ . Gives the highest estimate.

3. Incremental increase method — combines arithmetic and geometric trends (average cities):

P_{n} = P_{0} + n \overset{x}{ˉ} + \frac{n ( n + 1 )}{2} \overset{y}{ˉ}

$\overset{y}{ˉ}$ = average of the increments (change in decadal increases).

4. Decreasing rate of growth method — the percentage growth rate decreases steadily; the average decrease in rate is subtracted from the latest rate each decade.

5. Logistic curve (S-curve) method:

P = \frac{P _{s}}{1 + m e ^{n t}}, P_{s} = \frac{2 P _{0} P _{1} P _{2} - P _{1}^{2} ( P _{0} + P _{2} )}{P _{0} P _{2} - P _{1}^{2}}

using three census populations $P_{0}$ , $P_{1}$ , $P_{2}$ at equal time intervals; $P_{s}$ = saturation population.

Other methods:

Simple graphical method — extend the population–time curve by eye.
Comparative graphical method — the city's growth is assumed to follow that of larger, similar cities when they were of the same population.
Master plan / zoning method — population based on planned densities of zones in the city's master plan.
Ratio and correlation method — city population as a ratio of the state or national population forecast.

Arithmetic increase generally gives low estimates, geometric increase high estimates, and incremental increase intermediate values.

Worked examples

Worked ExampleExample 1 — population forecasts

Census populations of a town are 40 000 (1981), 50 000 (1991), 62 000 (2001) and 76 000 (2011). Estimate the 2041 population by the arithmetic, geometric and incremental increase methods.

Solution. Decadal increases: 10 000, 12 000, 14 000 → $\overset{x}{ˉ}$ = 12 000; increments: 2000, 2000 → $\overset{y}{ˉ}$ = 2000; $n$ = 3.

Arithmetic: $P = 76000 + 3 \times 12000 = 112000$

Incremental: $P = 76000 + 36000 + \frac{3 \times 4}{2} \times 2000 = 124000$

Geometric: rates 25%, 24%, 22.58% → $r = (0.25 \times 0.24 \times 0.2258)^{1/3} = 0.2384$ $P = 76000 \times 1.238 4^{3} = 76000 \times 1.899 = 144300$ (approx.)

Worked ExampleExample 2 — fire demand

Find the fire demand for a city of 1 lakh population by Kuichling's and Freeman's formulas and the CPHEEO formula.

Solution. $P$ = 100 (thousands) Kuichling: $3182100 = 31820 L/min$ Freeman: $1136 (10 + 10) = 22720 L/min$ CPHEEO: $100100 = 1000 kL/day$

Worked ExampleExample 3 — design demands

A city with sewerage has a design population of 1 lakh. Find the average daily demand, maximum daily demand and maximum hourly demand (use 135 lpcd).

Solution. Average $= 100000 \times 135 = 13.5 \times 1 0^{6}$ L/day = 13.5 MLD Maximum daily $= 1.8 \times 13.5 = 24.3 MLD$ Maximum hourly rate $= 2.7 \times 13.5 = 36.45 MLD$ (as a rate)

Frequently tested points

CPHEEO: 70 lpcd (no sewerage), 135 lpcd (with sewerage), 150 lpcd (metro), 40 lpcd (stand posts); JJM rural 55 lpcd.
Metering and lower pressure reduce per capita demand; flush toilets increase it.
Kuichling $3182 P$ ; Freeman $1136 (P /10 + 10)$ ; CPHEEO $100 P$ kL/day.
Max daily = 1.8 × average; max hourly = 2.7 × average; Goodrich $p = 180 t^{- 0.1}$ .
Peak factor: 3.0 (≤ 50 000), 2.5 (50 000–2 lakh), 2.0 (> 2 lakh).
Design periods: dams 50 y; intake, mains, distribution 30 y; pumps, treatment plants, service reservoirs 15 y.
Arithmetic (old cities, lowest), geometric (young cities, highest), incremental (intermediate).
Treatment plant designed for maximum daily demand; distribution for maximum hourly demand.

Common MistakeCommon mistakes

Using the arithmetic mean of growth rates in the geometric method (use the geometric mean).
Taking $n$ in years instead of decades.
Designing distribution pipes for average daily demand.

Revision SummaryChapter summary

Water demand combines domestic, industrial, commercial, public and fire uses plus losses.
CPHEEO norms and local factors fix per capita demand.
Demand varies seasonally, daily and hourly; peak factors size different components.
Design periods depend on component life and expandability.
Population is forecast by arithmetic, geometric, incremental, logistic and graphical methods.

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Water Demand & Population Forecasting