Water Supply

Water Demand, Design Periods and Demand Variation

PSC Subjective Note · Nepal Engineering Service (Civil), Second Paper

Study note: The numerical values below are illustrative assumptions, not universal design standards. For actual projects, verify current applicable guidance and local demand data.

Per Capita Demand of Water

Per capita demand, also called the rate of demand, is the annual average daily water consumption per person, including all demands. It is expressed in litres per capita per day (lpcd).

For the Nepalese context, illustrative values are 40–45 lpcd for rural water supply and 100–160 lpcd for urban water supply.

Per capita demand = \frac{Q}{365 \times P}

Where Q is the quantity of water required by the community per year (litres/year) and P is the community population (persons). The result is in lpcd.

Base and Design Periods

Base period: The time required for surveying, designing and constructing the water supply system. A period of 2–3 years is commonly adopted in these notes.

Design period: The future period for which provision is made when planning and designing the water supply system. A period of 15–20 years is often adopted. A shorter period may be used for a developing community with rapid, uncertain population growth; a longer period may be suitable where growth is lower and more predictable.

Table 1: Typical base and design periods
System	Base period	Design period
Rural, high population growth	2–3 years	15 years
Rural, low population growth	2–3 years	20 years
Urban	2–3 years	Up to 30 years

Base Year and Design Year

The base year is the year in which the completed system begins delivering water to the community. The design year is the year for which the system is designed.

Base year = Survey year + Base period

Design year = Base year + Design period

Periods in these expressions are measured in years.

Types of Water Demand

1. Domestic Demand (DD)

Domestic demand includes drinking, cooking, bathing, washing houses, utensils and clothes, sanitation, gardening and similar household uses. The study values are:

Fully plumbed houses: 112 lpcd.
Partly plumbed houses: 45 lpcd.
Rural areas served by public standposts: 45 lpcd.

Another illustrative domestic demand figure is 135 lpcd; the appropriate allowance depends on local conditions and applicable design guidance.

2. Livestock Demand (LD)

Livestock demand covers water consumed by domestic animals and birds. Illustrative allowances are:

Large animals, such as cows and horses: 45 litres/animal/day.
Medium-sized animals, such as pigs, sheep and goats: 20 litres/animal/day.
Birds, such as chickens and ducks: 20 litres/100 birds/day.

An illustrative assumption limits livestock demand to 20% of total domestic demand. It may be small relative to domestic demand in urban areas; assess it for the community being designed.

3. Commercial Demand (CD)

Commercial and institutional demand includes water used by offices, restaurants, schools, colleges, hospitals and hotels.

Table 2: Illustrative commercial and institutional water demands
Use	Allowance
Offices, depending on size	500–1,000 litres/day
Hospitals with beds	500 litres/bed/day
Hospitals without beds and health clinics	2,500 litres/day
Hotels with beds	200 litres/bed/day
Hotels without beds	500–1,000 litres/day
Restaurants and tea stalls	500–1,000 litres/day
Day scholars	10 lpcd
Boarders	65 lpcd

4. Public / Municipal Demand (PD)

This includes watering public parks and gardens and cleaning roads and sewers, mainly in urban communities. An illustrative assumption is 5–10% of total consumption.

5. Industrial Demand (ID)

Industrial demand varies with the size and type of industry. An illustrative assumption is 20–25% of total consumption.

6. Fire Demand (FD)

Fire demand is water required for firefighting. Provision should be considered in water supply schemes for cities and towns. The following empirical formulas use P as population in thousands; the unit of Q is specified for each formula.

Indian Water Supply Manual and Treatment Formula

Q = 100 \sqrt{P}

Q is in kilolitres/day.

Buston’s Formula

Q = 5663 \sqrt{P}

Q is in litres/minute.

Kuichling’s Formula

Q = 3182 \sqrt{P}

Q is in litres/minute.

Freeman’s Formula

Q = 1136 (\frac{P}{5} + 10)

Q is in litres/minute.

National Board of Fire Underwriters’ Formula

Q = 4637 \sqrt{P} (1 - 0.01 \sqrt{P})

Q is in litres/minute. Use empirical formulas within their applicable ranges and verify the relevant fire-design requirements.

7. Loss and Wastage (LW)

Water may be lost through leaking valves, mains and fittings, unauthorized connections and other wastage. An illustrative allowance is 15% of total supply; actual losses should be assessed for the system.

8. Total Water Demand

For a common demand basis, the components can be expressed as:

Total demand = DD + LD + CD + PD + ID + FD + LW

Use consistent units and time periods when combining components. Fire flow is an event-based requirement, not necessarily a continuous daily demand; apply the appropriate fire-flow duration and design scenario rather than adding a flow rate directly to daily volumes.

Variation in Water Demand

Water consumption varies seasonally, daily and hourly. Compare demand rates on the same time basis when applying peak factors.

1. Seasonal Variation

Demand may be higher in summer for drinking, bathing and washing and lower in winter. An illustrative seasonal peak factor is 1; local conditions may differ.

Maximum seasonal demand = Seasonal peak factor \times Annual average demand

2. Daily Variation

Demand changes from day to day and may increase during festivals such as Dashain and Tihar, weddings and other functions. The notes also identify Saturday as a potentially higher-demand day. An illustrative daily peak factor is 1, but this is not a universal value.

Maximum daily demand = Daily peak factor \times Annual average daily demand

3. Hourly Variation

Demand often peaks in the morning around 5–7 AM and again around 5–7 PM, including cooking and cleaning periods. It is typically lower overnight. An illustrative hourly peak factor is 3; actual patterns vary.

Maximum hourly demand = Hourly peak factor \times Annual average hourly demand

Annual average hourly demand is the annual average daily demand divided by 24 when expressed in litres/hour.

Peak Factor

A peak factor relates maximum demand to average demand on a consistent rate basis and helps account for demand fluctuations.

Peak factor = \frac{Maximum demand}{Average demand}

Maximum demand = Peak factor \times Average demand

When the individual factors are defined as successive, non-overlapping adjustments, the combined factor can be expressed as:

Combined peak factor = K_{s} \times K_{d} \times K_{h}

Here K_s, K_d and K_h represent seasonal, daily and hourly factors respectively. Do not multiply factors that already include the same variation.

Continuous system: illustrative peak factor of 2–4.
Intermittent system: illustrative peak factor of 4–6.

Choose project-specific factors according to the applicable design guidance and supply pattern.

Factors Affecting Water Demand

Size and type of community: Larger communities may have greater total demand because of their population and municipal, commercial and other uses.
Climatic conditions: Demand for drinking, washing and bathing may rise in hot weather and fall in cooler weather.
Standard of living: Higher standards of living and greater use of water-consuming amenities can increase demand.
Water quality: Consumers may use more supplied water when they consider its quality good and safe, and less if they distrust it.
System of supply: Consumption depends on the hours and reliability of supply. Continuous supply can permit greater use than intermittent supply.
Sewerage system: A sewerage system can increase water use for flushing sanitary units.
Metering and cost: Metering and higher tariffs may encourage conservation, while unmetered supply or lower prices may lead to higher consumption.
Other factors: Distribution pressure, industrial and commercial activity, public and private connections, distance to a tap stand, and the community's age distribution can also affect demand.

Population Forecasting

Population forecasting estimates the future population served by a water supply system. For a given per capita demand, the community's average water demand can be estimated as:

Q_{avg} = P \times q

Where P is the population, q is per capita demand (litres/person/day), and Q_avg is average daily demand (litres/day).

1. Mathematical Methods

Arithmetical Increase Method

This method assumes a constant numerical increase in population in each successive decade.

P_{n} = P_{0} + n C

Where P₀ is the population at the base year, P_n is the population after n decades, and C is the average numerical increase per decade.

Geometrical Increase Method

This method assumes a constant percentage increase in population for each future decade.

P_{n} = P_{0} {(1 + \frac{r}{100})}^{n}

Here r is the average percentage population increase per decade; P₀, P_n and n have the meanings above.

Incremental Increase Method

This method takes account of both the average decadal increase and the average change in that increase.

P_{n} = P_{0} + n C + \frac{n C^{'} (n + 1)}{2}

C′ is the average incremental increase in population per decade, and C is the average numerical increase per decade.

Decreased Rate of Growth / Changing Rate of Increase Method

Starting with the last observed decadal percentage increase, this method reduces the growth rate by the average decrease for each successive decade.

P_{n} = P_{0} \prod_{i = 1}^{n} (1 + \frac{r_{n} - i r^{'}}{100})

Here r_n denotes the percentage increase in the last observed decade, r′ is the average decrease in the percentage increase per decade, and i counts future decades from 1 to n. Assess whether extrapolated rates are reasonable for the community.

Other Mathematical Methods

Saturation limit method: Considers an upper limit to population growth.
Logistic curve (S-curve) method: Uses an S-shaped growth curve that approaches a saturation level.

2. Graphical Methods

Extension method: Extends the historical population trend graph to estimate future population.
Comparison method: Estimates future growth by comparing the community's trend with those of similar communities.

Compare forecasts with local development conditions and use judgement when selecting a design population.