Factors affecting water demand; variation

Factors affecting the rate of demand

1. Climate: Hot and dry weather increases the use of water for drinking, bathing, cooling and gardening.
2. Size and type of community: Large cities use more water per head than villages; industrial or commercial cities use more.
3. Standard of living: Rich households with flush toilets, baths and appliances use much more than poor households.
4. Industrial and commercial activities: Factories, hotels, and offices raise the demand.
5. Quality of water: Better and safer water encourages use.
6. Pressure in the distribution system: High pressure increases consumption and leakage.
7. Cost (tariff) and metering: Metered supply and higher tariff reduce wastage; flat rate and free taps increase it.
8. System of supply: Continuous supply uses more water than intermittent supply.
9. Water loss and wastage: Leaky pipes and careless use increase demand.
10. Other: Habits and culture, availability of sewerage, and fire demand.

Variation of demand

• Seasonal variation: Demand is highest in summer (about 1.1 to 1.2 times the average) and low in winter.
• Daily variation: Higher on holidays, hot days and during festivals; maximum daily demand is about 1.8 times the average daily demand.
• Hourly variation: Demand is low at night and high in the morning (6 to 9 am) and evening (5 to 8 pm). The peak hour demand is about 1.5 times the maximum daily demand, or 2.7 times the average.

 Demand
   |      /\        /\
   |     /  \      /  \
   |    /    \____/    \
   |___/                \___
   0   6   12   18   24  hour
      (morning & evening peaks)

Peak demand

Peak factors are applied to the average daily demand $Q_{avg}$:

• Maximum daily demand $= 1.8\,Q_{avg}$ (Nepal rural: about 1.5)
• Peak hourly demand $= 1.5\times$ maximum daily demand $\approx 2.7\,Q_{avg}$ (population 20,000 to 50,000: factor about 2.0 to 3.0 in Nepal practice).

The distribution mains are designed for the peak hourly demand, while the source and transmission main are designed for the maximum daily demand.