Variation in water demand is observed at different point of time of a day, month, or season. Speaking clearly, water demand varies from minute to minute in an hour, from hour to hour in a day, from day to day in a month, from month to month in a year, and even in different seasons of the year.
Variation in Water Demand
As you may have noticed, the usage of water is never constant. Sometimes, the usage of water increases while there are some periods of time when water usage is almost zero. Now water demand increases with increase in usage of water while it decreases when water usage decreases.
Thus, it is established that variation in demand of water is prevalent everywhere and in everyone’s routine.
Types of Variation in Water Demand
Variation in water demand are observed on hourly, daily, monthly and seasonal basis as follows:
- Hourly variation in water demand
- Daily variation in water demand
- Seasonal variation in water demand
1. Hourly Variation in Water Demand
Hour to hour variation in water demand can be observed with the variations in water consumption at different hours of the day.
The water demand is quite low in the early hours of the day, i.e., till morning 06:00 AM. Water demand increases gradually post that and reaches peak value in-between 08:00 to 11:00 AM. The water demand decreases post 11:00 PM till afternoon 01:00 PM. Then it remains constant till 04:00 PM. In the evening time, water demand increases reaching peak in-between 07:00 to 09:00 AM. Late night, water demand again decreases.
(If the industrial water demand is kept aside, then the night consumption of water normally represents the magnitude of loss and theft.)
The water demand described in the above paragraph in different hours of the day is represented graphically below:
2. Daily Variation in Water Demand
With the variation in demand of water at household or industrial level with respect to different days of the months, daily variation in water demand is observed.
Day to day variation in water demand can be observed with the increased consumption of water on Sundays, holidays, or on the days when storm is experienced.
3. Seasonal Variation in Water Demand
As a result of difference in usage of water with respect to seasons, seasonal variation in water demand is observed.
In summer season, water consumption is increased while water consumption decreases in winter season. This leads to increased water demand in summer season while water demand decreases in winter.
Additionally, sudden increase in demand of water due to processing of cash crops by industries during harvesting and such other reasons also lead to seasonal variation in water demand.
Importance of Variation in Water Demand
Studying the variation in water demand is quite important for the design of water supply system. Smaller towns have larger variations from their average water demand. While designing components of water supply system, the maximum demand needs to be considered too.
The maximum demand of water assumed during the design of water supply structures is given below:
- Maximum daily consumption of water
- Maximum hourly consumption of water
1. Maximum Daily Consumption of Water
Max daily consumption of water is normally taken as 180% of the mean daily water consumption.
2. Maximum Hourly Consumption of Water
Max hourly consumption of water is normally taken as 150% of the mean hourly water consumption.
Peak Demand is thus maximum hourly consumption of the maximum day.
Peak Demand of Water
Peak demand can be derived from the following equation:
![\!\!\!\!\!\!\!\!\!\!Peak\:Demand\\\\=1.5\times Avg\;hourly\;consumption\;of\;maximum\;day\\\\=1.5\times\left[\frac{Maximum\:daily\:demand}{24}\right]\\\\1.5\times\left[\frac{1.8q}{24}\right]\\\\=2.7\left[\frac{q}{24}\right]](https://s0.wp.com/latex.php?latex=%5C%21%5C%21%5C%21%5C%21%5C%21%5C%21%5C%21%5C%21%5C%21%5C%21Peak%5C%3ADemand%5C%5C%5C%5C%3D1.5%5Ctimes+Avg%5C%3Bhourly%5C%3Bconsumption%5C%3Bof%5C%3Bmaximum%5C%3Bday%5C%5C%5C%5C%3D1.5%5Ctimes%5Cleft%5B%5Cfrac%7BMaximum%5C%3Adaily%5C%3Ademand%7D%7B24%7D%5Cright%5D%5C%5C%5C%5C1.5%5Ctimes%5Cleft%5B%5Cfrac%7B1.8q%7D%7B24%7D%5Cright%5D%5C%5C%5C%5C%3D2.7%5Cleft%5B%5Cfrac%7Bq%7D%7B24%7D%5Cright%5D&bg=ffffff&fg=000&s=0&c=20201002)
Goodrich Formula for Peak Water Demand
The ratios of peak demand rate of water to its corresponding average is obtained with the help of Goodrich formula.
Let us see for daily variation of water demand first, then weekly and finally conclude for monthly variation.
Government of India has released a manual providing peak values of water demand for different population ranges.
| Population | Peak Factor |
|---|---|
| Up to 50,000 | 3.0 |
| 50,001 – 2,00,000 | 2.5 |
| Above 2,00,000 | 2.0 |
| For rural water supply scheme, when supply is effected through stand post for only hr | 3.0 |
As it can be observed from the above table, peak factor reduces with the increase in population as large group of people having varying habits tends to have different usage patterns of water. This reduces the variations in water demand reducing peak factor.
Role of Variation in Water Demand w.r.t. Various Units of Water Supply Scheme
As we already know by now, variation in water demand affects the design of different units/components of the water supply scheme.
The factors considered during the design of some of these units are tabulated here:
| 1 | Water supply sources (wells, etc.) | Maximum daily consumption (sometimes avg daily consumption) |
| 2 | Pipe Mains (connecting source to treatment plant, and treatment plant to service reservoir) | Maximum daily consumption |
| 3 | Filter and other units at treatment plant | Maximum daily consumption + Additional provision for breakdown & repair (Hence instead of 1.8, 2.0 is taken) |
| 4 | Pumps | Maximum daily consumption + Additional provision for breakdown & repair |
| 5 | Pipe Mains connecting service reservoir to distribution system | Maximum hourly consumption of maximum day |
| 6 | Distribution System | Maximum hourly consumption of maximum day |
| 7 | Service Reservoir | Day’s Consumption |
Summary
The annual average water demand is the quotient obtained by dividing water quantity supplied during the year by the number of days in a year and number of people the water is supplied.
The actual water demand is different from the annual average water demand obtained from the above relation. Hence, the maximum water demand is calculated which is recognized as peak demand.
Maximum daily demand = 1.8 q
Maximum hourly consumption = 2.7 annual avg hrly water demand