OHSR Capacity & Population Forecasting for Rural Water Supply
Reservoir sizing starts long before the structure: it starts with an honest forecast of how many people the scheme must serve, and how much water each will need.
Start with the design period
A water supply scheme is not built for today's population but for the population at the end of its design period, often taken as around thirty years for major components. Designing for today guarantees the scheme is inadequate within a few years of commissioning.
The first decision, then, is the design period for each component, because a pump may be sized for a shorter horizon than a reservoir or a main that is expensive to replace.
Forecasting the population
The design population is projected from past census figures using a recognised method such as the arithmetic increase, geometric increase or incremental increase approach, chosen to suit the village's growth pattern. A fast-growing peri-urban village and a stable remote hamlet should not be projected the same way.
The projection should be sensible: it is checked against local knowledge of likely growth, because a mechanically applied formula can produce an absurd number for a small base population.
From population to daily demand
Design population is converted to water demand using a per-capita norm expressed in litres per capita per day (LPCD). The norm reflects the service level targeted, such as household connections versus public stand posts, and whether the habitation is fully covered or partially covered.
Multiplying the design population by the LPCD norm gives the average daily demand. Allowances for losses and, where relevant, for livestock or institutional needs are added so the figure reflects what the scheme must actually deliver.
Sizing the overhead service reservoir
The OHSR does not store a full day's water; it balances the difference between a steady pumping pattern and a peaky demand pattern. Its capacity is driven by the balancing storage required, commonly a fraction of the daily demand determined from the pumping hours and the demand curve.
A reservoir sized purely as a round fraction of daily demand without considering pumping hours is a guess. The honest approach relates storage to how many hours the scheme pumps and how demand is distributed through the day.
Staging height and the downstream link
The reservoir's staging height is set so that its low water level still commands enough residual head across the distribution network. Capacity and staging are therefore not independent: the tank must hold the right volume and sit at the right height.
This is where reservoir sizing hands over to the distribution hydraulics. The low water level of the OHSR becomes the starting hydraulic grade line for the network below it, which is why the two calculations must be done as one connected exercise rather than in isolation.
What design period is used for a water supply scheme?
Major, expensive-to-replace components such as reservoirs and mains are commonly designed for a horizon of around thirty years, while components like pumps may use a shorter period.
How is daily water demand calculated?
Multiply the forecast design population by the per-capita norm in litres per capita per day (LPCD) for the targeted service level, then add allowances for losses and any institutional or livestock needs.
Does an OHSR store a full day of water?
No. It provides balancing storage to absorb the mismatch between steady pumping and peaky demand, so its capacity is a fraction of daily demand determined from pumping hours and the demand pattern.