Learning Objective for this module:
You will be able to design the concept for a rainwater harvesting system when supplied with the necessary data.
Sizing rainwater systems
Reliable rainfall data for a period of 10 years (longer in drought-prone areas) is ideally required to get an accurate estimate of the potential rainwater supply from a given catchment.
Estimating household water demand must be done with care because demand may vary over time as consumption rates may change in different seasons. The total daily water demand can vary widely, however the essential daily water requirements for drinking, cooking and personal hygiene are less variable, and 15-25 litres per person can normally suffice.
Estimating the potential maximum supply of rainwater runoff is straightforward if good data are available. Simply multiply the mean annual rainfall by the horizontal catchment area and runoff coefficient.
Runoff coefficients, when averaged over the long term, range from as high as 0.8-0.85 for a well-constructed corrugated-iron roof to 0.1-0.2 for a compacted soil surface.
Several techniques are available for sizing rainwater storage tanks. These approaches are described in more detail on the following pages:
- demand side approach (sizing I)
- supply side approach (sizing II)
- computer-based methods (sizing III)
Any decision regarding rainwater catchment system dimensions will depend on a range of factors, including the rainfall amount and distribution, the demand schedule and willingness to ration, and the affordability of the catchment/storage size options.