Rainwater Harvesting

A joint effort of:

This lesson gives a simple way to calculate the necessary rainwater storage requirement (assuming sufficient rainfall and catchment area)

# Sizing a domestic RWH system: Demand-side approach

Usually, the main calculation when designing a domestic RWH system will be to size the water tank to give adequate storage capacity. The storage requirement will be determined by a number of interrelated factors.
They include:

• Local rainfall data and weather patterns
• Roof (or other) catchment area
• Runoff coefficient (this varies between 0.5 and 0.9 depending on roof material and slope)
• User numbers and consumption rates

A very simple method is to calculate the largest storage requirement based on the consumption rates and occupancy of the building.

As a simple example we can use the following typical data:

Consumption per capita per day, C = 20 litres
Number of people per household, n = 6
Longest average dry period = 25 days

Storage requirement T = C * n * 25 = 3,000 litres

This simple method assumes sufficient rainfall and catchment area which is adequate, and is therefore only applicable in areas where this is the situation. It is a method for acquiring rough estimates of tank size.

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# Formula 1

T = C * n * dpd

(T: storage requirement in litres
C: consumption per capita per day in litres
n: number of people per household
dpd: longest average dry period in days)