Rainwater harvesting: Sizing and modeling

Rapid

Five percent of the average annual yield (Y_0.05, in L) is calculated as the product of:

• The catchment area (A, in m²)
• The runoff coefficient for the catchment (C_vol)
• The average annual rainfall depth (R, in mm)
• the efficiency of the pre-storage filter (η), and
• 0.05

Five percent of the average annual demand (D_0.05) is calculated as the product of:

• The daily demand per person (P_d, in L)
• The number of occupants (n), and
• 0.05

The following calculation is based upon two criteria:

1. A design rainfall depth is to be captured entirely by the RWH system.
2. The average annual demand (D_0.05) is greater than the average annual yield (Y_0.05) coming from the catchment.

When Y_0.05/ D_0.05 <0.33
The total storage volume required (V_S, in L) is calculated as the product of:

• The catchment area (A, in m²)
• The design storm rainfall depth (R_d, in mm)
• The design storm runoff coefficient for the catchment (C_vol), and
• the efficiency of the pre-storage filter (η).

Good catchment selection means that the runoff coefficient (β) should work out to be 0.9 or greater. Filter efficiency can be reasonably estimated as 0.9 pending manufacturer’s information.

When 0.33 < Y_0.05/ D_0.05 <0.7
The total storage required is the sum of V_S and Y_0.05.

Total cistern volume can be estimated by multiplying the depth of design storm the catchment area.
One millimeter of rain landing on 1 m² results in 1 L of runoff.
For example, the 90th percentile event in Barrie is 26 mm, so every 1 m² of rooftop will generate 26 L during this storm event.
A 2000 m² building would generate approximately 54,000 L.
The designers have three choices:-

1. Construct a suitably sized concrete vault underground to capture all 54,000 L of the water
2. Alter the slope of the roof to create two or more catchments, the smaller catchments may be diverted to plastic or fiberglass cisterns
3. Design 1. or 2. slightly undersized for this storm, with additional capacity in an infiltration system to capture overflow. Examples include bioretention cells or infiltration chambers.

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  Schematic diagram of the inputs and outputs to a rainwater harvesting cistern

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STEP Rainwater Harvesting Tool

The Sustainable Technologies Evaluation Program have produced a rainwater harvesting design and costing tool specific to Ontario. The tool is in a simple to use Excel format and is free to download.

In a study of three sites in Ontario, STEP found the annual C_vol of the rooftops to be around 0.8 [1]. This figure includes losses to evaporation, snow being blown off the roof, and number of overflow events.

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The Treatment Train Tool

Once the size of cistern has been determined, it can easily be modelled in many open source and proprietary applications.

In addition to the cistern size, this watershed scale modelling requires input information about draw down. time i.e. the rate of use.

<btnPrimary>The Treatment Train Tool</btnPrimary>

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See Also

This list will be other 'Sizing and Modelling' pages

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External Links

• SWMM5.1

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The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Rainwater harvesting.

Tank used to store rainwater (typically roof runoff) for later use.

Tank used to store rainwater (typically roof runoff) for later use.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Sustainable Technologies Evaluation Program

Abiotic transfer of water vapour to the atmosphere.