Rainwater harvesting: Sizing and modeling

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

Simple

Five percent of the average annual yield can be estimated: $Y_{0.05}=A_{c}\times C_{vol,A}\times R_{a}\times e\times 0.05$

Where:

• Y_0.05 is five percent of the average annual yield (L)
• A_c is the catchment area (m²)
• C_{vol, A} is the annual runoff coefficient for the catchment
• R_a is the average annual rainfall depth (mm)
• e is the efficiency of the pre-storage filter

• Filter efficiency (e) can be reasonably estimated as 0.9 pending manufacturer’s information.
  
• In a study of three sites in Ontario, STEP found the annual C_{vol, A} of the rooftops to be around 0.8 [1]. This figure includes losses to evaporation, snow being blown off the roof, and a number of overflow events.

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Five percent of the average annual demand can be estimated: $D_{0.05}=P_{d}\times n\times 18.25$

Where:

• D_0.05 is five percent of the average annual demand (L)
• P_d is the daily demand per person (L)
• n is the number of occupants

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Then the following calculations are based upon two criteria:

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

When \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated: $V_{S}=A_{c}\times C_{vol,E}\times R_{d}\times e$

Where:

• V_S is the volume of storage required (L)
• A_c is the catchment area (m²)
• C_vol,E is the design storm runoff coefficient for the catchment
• R_d is the design storm rainfall depth (mm), and
• e is the efficiency of the pre-storage filter.

• Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (C_{vol, E}) should be 0.9 or greater.

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Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding Y_0.05: $TotalStorage=V_{S}+Y_{0.05}$

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STEP Rainwater Harvesting Tool

Quick reference table generated using STEP RWH tool, (data for the City of Toronto (median annual rainfall 678 mm). Optimal cistern size is that providing at least a 2.5% improvement in water savings following an increase of 1,000 Litres in storage capacity.

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.

STEP Treatment Train Tool

Rainwater harvesting: TTT

Cistern dimensions

The blue area indicates the only usable volume in this rainwater harvesting cistern, the depth between the pump inlet and the overflow.

The connections into and out of a rainwater cistern can have a dramatic effect on the actual usable volume. The only usable water within the cistern is that above the height of the pump intake, and below the invert of the overflow outlet. This dimension can easily become constrained where the outlet must lie beneath the frost line and where a high powered pump is required to elevate the water many storeys. Communication and understanding between design engineers is paramount in large installations. Quick notes:

1. The depth of storage between the elevation of the inlet and overflow is unusable space, so the overflow should be located towards the top of the cistern.
2. The depth of storage beneath the pump inlet is unusable, but may also be a useful zone for sediment to settle. A custom or cast-in-place vault could minimise this unused volume by tapering towards the the base.

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.

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

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.

Sustainable Technologies Evaluation Program

Abiotic transfer of water vapour to the atmosphere.

Rainwater harvesting.

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

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.