Rainwater harvesting: Sizing and modeling

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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_{vol, A} × R_a × e × 0.05
• Y_0.05 = Five percent of the average annual yield (L)
• A = The catchment area (m²)
• C_{vol, A} = The annual runoff coefficient for the catchment
• R_a = The average annual rainfall depth (mm)
• e = 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 number of overflow events.

Five percent of the average annual demand (D_0.05) can be estimated:

D_0.05 = P_d × n × 18.25
D_0.05 = Five percent of the average annual demand (L)
P_d = The daily demand per person (L)

n = The number of occupants

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_vol × R_d × e
V_S = Storage volume required (L)
A = The catchment area (m²)
C_{vol, E} = The design storm runoff coefficient for the catchment
R_d = The design storm rainfall depth (mm), and
e = The efficiency of the pre-storage filter. /*Good catchment selection means that the runoff coefficient, for a rainstorm event (C_{vol, E}) should be 0.9 or greater.

When 0.33 < Y_0.05/ D_0.05< 0.7, the total storage required can be estimated by adding Y_0.05:
Total storage = 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. Rainwater Harvesting Tool

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STEP Treatment Train Tool

Once the size of cistern has been determined, it can easily be modeled in many open source and proprietary applications. For planning purposes, a RWH system could be integrated into a site plan using STEP's Treatment Train Tool. In a typical configuration:

• The catchment (roof) would be 100% impervious
• The rainwater harvesting system would be a 'Storage' Element with the following properties:
  • Storage type = No removal
  • Lined
  • Underlying soil = doesn't matter, can ignore
  • Evaporation factor = 0
  • Suction head (mm) = 0
  • Saturated conductivity (mm/hr) = 0
  • Initial soil moisture deficit (fraction) = 0
• The dimensions of the rainwater cistern can be placed into the fields:

1. Bottom elevation (m)
2. Maximum depth (m)
3. Initial water depth (m)
4. The Curves table is designed to accommodate ponds of roughly conical dimensions. A rainwater cistern is usually cuboid or cylindrical in shape, so that the area (m²) will remain the same throughout the depth.

Treatment Train Tool

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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.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

Abiotic transfer of water vapour to the atmosphere.