Rainwater harvesting: Sizing and modeling

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

Simple[edit]

  • Five percent of the average annual yield can be estimated:
  • Y0.05= A × Cvol, A × Ra × e × 0.05
  • Y0.05 = Five percent of the average annual yield (L)
  • A = The catchment area (m2)
  • Cvol, A = The annual runoff coefficient for the catchment
  • Ra = 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 Cvol, 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 can be estimated:
  • D0.05 = Pd × n × 18.25
  • D0.05 = Five percent of the average annual demand (L)
  • Pd = 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 Y0.05/ D0.05 < 0.33, the storage volume required can be estimated:
  • VS = A × Cvol × Rd × e
  • VS = Storage volume required (L)
  • A = The catchment area (m2)
  • Cvol, E = The design storm runoff coefficient for the catchment
  • Rd = The design storm rainfall depth (mm), and
  • e = The efficiency of the pre-storage filter.

/*Careful catchment selection means that the runoff coefficient, for a rainstorm event (Cvol, E) should be 0.9 or greater.

When 0.33 < Y0.05/ D0.05< 0.7, the total storage required can be estimated by adding Y0.05:
Total storage = VS + Y0.05


STEP Rainwater Harvesting Tool[edit]

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


STEP Treatment Train Tool[edit]

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