Rainwater harvesting: Sizing and modeling

{{TOClimit|2}

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

---

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. Rainwater Harvesting Tool

---

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

---

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.