Difference between revisions of "Rainwater harvesting: Sizing and modeling"
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[[File:Cistern Size.png|thumb|Schematic diagram of the inputs and outputs to a rainwater harvesting cistern]] | [[File:Cistern Size.png|thumb|Schematic diagram of the inputs and outputs to a rainwater harvesting cistern]] | ||
===Simple=== | ===Simple=== | ||
*Five percent of the average annual yield can be estimated: | *Five percent of the average annual yield can be estimated: | ||
*<strong>Y<sub>0.05</sub>= A × C<sub>vol, A</sub> × R<sub>a</sub> × e × 0.05</strong> | *<strong>Y<sub>0.05</sub>= A × C<sub>vol, A</sub> × R<sub>a</sub> × e × 0.05</strong> | ||
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*C<sub>vol, A</sub> = The annual runoff coefficient for the catchment | *C<sub>vol, A</sub> = The annual runoff coefficient for the catchment | ||
*R<sub>a</sub> = The average annual rainfall depth (mm) | *R<sub>a</sub> = The average annual rainfall depth (mm) | ||
*e = The efficiency of the pre-storage filter | *e = The efficiency of the pre-storage filter | ||
Filter efficiency (e) can be reasonably estimated as 0.9 pending manufacturer’s information. | 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<sub>vol, A</sub> of the rooftops to be around 0.8 [http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/performance-evaluation-of-rainwater-harvesting-systems-toronto-ontario/]. This figure includes losses to evaporation, snow being blown off the roof, and number of overflow events.<br> | In a study of three sites in Ontario, STEP found the annual C<sub>vol, A</sub> of the rooftops to be around 0.8 [http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/performance-evaluation-of-rainwater-harvesting-systems-toronto-ontario/]. This figure includes losses to evaporation, snow being blown off the roof, and number of overflow events.<br> | ||
*Five percent of the average annual demand (D<sub>0.05</sub>) can be estimated: | *Five percent of the average annual demand (D<sub>0.05</sub>) can be estimated: | ||
*<strong>D<sub>0.05</sub> = P<sub>d</sub> × n × 18.25</strong> | *<strong>D<sub>0.05</sub> = P<sub>d</sub> × n × 18.25</strong> | ||
*D<sub>0.05</sub> = Five percent of the average annual demand (L) | *D<sub>0.05</sub> = Five percent of the average annual demand (L) | ||
*P<sub>d</sub> = The daily demand per person (L) | *P<sub>d</sub> = The daily demand per person (L) | ||
*n = The number of occupants | *n = The number of occupants | ||
Then the following calculations are based upon two criteria: | Then the following calculations are based upon two criteria: | ||
Revision as of 22:47, 11 August 2017
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 (D0.05) 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:
- A design rainfall depth is to be captured entirely by the RWH system.
- 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.
/*Good 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]
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:
- Bottom elevation (m)
- Maximum depth (m)
- Initial water depth (m)
- 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.