Rainwater harvesting: Sizing and modeling
Revision as of 18:59, 20 February 2018 by Jenny Hill (talk | contribs)
Simple[edit]
Five percent of the average annual yield can be estimated:
Where:
- Y0.05 is five percent of the average annual yield (L)
- Ac is the catchment area (m2)
- Cvol, A is the annual runoff coefficient for the catchment
- Ra 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 Cvol, 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.
Five percent of the average annual demand can be estimated:
Where:
- D0.05 is five percent of the average annual demand (L)
- Pd is the daily demand per person (L)
- n is 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 \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated:
Where:
- VS is the volume of storage required (L)
- Ac is the catchment area (m2)
- Cvol,E is the design storm runoff coefficient for the catchment
- Rd 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 (Cvol, E) should be 0.9 or greater.
Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding 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.