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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: |
| $$Y_{0.05} = A \times C_{vol,A}\times R_{a} \times e \times 0.05$$ | | $$Y_{0.05} = A \times C_{vol,A}\times R_{a} \times e \times 0.05$$ |
| where: | | where: |
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| *''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.<br> | | *Filter efficiency (''e'') can be reasonably estimated as 0.9 pending manufacturer’s information.<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 a number of overflow events. | | *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 a number of overflow events. |
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| Then the following calculations are based upon two criteria: | | Then the following calculations are based upon two criteria: |
| #A design rainfall depth is to be captured entirely by the RWH system. | | #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. | | #The average annual demand (''D'') is greater than the average annual yield (''Y'') from the catchment. |
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| When \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated: | | When \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated: |
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| *''e'' = The efficiency of the pre-storage filter.}} | | *''e'' = The efficiency of the pre-storage filter.}} |
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| *Careful catchment selection means that the runoff coefficient, for a rainstorm event (C<sub>vol, E</sub>) should be 0.9 or greater. | | *Careful catchment selection means that the runoff coefficient, for a rainstorm event (''C<sub>vol, E</sub>'') should be 0.9 or greater. |
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