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*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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Five percent of the average annual demand can be estimated:
Five percent of the average annual demand can be estimated:
<math>D_{0.05} = P_{d} \times n\times 18.25</math>
<math>D_{0.05} = P_{d} \times n\times 18.25</math>
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*''P<sub>d</sub>'' is the daily demand per person (L)
*''P<sub>d</sub>'' is the daily demand per person (L)
*''n'' is the number of occupants}}
*''n'' is the number of occupants}}
 
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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.  
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:
<math>V_{S} = A_{c} \times C_{vol,E}\times R_{d} \times e</math>
<math>V_{S} = A_{c} \times C_{vol,E}\times R_{d} \times e</math>
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*Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (''C<sub>vol, E</sub>'') should be 0.9 or greater.
*Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (''C<sub>vol, E</sub>'') should be 0.9 or greater.
       
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Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding ''Y<sub>0.05</sub>'':
Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding ''Y<sub>0.05</sub>'':
<math>TotalStorage = V_{S} + Y_{0.05}</math>
<math>TotalStorage = V_{S} + Y_{0.05}</math>
 
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==STEP Rainwater Harvesting Tool==
==STEP Rainwater Harvesting Tool==
[[File:RWH_tank_capacity_table.jpg|thumb|500 px|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.]]
[[File:RWH_tank_capacity_table.jpg|thumb|500 px|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.]]

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