Changes

*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_{vol, A}'' 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_{vol, A}'' 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.

 

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>

*''P_d'' is the daily demand per person (L)

*''P_d'' is the daily demand per person (L)

*''n'' is the number of occupants}}

*''n'' is the number of occupants}}

 

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>

*Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (''C_{vol, E}'') should be 0.9 or greater.

*Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (''C_{vol, 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 ''Y_0.05'':

Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding ''Y_0.05'':

<math>TotalStorage = V_{S} + Y_{0.05}</math>

<math>TotalStorage = V_{S} + Y_{0.05}</math>

 

==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.]]