Changes

[[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:

*''R_a'' =  The average annual rainfall depth (mm)

*''R_a'' =  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_{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.

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:

*''e'' &#61; The efficiency of the pre-storage filter.}}

*''e'' &#61; The efficiency of the pre-storage filter.}}

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

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