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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=== |
| | The following approximations |
| Five percent of the average annual yield can be estimated: | | Five percent of the average annual yield can be estimated: |
| <math>Y_{0.05} = A_{c} \times C_{vol,A}\times R_{a} \times e \times 0.05</math> | | <math>Y_{0.05} = A_{c} \times C_{vol,A}\times R_{a} \times e \times 0.05</math> |
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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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| <math>TotalStorage = V_{S} + Y_{0.05}</math> | | <math>TotalStorage = V_{S} + Y_{0.05}</math> |
| ---- | | ---- |
| | ===Example calculation=== |
| | |
| ==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.]] |