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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=== |
| <blockquote>Five percent of the average annual yield can be estimated: | | <blockquote> |
| <br><strong>Y<sub>0.05</sub>= A × C<sub>vol, A</sub> × R<sub>a</sub> × e × 0.05</strong>
| | *Five percent of the average annual yield can be estimated: |
| <br>Y<sub>0.05</sub> = Five percent of the average annual yield (L)
| | *<strong>Y<sub>0.05</sub>= A × C<sub>vol, A</sub> × R<sub>a</sub> × e × 0.05</strong> |
| <br>A = The catchment area (m<sup>2</sup>)
| | *Y<sub>0.05</sub> = Five percent of the average annual yield (L) |
| <br>C<sub>vol, A</sub> = The annual runoff coefficient for the catchment
| | *A = The catchment area (m<sup>2</sup>) |
| <br>R<sub>a</sub> = The average annual rainfall depth (mm)
| | *C<sub>vol, A</sub> = The annual runoff coefficient for the catchment |
| <br>e = The efficiency of the pre-storage filter</blockquote>
| | *R<sub>a</sub> = The average annual rainfall depth (mm) |
| | *e = The efficiency of the pre-storage filter</blockquote> |
| | |
| Filter efficiency (e) can be reasonably estimated as 0.9 pending manufacturer’s information. | | Filter efficiency (e) can be reasonably estimated as 0.9 pending manufacturer’s information. |
| 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 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 number of overflow events. |
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| <strong>Total storage = V<sub>S</sub> + Y<sub>0.05</sub></strong> | | <strong>Total storage = V<sub>S</sub> + Y<sub>0.05</sub></strong> |
| ---- | | ---- |
| | |
| ===STEP Rainwater Harvesting Tool=== | | ===STEP Rainwater Harvesting Tool=== |
| [[File:RWH_tank_capacity_table.jpg|thumb|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|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.]] |