View source for Rainwater harvesting: Sizing and modeling

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[[File:Cistern Size.png|thumb|Schematic diagram of the inputs and outputs to a rainwater harvesting cistern]]
===Simple===
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*Five percent of the average annual yield can be estimated:
*<strong>Y<sub>0.05</sub>&#61;  A × C<sub>vol, A</sub> × R<sub>a</sub> × e × 0.05</strong>
*Y<sub>0.05</sub> &#61;  Five percent of the average annual yield (L)
*A &#61;  The catchment area (m<sup>2</sup>)
*C<sub>vol, A</sub> &#61;  The annual runoff coefficient for the catchment
*R<sub>a</sub> &#61;  The average annual rainfall depth (mm)
*e &#61; The efficiency of the pre-storage filter
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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.<br>

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*Five percent of the average annual demand (D<sub>0.05</sub>) can be estimated:
*<strong>D<sub>0.05</sub> &#61; P<sub>d</sub> × n × 18.25</strong>
*D<sub>0.05</sub> &#61; Five percent of the average annual demand (L)
*P<sub>d</sub> &#61; The daily demand per person (L)
*n &#61; The number of occupants}}}}

Then the following calculations are based upon two criteria:
#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. 

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*When Y<sub>0.05</sub>/ D<sub>0.05</sub> < 0.33, the storage volume required can be estimated:
*<strong>V<sub>S</sub> &#61; A × C<sub>vol</sub> × R<sub>d</sub> × e</strong>
*V<sub>S</sub> &#61; Storage volume required (L)
*A &#61; The catchment area (m<sup>2</sup>)
*C<sub>vol, E</sub> &#61; The design storm runoff coefficient for the catchment
*R<sub>d</sub> &#61; The design storm rainfall depth (mm), and
*e &#61; The efficiency of the pre-storage filter.}}}}

/*Careful catchment selection means that the runoff coefficient, for a rainstorm event (C<sub>vol, E</sub>) should be 0.9 or greater.
        
When 0.33 < Y<sub>0.05</sub>/ D<sub>0.05</sub>< 0.7, the total storage required can be estimated by adding Y<sub>0.05</sub>:
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<strong>Total storage = V<sub>S</sub> + Y<sub>0.05</sub></strong>
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===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.]]
The Sustainable Technologies Evaluation Program have produced a rainwater harvesting design and costing tool specific to Ontario. The tool is in a simple to use Excel format and is free to download.<br>
'''[http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/rainwater-harvesting-design-and-costing-tool/| Rainwater Harvesting Tool]'''
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===STEP Treatment Train Tool===
Once the size of cistern has been determined, it can easily be modeled in many open source and proprietary applications. For planning purposes, a RWH system could be integrated into a site plan using STEP's Treatment Train Tool.
In a typical configuration:
*The catchment (roof) would be 100% impervious
*The rainwater harvesting system would be a 'Storage' Element with the following properties:
**Storage type = No removal
**Lined
**Underlying soil = <em>doesn't matter, can ignore</em>
**Evaporation factor = 0
**Suction head (mm) = 0
**Saturated conductivity (mm/hr) = 0
**Initial soil moisture deficit (fraction) = 0
*The dimensions of the rainwater cistern can be placed into the fields:
#Bottom elevation (m)
#Maximum depth (m)
#Initial water depth (m)
#The Curves table is designed to accommodate ponds of roughly conical dimensions. A rainwater cistern is usually cuboid or cylindrical in shape, so that the area (m<sup>2</sup>) will remain the same throughout the depth. 
<strong>[http://www.sustainabletechnologies.ca/wp/low-impact-development-treatment-train-tool/|The Treatment Train Tool]</strong>
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[[category:modeling]]