# Rainwater harvesting: Sizing and modeling

## Contents

Schematic diagram of the inputs and outputs to a rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. cisternTank used to store rainwater (typically roof runoff) for later use.

### Simple

The following approximations Five percent of the average annual yield can be estimated$Y_{0.05} = A_{c} \times C_{vol,A}\times R_{a} \times e \times 0.05$

Where:

• Y0.05 is five percent of the average annual yield (L)
• Ac is the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. area (m2)
• Cvol, A is the annual runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. coefficient for the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.
• Ra is the average annual rainfall depth (mm)
• e is the efficiency of the pre-storage filter
• Filter efficiency (e) can be reasonably estimated as 0.9 pending manufacturer’s information.
• In a study of three sites in Ontario, STEPSustainable Technologies Evaluation Program found the annual Cvol, A of the rooftops to be around 0.8 [1]. This figure includes losses to evaporationAbiotic transfer of water vapour to the atmosphere., snow being blown off the roof, and a number of overflow events.

Five percent of the average annual demand can be estimated$D_{0.05} = P_{d} \times n\times 18.25$

Where:

• D0.05 is five percent of the average annual demand (L)
• Pd is the daily demand per person (L)
• n is the number of occupants

Then the following calculations are based upon two criteria:

1. A design rainfall depth is to be captured entirely by the RWHRainwater harvesting. system.
2. The average annual demand (D) is greater than the average annual yield (Y) from the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale..

When $$Y_{0.05}/D_{0.05}<0.33$$, the storage volume required can be estimated$V_{S} = A_{c} \times C_{vol,E}\times R_{d} \times e$

Where:

• VS is the volume of storage required (L)
• Ac is the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. area (m2)
• Cvol,E is the design storm runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. coefficient for the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.
• Rd is the design storm rainfall depth (mm), and
• e is the efficiency of the pre-storage filter.
• Careful catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. selection means that the runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. coefficient, for an individual rainstorm event (Cvol, 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 Y0.05$TotalStorage = V_{S} + Y_{0.05}$

## STEPSustainable Technologies Evaluation Program Rainwater Harvesting Tool

Quick reference table generated using STEPSustainable Technologies Evaluation Program RWHRainwater harvesting. tool, (data for the City of Toronto (median annual rainfall 678 mm). Optimal cisternTank used to store rainwater (typically roof runoff) for later use. 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 harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. design and costing tool specific to Ontario. The tool is in a simple to use Excel format and is free to download.

## CisternTank used to store rainwater (typically roof runoff) for later use. dimensions

The blue area indicates the only usable volume in this rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. cisternTank used to store rainwater (typically roof runoff) for later use., the depth between the pump inlet and the overflow.

The connections into and out of a rainwater cisternTank used to store rainwater (typically roof runoff) for later use. can have a dramatic effect on the actual usable volume. The only usable water within the cisternTank used to store rainwater (typically roof runoff) for later use. is that above the height of the pump intake, and below the invert of the overflow outlet. This dimension can easily become constrained where the outlet must lie beneath the frost line and where a high powered pump is required to elevate the water many storeys.

• The depth of storage between the elevation of the inlet and overflow is unusable space, so the overflow should be located towards the top of the cisternTank used to store rainwater (typically roof runoff) for later use..
• The depth of storage beneath the pump inlet is unusable, but may also be a useful zone for sedimentSoil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls. to settle. A custom or cast-in-place vault could minimise this unused volume by tapering towards the the base.