Rainwater Harvesting System Design and Cistern Sizing

Collection Area and Annual Yield

The fundamental collection equation:

Volume (gallons) = Rainfall (inches) × Collection Area (ft²) × 0.623 × Efficiency

Where 0.623 converts inch-ft² to gallons (1 inch of rain on 1 ft² = 0.623 gallons).

Collection efficiency accounts for losses from evaporation, splash, gutter overflow, first-flush diversion, and surface absorption. Typical values:

• Metal roofing: 0.90–0.95
• Asphalt shingles: 0.80–0.85
• Concrete tile: 0.75–0.85
• Clay tile: 0.75–0.80
• Green (vegetated) roof: 0.20–0.40

Example: A 2,000 ft² metal roof in an area receiving 40 inches of annual rainfall:

Annual Yield = 40 × 2,000 × 0.623 × 0.90 = 44,856 gallons

That is roughly 3,738 gallons per month on average -- but rainfall is not uniform. The monthly distribution matters enormously for cistern sizing. A region with 40 inches distributed evenly (3.3"/month) requires a much smaller cistern than one with 40 inches concentrated in 6 wet months and 6 dry months.

Cistern Sizing: Supply-Demand Balance

Cistern sizing is a mass balance problem. The cistern must store enough water during wet months to bridge the deficit during dry months. The proper method is a month-by-month simulation:

For each month, calculate:

Supply = Monthly Rainfall × Collection Area × 0.623 × Efficiency
Demand = Monthly water usage from the harvested supply
Net = Supply − Demand
Running Storage = Previous month's storage + Net (clamped at 0 and cistern capacity)

The cistern must be large enough that running storage never reaches zero during the dry season. Iterate the cistern size until the simulation shows no shortfall, or determine the acceptable shortfall probability for supplemental supply.

Quick-estimate methods (for preliminary sizing):

• Longest dry spell method: Cistern = Daily demand × days in longest expected dry spell. Conservative but simple.
• Seasonal storage method: Cistern = total demand during the dry season minus total supply during the dry season. Assumes the cistern starts full at the end of the wet season.

Practical considerations: cisterns are available in standard sizes (500, 1,000, 1,500, 2,500, 5,000, 10,000 gallons for above-ground; custom sizes for below-ground concrete or fiberglass). Round up to the next available standard size. Underground cisterns offer freeze protection and space savings but cost significantly more to install.

First Flush Diversion and Water Quality

The first rain after a dry spell washes accumulated dirt, bird droppings, pollen, and debris off the collection surface. This first flush is the most contaminated water in the collection cycle and should be diverted away from the cistern.

The standard first-flush diversion volume is 1 gallon per 100 ft² of collection area (about 0.01 inches of rainfall equivalent). Some sources recommend up to 2 gallons per 100 ft² in areas with heavy tree cover or bird activity. A 2,000 ft² roof needs a 20–40 gallon first-flush diverter.

First-flush diverters work by filling a standpipe or chamber with the initial rainfall. Once the chamber is full, subsequent rainfall overflows into the cistern. The chamber drains slowly (over 12–24 hours) through a small orifice so it is ready for the next rain event.

Beyond first-flush diversion, water quality treatment depends on the intended use:

• Irrigation only: Screening (leaf filter + mosquito screen) and first-flush diversion are typically sufficient
• Toilet flushing, laundry: Add sediment filtration (5–20 micron) and UV disinfection or chlorination
• Potable use: Multi-barrier treatment: sediment filtration, activated carbon, UV disinfection, and chlorine residual. Must comply with state/local health department requirements -- many jurisdictions require a licensed engineer's design.

Pump Selection and Distribution

If the cistern is above the point of use (gravity-fed), no pump is needed -- 1 foot of head provides 0.433 psi. Most cistern installations require a pump to pressurize the distribution system.

Pump sizing for rainwater systems:

• Irrigation only: A 1/2 HP shallow-well jet pump or submersible pump typically provides 5–10 GPM at 30–40 psi. Adequate for most residential irrigation.
• Indoor non-potable (toilet flushing): A variable-speed booster pump with a pressure tank maintains consistent 40–60 psi. Size for the peak simultaneous demand of all connected fixtures.
• Whole-building: Booster pump system sized per plumbing fixture unit method, same as a well system.

A pressure tank (diaphragm type, 20–80 gallon capacity) is essential for indoor systems. It prevents the pump from cycling on and off with every toilet flush and provides consistent pressure. The tank pre-charge should be set 2 psi below the pump cut-in pressure.

For freeze-prone climates, protect the pump, pressure tank, and piping from freezing. Below-ground cisterns are naturally protected, but the pump house and exposed piping need insulation and heat trace. Many installations use a submersible pump inside the cistern to eliminate the pump house entirely.