Quick Answer
Arizona averages around 12-15 inches of annual rainfall, resulting in varying cistern needs based on location and intended use, but generally requiring 3,000-5,000 gallons for a small household with minimal irrigation needs.
Cistern Sizing Considerations
When determining cistern needs in Arizona, it’s essential to consider the location’s average annual rainfall. For example, the Tucson area receives around 12 inches, while Flagstaff receives about 20 inches. To account for these variations, you can use a general rule of thumb: for every inch of rainfall, you can collect approximately 900-1,000 gallons per 1,000 square feet of roof area.
Effective Roof Collection
To maximize roof collection, consider the following:
- Use a minimum of 2,000 square feet of roof area for a small household.
- Ensure the cistern is sized to collect at least 50% of the roof’s annual rainfall.
- Account for 1-2 inches of rainfall per year for irrigation needs.
- Factor in potential evaporation and wind loss.
For instance, if you have a 2,000 square foot roof in Tucson, you can collect around 12 inches * 0.9 gallons/ft^2/inch * 2,000 ft^2 = 22,800 gallons per year. A 3,000 gallon cistern would provide about 13% of the roof’s annual collection, while a 5,000 gallon cistern would provide around 22%.
Additional Factors
When sizing your cistern, remember that local regulations, water quality, and intended use may impact your needs. Consider the following:
- Storage requirements for water treatment systems.
- Additional storage for greywater or rainwater reuse systems.
- Local building codes and regulations regarding cistern size and placement.
- Water quality and treatment requirements to ensure safe drinking water.
Keep in mind that these are general guidelines, and a more detailed site-specific analysis may be necessary to accurately determine your cistern needs.
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