Factors Influencing Cistern Sizing: What Should You Keep in Mind?

Factors to Consider for Rainfall Data

When sizing a cistern, it’s essential to determine the average annual rainfall for your location. A rainfall data collector or a rainfall calculator can help you estimate the average annual rainfall in inches or millimeters. Use this data to calculate the roof’s catchment area and determine the cistern size based on the desired storage capacity. For example, if you live in an area with an average annual rainfall of 40 inches and want to store at least 2 months’ worth of water, you’ll need to calculate the cistern size accordingly.

Cistern Sizing Formulas and Techniques

To size a cistern, you can use the following formula: Cistern size (gallons) = Roof catchment area (square feet) x Average annual rainfall (inches) x 0.625 (conversion factor). Alternatively, you can use the “2/3 rule,” which estimates the cistern size based on the roof’s square footage and a factor that accounts for rainfall variability. For example, if the roof is 1,000 square feet, you can use the 2/3 rule to estimate the cistern size: Cistern size (gallons) = 1,000 square feet x 0.67 (2/3 factor) = 670 gallons.

Roof Material and Collection Efficiency

When sizing a cistern, it’s also essential to consider the roof material and collection efficiency. Different roof materials have varying surface roughness and porosity, affecting the collection efficiency. For example, a roof with a smooth surface may have a lower collection efficiency compared to a roof with a textured surface. To account for this, you can use a collection efficiency factor, which ranges from 0.7 to 1.0, depending on the roof material and surface characteristics.