Average Rainfall in the Pacific Northwest: How Does It Affect Cistern Size?

Calculating Cistern Size

To determine the ideal cistern size for your Pacific Northwest home, you’ll need to calculate the rainfall collection potential from your roof. A common rule of thumb is to use 0.7 to 0.8 inches of roof area for every gallon of cistern capacity. For example, a 2,000 square foot roof with a 0.8 inch rainfall per hour would yield around 1,600 gallons of water per hour. With an average rainfall of 35 inches per year, this translates to approximately 56,000 gallons of water per year. A 2,000 gallon cistern can hold about 3.6% of this amount, providing a 5-6 month supply of water in the dry season.

Factors Influencing Cistern Size

In addition to rainfall collection potential, several factors will influence your cistern size. These include household water usage, intended use of the collected water (e.g., irrigation, drinking, washing), and local climate and precipitation patterns. It’s essential to consider these factors when sizing your cistern to ensure it meets your needs. For example, if you plan to use the collected water for irrigation and expect to use up to 5 gallons of water per minute, your cistern capacity should be significantly larger than if you only plan to use it for drinking and cooking.

Designing and Installing a Cistern

When designing and installing a cistern, it’s crucial to consider factors such as site selection, cistern material, and safety features. A well-designed cistern can last for decades with proper maintenance, providing a reliable source of water for your household. When selecting a site for your cistern, ensure it is level, accessible, and protected from potential contaminants such as animal waste and debris. For cistern material, consider durable, non-toxic options such as concrete or stainless steel.