Annual rainfall averages — how do they impact water collection systems?

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Determining Collection Area When designing a rainwater collection system, the first step is to calculate the collection area, which is typically the roof area that will collect rainfall. A general rule of thumb is to use a roof area of at least 1,000 square feet (93 square meters) to collect 1,000 gallons (3,785 liters) of water. The actual collection area will depend on the roof material and slope, as well as the desired collection volume. For example, a roof with a 1,000 square foot (93 square meter) area and a 50% collection efficiency (due to gutter and downspout losses) will collect approximately 500 gallons (1,893 liters) of water per inch (25.4 millimeters) of rainfall.
Calculating Storage Tank Size Once the collection area is determined, the next step is to calculate the storage tank size. The storage tank should be sized to hold at least 1,000 gallons (3,785 liters) of water to provide a buffer against droughts and other dry periods. A general rule of thumb is to use a storage tank size of 1.5 to 2 times the collection area in square feet. For example, if the collection area is 1,000 square feet (93 square meters), the storage tank should be sized to hold 1,500 to 2,000 gallons (5,678 to 7,571 liters) of water.
Household Use and Storage Requirements The final step in designing a rainwater collection system is to determine the household use and storage requirements. This will depend on the number of people in the household, the type of appliances used, and the desired level of water conservation. A general rule of thumb is to use 50 gallons (189 liters) of water per person per day for household use. For example, a household of four people would require a minimum of 200 gallons (757 liters) of water per day. The storage tank should be sized to meet this demand, as well as account for losses due to evaporation, leakage, and other factors.