Cistern Sizing Considerations for Large Family Gatherings: What’s Needed?

Calculating Total Water Needs

When sizing a cistern for large family gatherings, it’s essential to calculate the total water needs based on the number of attendees, duration of the event, and desired water storage for potential rain events. A good starting point is to assume 10 gallons of water per person per day for drinking, cooking, and hygiene. For example, a family gathering with 20 attendees for a 3-day event would require 20 x 10 x 3 = 600 gallons of water. This calculation only accounts for potable water needs and does not include additional storage for irrigation or other non-potable uses.

Considering Rainwater Harvesting

To determine the required cistern size, consider the potential for rainwater harvesting during the event. A general rule of thumb is to store at least 1-2 inches of rainfall on the roof area per day. Assuming a roof area of 2,000 square feet, a 1-inch rainfall event would yield approximately 14,000 gallons of rainwater. This can be stored in the cistern to supplement potable water needs. To account for potential evaporation and water loss, it’s essential to add 10-20% to the calculated cistern size.

Cistern Sizing Formulas and Techniques

For a more accurate cistern sizing calculation, use the following formula: Cistern size (gallons) = (Total water needs / (Rainwater yield per day x Harvest duration)) + (10-20% for evaporation and loss). For example, if the total water needs are 600 gallons and the rainwater yield per day is 14,000 gallons, the cistern size would be (600 / (14,000 x 3)) + (10-20%) = 13.57 gallons + (10-20%) = 14.7 gallons to 16.4 gallons.