Supplying Rainwater Harvesting Systems with Clean Storage
The size of a rainwater tank depends on the average annual rainfall in your area, the rooftop collection area and the capacity needs of your household. It’s also important to research the local authority regulations on water tanks, including permitting requirements.
There are resources such as the Current Results website that break down rainfall averages by State, and then by major city:
US Climate Data by State:
Local rainfall data is also sometimes provided by the County or public education colleges in the area if you require more detailed, accurate data. It’s safe to say that rainwater systems should prepare for an excess and a recess of rainfall in all cases, with enough capacity to hold rain in changing the weather.
The basic equation of calculating how much rainwater you can collect off of your roof is the catchment area (ft 2 ) multiplied by the rainfall depth (inches of rain) multiplied by a 0.623 conversion factor.
There are many factors that affect this, including what materials the roof is made of, the slope of the roof, and rate of the rainfall. The Texas AgriLife Extension Office has created an extensive spreadsheet calculation that breaks down rainwater collection by the month.
The capacity needs of your household or business depend on a lot of factors, including what this will be a water source. If the drinking water system is also being replaced by rainwater, the capacity needs should provide for the longest anticipated time without rain with a backup plan for if it ever goes dry. The average water-conserving household will use between 20 and 25 gallons per person, per day – but this estimate goes as high as 80 to 100 gallons per person, per day.
The basic rule for sizing a rainwater system capacity is finding the volume of water that equals or exceeds domestic end uses or the water demand. If the household is using a utility water system, water usage is easier to gauge based on past billing. Irrigation typically takes 40% of water, but it depends on your specific location and general needs. The Texas Water Development Board gauges that irrigation can take up to 60% of water usage during peak summer months.
One rough estimation method for storage capacity is to size the tank to meet the quarterly demand or a three-month period. If you can take three of the highest water usage months’ needs, that can be a rough estimate for a residential system. Commercial systems should be more detailed to provide for the sustainable goals set forth for the system.
Another capacity estimation method is to look at the rainwater system like a monthly checkbook balance. The volume captured every month is added to the previous water capacity “balance” and the water usage is subtracted. This system would require an initial volume of water in the tanks before the system is put into place since it relies on collecting rainwater to replace the water used.