Rainfall Capture
Rainfall capture systems have been used exhaustively in places like the Bahamas where there is no other source of potable water. For climates that have abundant rains, rainfall capture can be an attractive alternative for sustainable home water supply. The key variables governing the design of such a system include the following:
- Home occupancy and anticipated water usage
- Year round rainfall distribution patterns
- Roof area
- Storage tank size
- Availability of a supplemental water source to augment rainfall capture as needed.
Stand-alone rainfall capture systems tend to be very costly because the tank size must be big enough to get through the longest dry period that can be anticipated. In Costa Rica, the dry season historically lasts 4 months. Hence for a family of four with an conservative usage of 50 gallons per person per day, this amounts to 24,000 gallons. Using a construction rule of thumb of $1 per gallon of storage, this is $24,000 in the tank alone. If rainfall distribution is even throughout the year and especially if a supplemental water source can be used periodically as needed, then the tank size can be brought down to a size considerably lower, making the investment and space requirements much more manageable.
Abstracting for a moment the storage tank, the other system components include the following:
- Gutters with trash racks to exclude leaves
- Downspouts that pass through washable pre-filters and are plumbed into the storage tank.
- A pressure pump/tank system to pressurize the water collected in the cistern.
- A home purification system that includes particulate filtration, granular activated carbon adsorption, and ultraviolet disinfection.
Overall system costs vary—as already mentioned—mostly as a function of tank size. Also, the gutter costs and trash racks vary according to the lengths of the roof that is to be used to capture rain. For a water supply for a modest two bedroom home a one half horsepower pump and a 35 gallon pressure tank are adequate for home pressurization ($750 in materials). Osa Water offers the Pura-III 15 gpm home purification system for water quality control at a price of $1150. For additional information on sizing and costs of a rainfall capture system, we require more information on the location, rainfall patterns, roof area, and if a municipal water supply can be accessed to provide supplemental water supply as needed to keep the tank cost down to the minimum possible.
