Rainwater Harvesting

Rainwater harvesting in the United States is of sufficient importance to inspire a publication: "Raindrop--Rainwater Harvesting Bulletin."  In a recent issue Dennis Lye, a microbiologist with the U.S. Environmental Protection Agency, reported that many individuals, and even communities regularly use stored rainwater to meet or supplement clean water needs.  He stated that about 200,000 cisterns are in use in the U.S. collecting and storing rainwater.

Water is collected or harvested from concrete patios, driveways and other paved areas.  Also harvested is the flow from the roof and from catchments such as gutters.  Houses can be designed to maximize the amount of catchment area, thereby increasing rainwater harvesting possibilities.

Rainfall harvesting has limitations not shared by graywater use.   Rainwater harvesting lacks the dependability, relying instead on the variability and vicissitudes of climate for its water source.

Also, graywater is more readily stored than rainwater, with less cost.  Since it is a dependable source, its storage system does not have to be very large.  A typical residence could get by with about a 1200-gallon tank, the size of the standard concrete septic tank.  Being infrequent, rainfall requires a larger storage capacity so supplies can carry over between desert rainfall events.

Collected and stored rainwater can be used in evaporative coolers, toilet flushing, car washing, chlorinated swimming pools, and surface irrigation, especially in food gardens.  In the United States harvested rainfall mostly is used for irrigation, with limited other domestic uses.

Rainwater harvesting systems vary from the simple and inexpensive to the complex and very costly. Directing rainfall to plants located at contoured low points is a very simple rainwater harvesting system.  Falling rain then flows to areas with vegetation.  No rain escapes property boundaries. 

More complex rainwater harvesting systems include storage of rainfall.  Tanks are an expensive item, with th e excavation cost to install a tank sometime equal to the cost of the tank.  Rainfall is stored in steel or concrete tanks. About one dollar per gallon is the average cost for most underground storage units.

The least expensive rainwater storage system makes use of an above-ground swimming pool, with a lid or cover to reduce evaporation.  Rainwater then can be stored for about .07 cents a gallon.  The financial advantages however may not outweigh the inconvenience of living with this unsightly above-ground unit taking up a large area of the backyard.

Rainwater harvesting is an everyman's water augmentation method. Any container capable of holding rain dripping from roof or patio can be a rainwater harvesting system. A plastic garbage barrel is sufficient. Sturdier and more elegant containers create a more pleasing effect and some are built using backyard technology. For example, small ferro cement tanks can be constructed to collect water at the base of down spouts.

Q.  What about the quality of rainwater?

A.  The image of falling rain may be pure and refreshing but harvested rain is not without water quality concerns. Rain in certain urban areas may contain various impurities absorbed from the atmosphere, including arsenic and lead.

Certain desert conditions also can cause rainwater quality concerns. Desert rain is infrequent and, therefore, bird droppings, dust and other impurities accumulate between rain events.  They then occur in high concentrations in runoff when it does rain.  As a result, the quality of harvested rainfall needs frequent monitoring if it is used for potable uses.

Q. Can I purify rainwater?

A.  Various methods are used to purify rainwater.  First-flush devices ensure a certain degree of water quality in harvested rainwater.  The first five gallons of runoff from a gutter, roof or other surface is likely to contain various impurities such as bird droppings and dust.  A first-flush device prevents this initial flow from draining into the storage tank.

Many first-flush devices are simply and cleverly designed.  Such devices include tipping buckets that dump when water reaches a certain level.  Also there are containers with a ball that floats with the rising water to close off an opening after an inflow of five gallons.  Water then is diverted to another pipe leading to the cistern.  This use of simple technology is an attractive feature of rainwater harvesting.  (Water harvesting systems are not readily available on the market.)

Updated February 2008. We will be adding additional FAQs soon!