By Demi Fortuna, Atlantic Water Gardens

rainwater harvesing salt vs fresh water

From space ‘the earth’ is a giant blue marble, with water so prevalent it literally colors our very perception of the globe. In the face of such abundance, it is hard to imagine how scarce a resource fresh water is. 97.3% of the water on the planet is salty, useless to terrestrial plants and animals. Of the 2.7% left, more than 2% is frozen. Paradoxically, on a world three-quarters covered by water, less than three-quarters of one percent is available for the needs of every land organism on the planet.

We have always known rain as a most precious gift from the heavens. The earliest evidence we have of rainwater collection waterworks dates back 5,000 years, at Jawa Dam in Jordan. The fabled Biblical cisterns of King Herod at Masada are still in perfect condition. From the roof-tops of Pompei to the Stepped Wells of India, rainwater harvesting has always been a crucial technology, even now.

Rainwater Harvesting has roots in America extending back to the founding fathers. In 1791 Thomas Jefferson installed 4 interconnected masonry tanks, 15,000 gallons in all, for making ale (contact me for the recipe!). Every town square had a firefighting reservoir, and even a cistern, right through the beginning of the 20th Century. Rainwater Harvesting isn’t new, but new technologies are bringing it once more to the fore.

Rainwater Harvesting equipment has come a long way in the last 100 years. Although the basic ideas haven’t changed, the way that rainwater is filtered and stored has entered the twenty first century.

Water Storage

In place of the stone, brick and concrete cisterns of the past, plastic storage containers in a range of sizes from rain barrels to 10,000 gallon tanks offer strong and convenient storage above and below ground. Even more versatile are lined and buried modular reservoirs filled with water matrices, because the space above them is available for landscaping, hardscapes or even light vehicle traffic. These are no mere ‘milk crates’ – for example, Eco-Blox matrices by Atlantic Water Gardens support over 14,000 pounds – each! The complete modularity they provide means that buried storage can be custom built to any size or shape, one piece at a time, perfect for restricted access areas and irregular spaces. Some of the latest devices might best be described as "rain pillows", large watertight bags that slowly empty to drip-irrigate gardens.


Although rainwater is almost by definition “clean” water, the process of collecting it usually introduces pollutants. Water coming off a roof may wash pollen, dust, leaves, droppings, even roofing debris into the stored water. Sediment traps work by collecting the water in a series of wells, with water filling then overflowing into the next, leaving heavy sediments behind while allowing relatively cleaner water to overflow into storage, while a screen excludes lighter debris. Sediment traps have been in existence for millennia. More sophisticated “First Flush” devices work by diverting the initial rainfall to waste, then after the roof is washed clean, a valve shunts the now-clean runoff into storage. The two types differ in two important ways: sediment traps are typically installed at soil level and can gravity feed only buried reservoirs, and need to be cleaned out periodically. First flush diverters are usually mounted on downspouts as high as desired, so they can gravity feed both above-ground tanks and barrels as well as below-ground storage systems, and they are typically self-cleaning.

Potential for Collection

Regardless of the hardware chosen, two questions always arise – how much water can actually be collected and stored, and why would anyone bother with water costs typically very low?

With 27,000 gallons per acre in every inch of rainfall, even at the household level, every 1,000 square feet of roof area delivers 600 gallons per inch of rain. The harvested rainwater goes farthest with drip irrigation of low demand native landscapes and xeriscapes, but there’s an essential disconnect between what is needed for irrigation of high water demand landscapes and what can economically be stored.

A 2,000 square foot house with a quarter-acre of bluegrass lawn can conceivably collect 1,200 gallons per inch of rainfall, while requiring about one inch a week, well over 6,000 gallons of irrigation. It would take five inches of rain to fill a 6,000 gallon reservoir with enough for a week’s worth of watering, and that reservoir would have to measure about 800 cubic feet, say 10'x 20'x 4'. Obviously that’s a lot of costly storage for a very thirsty lawn, and with water so inexpensive, the return on investment just isn’t there… UNLESS legislation is factored in.

Current Trends in Legistlation

The actual cost of water is still relatively low in most areas (though that may not be true for much longer), but there is another factor that has begun to make Rainwater Harvesting a sound investment with a tangible return on investment. In many areas around the country the need for new and updated infrastructure has led to the adoption of stormwater mitigation fees and taxes. Those who install rainwater harvesting systems, and their close companions rainwater retention systems that percolate harvested water back into the water table, may be eligible for rebate or even complete exemption from annual fees, which can mean substantial savings. There are no hard and fast guidelines as to which municipalities might offer programs or rebates, so the installer or property owner must investigate whether such opportunities exist in their local area, but there is no dispute that these taxes, and rebates, are increasing in amount, frequency and distribution.

Some Examples

ARIZONA – One-time 25% Rainwater Harvesting credit (max $1000 residents, $200 builders)

Tucson, AZ – First to require Rainwater Harvesting to supply 50% of commercial irrigation

COLORADO – changed “first in time, first in right” doctrine to allow rural harvesting

TEXAS – allows State taxing orgs to exempt all or part of properties with Rainwater Harvesting

Austin, TX – Subsidizes rain barrels and cisterns, up to $40K

San Antonio, TX – 50% rebates to commercial, industrial, institutional installations

Albuquerque, Bernalillo and Santa Fe Co, NM – Residences with 2,500 sq ft or more area must install an active rainwater catchment system comprised of cisterns. All commercial developments are required to collect all roof drainage into cisterns to be reused for landscape irrigation

CALIFORNIA – several water districts offer rebates, San Francisco discounts Rainwater Harvesting cisterns

OREGON – State Code Appendix M sets potable/non-potable guidelines

ILLINOIS – SB 2549 still pending, allows for Rainwater Harvesting in plumbing codes

FL, HI, NC, OH, UT, VA, WA – all adopting either Rainwater Harvesting or Stormwater Management regulations

Two great ideas that work great together

Rainwater Harvesting and Pond-free Water Features go hand in hand, sharing 90% of their components and construction procedures, and perfectly complementing each other. Each system requires a pump and the construction of a lined reservoir, and each adds beneficial qualities to the other. The action of a small bubbler, or waterfall, oxygenates and keeps stored rainwater fresh, while the reservoir of a pond-free system needs only a First Flush Diverter to become an ideal, attractive rainwater storage tank. With most of the work done either way, the addition of a Rainwater Harvesting system to a Pond-free one or vice versa offers the potential of doubling profits with little additional expense to the installer.

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