On-Site Alternate Water Sources & Use Introduction

There are many water sources available on building sites that can supplement the more traditional water sources.  These water sources are non-potable, and vary greatly in quality.  Most of the on-site water resources have limited application, and are never suitable for human consumption.  The following is a brief overview of such resources that may be available in both residential and non-residential settings.

Rain Water Harvesting

Probably the original alternate water source, rainwater harvesting dates back to pre-historic times in rudimentary forms.  Some plants and animals use the practice to enhance survival.  Desert plants, such as agave, form concave appendages to capture rain drops and direct the water towards the root zone.  Porcine root out pits capturing rain water to form small ponds.  By 300 B.C., the Nabataeans had already built miles of an elaborate rain water collection system and storage cisterns to supply water to their city of Petra, in what is now the country of Jordan. 


In whatever size and form, rainwater harvesting usually is one of the purest waters available of all the alternate water sources.  Despite its imitial purity, rain water can be easily contaminated by: air-pollution; heavy metals and other toxins in roofing materials, paints, solvents, lead soldering, zinc coatings and adhesives; bird and rodent feces; and mosquitoes.  It is never advised to use untreated rainwater for direct human consumption, though human have been consuming it for centuries with little documented ill-effects when precations are taken to prevent contamination.  It is often advised for people to avoid prolonged skin contact, and be wary of the stored water becoming septic or a breeding place for mosquitoes and pathogens.  It has also been observed that residents of home cisterns tend to be more tolerant of their own cistern water quality than visitors that imbibe in the cistern water..

Rain Barrels

There is a growing trend of water utilities offering free or price-discounted pre-fabricated plastic rain barrels to residential customers.  These barrels are connected to the downspouts of the home’s roof gutters to collect the rain water from the roof.  The rain barrels include a tap at the bottom of the barrel for users to attach garden hoses as a means to convey the water to landscape plants.  (Rain water is superior to treated potable water for irrigating plants.)    The barrels also have an overflow spout and hose to divert excess water away from the home’s foundation.

A typical 1,000 sq, ft. (92.9 square meters)roof can collect 624 gallons (2.4 m3) during a 1 inch (2.54 cm) rainfall, but it is seldom practical or possible to harvest and store this volume of water with rain barrels.  Roofs very seldom direct all of the water to one downspout.  Most often there are four or more downspouts, one at each corner of the house.  Capturing all of the rainwater requires the resident to attach rain barrels at every downspout.   Each rain barrel will store only 50 to 60 gallons (189.2 L to 227.1 L) of water.   Capturing the full 624 gallons (2.4 m3) requires more than 12 rain barrels.  At a cost of $80 to $120 per purchased rain barrel, a system with a 624 gallon (2.4 m3) capacity will cost more than $1,200.  (A do-it-yourself project will cost about $20/barrel in hardware, assuming a used plastic barrel is available free of charge)

The economic benefit-cost ratio of rain barrel systems is not favorable when compared to most potable water prices.  Even when a ten-year lifecycle is used for the barrels in a rainy US climate, the water collected over the ten years would cost approximately $4.55 per CCF, and this assumes the water is put to beneficial use and replacing potable water use.  The problem is the barrels are filled when it rains; a time the landscape probably does not need additional water.  Once the barrels are full, they cannot collect additional water until emptied.  When there is little beneficial use for the water during the rainy season; thus, the rainwater is of little economic value.  During parts of the country with dry summers, the water is valuable, but rainfall is too infrequent to regularly refill the barrels as needed.

In most climates, rain barrel systems should be valued for the rainwater’s superior quality (no chlorine, chloramines and other treatment chemicals) as irrigation water.   Some caution should be exercised for using the collected rainwater to irrigate crops intended for human consumption.  The water might absorb heavy metals from the roofing materials and gutters; and the water might contain E. coli, absorbed from bird and animal droppings from the roof surface. 

Rain barrel systems can be a great way to harvest rain water to supplement irrigation needs.  To properly evaluate its application and benefits, one must consider the frequency of local rain events that occur during the irrigation season.  Also consider the need to drain and bypass the rain barrels during monsoon seasons of freezing weather.   While it is true that the rainwater can be used to flush toilets, the added cost of equipment to convey the water to the toilets might hamper overall cost effectiveness; one barrel of water is unlikely to supply more than 3 days worth of flushing water for the typical home.  In general, larger rainwater storage systems will often be more cost effective than rain barrels if the water is to be used beyong local landscape irrigation.   


A cistern is defined as any tank used to store water.  Most often the word “cistern” refers to an underground tank storing rainwater collected from roofs.   Cisterns were once very common in the US, especially in rural areas where homes relied on private wells for water.   The cisterns were used to supplement the well water, especially during extended droughts when groundwater levels receded below well depth.  Except in emergencies, the cistern water was not used for drinking, food preperation or bathing.  Harvested rainwater is considered potable only after proper treatment and disinfection.

Cisterns fell out of favor in the US as water utilities provided treated potable water at extremely low cost.  In rural areas, the advances in well drilling technology allows for deeper wells, where groundwater levels seldom fall below well depth.  Many older cisterns were once considered a health and safety hazard, and were backfilled or removed.    In the past few years, as water becomes scarce and more precious, cisterns have re-emerged with new technological advances and safety.   Harvesting rainwater is no longer limited to rural areas or water conservation practitioners.

The green building industry is slowly but steadily advancing water conservation and rain water collection in its strategies; and cisterns provide a means to store the rainwater for later use.  Even high-rise office buildings are now collecting rain water in cisterns.   Reducing storm water flows is often a main impetus of rainwater roof collection and cisterns in urban settings.  In some designs, the cisterns hold additional alternate water resources (gray water, condensate, cooling tower blow-down, etc.) along with the rain water for uses beyond irrigation of landscape.  The various alternate water sources are collected in the cistern, and often filtered and sanitized for use in flushing urinals and toilets throughout the building.

Cisterns come in all shapes and sizes.  Historically, cisterns were made underground pits lined with stone, mortar, plaster or cement; topped with a reinforced concrete roof, at grade level.  Today, cisterns are often prefabricated from cement, fiberglass, or plastic; usually placed underground to hide from view and prevent freezing in colder climates.  There are some above grade installations made of steel or fiberglass tanks.  New inventive designs are using corrugated steel bins (originally fabricated for grain storage) with a plastic liner to create a water vessel.   The typical cistern storage capacity ranges from 500 to 10,000 gallons per cistern (1.9 m3 to 37.8 m3) ; although multiple cisterns are often interconnected in large building sites.

Additional information:  American Rainwater Catchment Association (ARCSA) 


Stormwater Harvesting

Generally, the term stormwater harvesting refers to rainwater collected from non-roof surfaces, such as parking lots, hardscapes, and landscapes surrounding urban buildings.  Managing stormwater in urban environments has reached near crises levels in some US cities as urban sprawl grows and landscapes are paved over with non-porous materials (concrete and asphalt).  In typical US cities, where 50% of the landscape is paved over, a 1 inch rain event produces more than 150 million gallons (567,811.8 m3) of stormwater per square mile.  Strategies to capture and utilize this water include: landscaping designs to retain water in soil; semi-porous hardscape material; diverting the water to holding tanks or retention ponds for irrigation use; and diverting stormwater to groundwater recharge sites. 

Government agencies encourage or require implementing many of the stormwater reuse strategies.  In Florida and California stormwater is being diverted to retention basins at groundwater recharge sites.  Tucson Arizona requests residents to build berms and swales in their landscape to retain the rainwater.   Texas provides sales tax exemptions for rainwater capture and reuse systems.  The Texas Cities of Austin and San Antonio offer rebates for rainwater capture and reuse.1 

It is important to be cautious when using stormwater as an alternative water supply.  This water may collect many pollutants as it travels across landscapes and hardscapes.  Landscapes often contain fertilizers, herbicides, pesticides, fungicides and animal feces.  Parking lot and roadway surfaces are coated in motor oil, antifreeze, exhaust soot, and other automobile fluids.  Sidewalks surfaces often contain food waste, especially near fast food establishments.  Some water agencies have determined landscape water run-off to be a major cause of non-source point pollution in groundwater supplies.   The quality of stormwater varies greatly from different sites; the water quality determines the appropriate uses, or if water treatment is needed before the water is utilized.

1 H.W. (Bill) Hoffman, Using Alternate Onsite Sources, AWWA Journal. 2008


Additional Resource Links:

U.S. Department of Energy Alternative Water Sources Map - Rain Water Harvesting Regulations by State 

Blow-down Water Introduction

Center for Neighborhood Technology and American Rivers (2010) The Value of Green Infrastructure: A Guide to Recognizing Its Economic, Environmental and Social Benefits 

Condensate Water Introduction 

Graywater Introduction 

University of Florida IFAS Extension (2007) A Review of Applicable Policies and Permitting Requirements for Non-Potable Use of Cisterns 

Texas Rainwater Harvest Guide 

Maroochy Water Services Rainwater Tank Rebate Scheme (March 2004)