Rainwater Harvesting

General Questions from
American Rainwater Catchment Systems Association (ARCSA)

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metal-tankWhy is rainwater harvesting important?

One of the principles of rainwater management is to slow the water by collecting and using it onsite and slowly infiltrating any excess overflow. Collected rainwater can be an important source of high-quality water for crops, greenhouses, livestock and humans even in very arid climates.

What is the history of rainwater harvesting?

Rainwater harvesting has been used for thousands of years. The Old Testament mentions cisterns at least ten times, and rainwater harvesting systems in the Middle East date to 5,000 years ago. In the U.S., water shortages, quality concerns and storm-water mitigation have renewed interest in this ancient practice.

Besides the obvious environmental benefits, what other advantages does rainwater harvesting offer?

The most enlightened rainwater harvesting practices involve a recognition and exploitation of the relationships among the four components of ARCSA’s mission: potable, non-potable, storm-water and energy. For example, sustainable harvesting of rainwater for gravity-fed non-potable drip irrigation diminishes the storm-water impact of rain events, decreases the water-energy footprint and displaces the need for an equal amount of highly treated potable utility water.

What are some other uses for harvested rainwater other than irrigation and drinking water?

Rainwater can be used for any potable or non-potable use.

contain-metal-tankWater from my utility is very cheap. How does a rainwater harvesting business compete with subsidized, inexpensive utility water?

There are many reasons to harvest rainwater: quality of the resource; reduced appliance maintenance due to low mineral content; availability of a secure, onsite water source; and greater nutritional value for crops and gardens. Some successful rainwater harvesting businesses are growing because they make Return on Investment arguments to potential clients. Their rationale is based on the current and growing cost of utility water, the value of a rainwater harvesting system relative to other potential investments, the ability to mitigate costly storm-water effects, the quality of rainwater relative to utility water, the relative security of distributed water systems and — for a select few — the altruistic benefits.

How does the quality of rainwater as a resource compare to ground- and surface-water used by utilities?

Increasingly, consumers are attracted to rainwater for the quality of the resource. While surface and ground waters suffer from contact with the contaminants that are plentiful in those water sources, rainwater has fewer contaminants to deal with, and those can be reduced or eliminated by discarding the first portion of each rain event by using a “first divert” system, and by using the components and materials approved by the National Sanitation Foundation for rainwater collection. As a testament to the quality of rainwater, ARCSA endorsed six rainwater entries in the prestigious Berkeley Springs [West Virginia] International Water Tasting competition held in February, 2011. Rainwater from Texas-based SparkleTap won First Prize in the Purified Water category, and Virginia-based RMS took fifth place.

Shouldn’t we be conserving water by decreasing the demand before we go to the expense of increasing the supply through rainwater harvesting?

Rainwater harvesters are first conservation and efficiency advocates. It makes no sense to spend precious funds to augment water supplies if one has not first extracted the greatest benefit from water efficiency and conservation opportunities.

In a related sustainability field, the same is true of exhausting energy efficiency and conservation opportunities before installing a photovoltaic system. If one was driving down the road and found that one’s parking brake was on, the first solution would be to release the parking brake, not to add more power by buying a bigger engine. Thus if one has not implemented water conservation and efficiency opportunities, it is premature to harvest rainwater.

Over the past few years, have you noticed increased public interest in rainwater harvesting? Do you think a water treatment business would be wise to expand into rainwater harvesting?

A recent survey of the US rainwater harvesting market reveals substantial growth in the face of an otherwise stagnant economy. While it may appear that harvesting rainwater is simple, the best rainwater harvesters are multi-disciplined professionals. We strongly recommend that aspiring rainwater harvesters join ARCSA, avail themselves of one of ARCSA’s periodic rainwater harvesting Accreditation workshops and apply for Accredited Professional designation. The two-day program is designed to take someone who is aware of the fundamentals of rainwater harvesting to the next level through instruction in the proper design, installation and maintenance of a rainwater harvesting system. ARCSA has also created a “graduate” level” certification — the Certified Professional — which is a designation available to an Accredited Professional who fulfills 48 hours of classroom or webinar instruction, attends a two-day Design and Construction workshop and passes a rigorous examination.

Information and Answers from Wikipedia on Rainwater Catchment Systems

What are the major components of a rainwater harvesting system?

The major components of a rainwater harvesting system are the collection surface, gutters, downspouts, pre-filtration systems or first-flush devices, storage tanks and distribution systems — which can include sanitization.

I’m familiar with rain barrels. Can rainwater be harvested on a large scale?

Modern tanks used for residential and commercial applications are available in all sizes and can exceed one million gallons. The Romans built huge cisterns under Istanbul; the largest stored 80,000 cubic meters (21 million gallons).

What special components or treatments are necessary to make rainwater safe to drink?

Rainwater must be treated to be safe to drink. The U.S. Environmental Protection Agency publishes drinking water standards that define the maximum contamination level for potable water in the U.S., and while they apply only to public water supplies serving more than 25 people, it is prudent to apply the same requirement to smaller systems

Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to run off. Rainwater can be collected from rivers or roofs, and in many places the water collected is redirected to a deep pit (well, shaft, or borehole), a reservoir with percolation, or collected from dew or fog with nets or other tools. Its uses include water for gardens, livestock, irrigation, domestic use with proper treatment, and indoor heating for houses etc. The harvested water can also be used as drinking water, longer-term storage and for other purposes such as groundwater recharge.


Rainwater harvesting provides an independent water supply during regional water restrictions and in developed countries is often used to supplement the main supply. It provides water when there is a drought, can help mitigate flooding of low-lying areas, and reduces demand on wells which may enable groundwater levels to be sustained. It also helps in the availability of potable water as rainwater is substantially free of salinity and other salts. Application of rainwater harvesting in urban water system provides a substantial benefit for both water supply and wastewater subsystems by reducing the need for clean water in water distribution system, less generated storm-water in sewer system, as well as a reduction in storm-water runoff polluting freshwater bodies.

There has been a large body of work focused on the development of Life Cycle Assessment and Life Cycle Costing methodologies to assess the level of environmental impacts and money that can be saved by implementing rainwater harvesting systems.

More development and knowledge is required to understand the benefits rainwater harvesting can provide to agriculture. Many countries especially those with an arid environment use rainwater harvesting as a cheap and reliable source of clean water. To enhance irrigation in arid environments, ridges of soil are constructed in order to trap and prevent rainwater from running down hills and slopes. Even in periods of low rainfall, enough water is collected in order for crops to grow. Water can be collected from roofs, dams, and ponds can be constructed in order to hold large quantities of rainwater so that even on days where there is little to no rainfall, there is enough available to irrigate crops.


The concentration of contaminants is reduced significantly by diverting the initial flow of run-off water to waste. Improved water quality can also be obtained by using a floating draw-off mechanism (rather than from the base of the tank) and by using a series of tanks, with draw from the last in series. Pre-filtration is a common practice used in the industry to ensure that the water entering the tank is free of large sediment. Pre-filtration is important to keep the system healthy.

Conceptually, a water supply system should match the quality of water with the end use. However, in most of the developed world high quality potable water is used for all end uses. This approach wastes money and energy and imposes unnecessary impacts to the environment. Supplying rainwater that has gone through preliminary filtration measures for non-potable water uses, such as toilet flushing, irrigation, and laundry, may be a significant part of a sustainable water management strategy.

System setup

Rainwater harvesting systems can range in complexity, from systems that can be installed with minimal skills, to automated systems that require advanced setup and installation. The basic Rainwater harvesting system is more of a plumbing job than a technical job as all the outlets from the building terrace are connected through a pipe to an underground tank that stores water.

Systems are ideally sized to meet the water demand throughout the dry season since it must be big enough to support daily water consumption. Specifically, the rainfall capturing area such as a building roof must be large enough to maintain adequate flow. The water storage tank size should be large enough to contain the captured water.

For low-tech systems, there are many low-tech methods used to capture rainwater: rooftop systems, surface water capture, and pumping the rainwater that has already soaked into the ground or captured in reservoirs and storing it into tanks (cisterns).

New approaches

Instead of using the roof for catchment, the RainSaucer, which looks like an upside down umbrella, collects rain straight from the sky. This decreases the potential for contamination and makes potable water for developing countries a potential application. Other applications of this free standing rainwater collection approach are sustainable gardening and small plot farming.

A Dutch invention called the Groasis Waterboxx is also useful for growing trees with harvested and stored dew and rainwater.

Traditionally, storm water management using detention basins served a single purpose. However, Optimized Real-Time Control (OptiRTC) lets this infrastructure double as a source of rainwater harvesting without compromising the existing detention capacity. This has been used in the EPA headquarters to evacuate stored water prior to storm events, thus reducing wet weather flow while ensuring water availability for later reuse. This has the benefit of increasing water quality released and decreasing the volume of water released during combined sewer overflow events.

Generally, check dams are constructed across the streams to enhance the percolation of surface water in to the sub soil strata. The water percolation in the water impounded area of the check dams, can be enhanced artificially many folds by loosening the sub soil strata / overburden by using ANFO explosives as used in open cast mining. Thus local aquifers can be recharged quickly by using the available surface water fully for using in the dry season.



Around the third century BC, the farming communities in Balochistan (now located in Pakistan, Afghanistan and Iran), and Kutch, India, used rainwater harvesting for agriculture and many uses also. In ancient Tamil Nadu , rainwater harvesting was done by Chola kings.  Rainwater from the Brihadeeswarar temple (located in Balaganpathy Nagar, Thanjavur, India) was collected in Shivaganga tank. During the later Chola period, the Vīrānam tank was built (1011 to 1037 CE) in Cuddalore district of Tamil Nadu to store water for drinking and irrigation purposes. Vīrānam is a 16 km (9.9 mi) long tank with a storage capacity of 1,465,000,000 cubic feet (41,500,000 m3).

Rainwater harvesting was done in the Indian states of Madhya Pradesh, Maharashtra, and Chhattisgarh in the olden days Ratanpur, in the state of Chhattisgarh, had around 150 ponds. Most of the tanks or ponds were utilized in agriculture works.

It is a little know fact that the town of Venice depended for centuries on rainwater harvesting. The lagoon which surrounds Venice is made of brackish water which is not suitable for human drinking. The ancient inhabitants of Venice established a system of rainwater collection which was based on man-made insulated collection wells. Water would percolate down the specially designed stone flooring, and be filtered by a layer of sand, then collecting at the bottom of the well. Later, as Venice acquired territories on the mainland, it started to import water by boat from local rivers, but the wells remained in use, and were especially important in time of war when access to the mainland water could be blocked by an assailant.

In the United States: until 2009 in Colorado, water rights laws almost completely restricted rainwater harvesting; a property owner who captured rainwater was deemed to be stealing it from those who have rights to take water from the watershed. Now, residential well owners that meet certain criteria may obtain a permit to install a rooftop precipitation collection system (SB 09-080). Up to 10 large scale pilot studies may also be permitted (HB 09-1129). The main factor in persuading the Colorado Legislature to change the law was a 2007 study that found that in an average year, 97% of the precipitation that fell in Douglas County, in the southern suburbs of Denver, never reached a stream—it was used by plants or evaporated on the ground. In Colorado you cannot even drill a water well unless you have at least 35 acres (14 ha). In New Mexico, rainwater catchment is mandatory for new dwellings in Santa Fe. Texas offers a sales tax exemption on the purchase of rainwater harvesting equipment. Both Texas and Ohio allow the practice even for potable purposes. Oklahoma passed the Water for 2060 Act in 2012, to promote pilot projects for rainwater and graywater use among other water saving techniques.

Information from Steven Sweeny, President of Rain Harvesting Supplies, Inc.

There aren’t a lot of U.S. homes with a 10,000 gallon water tank on the lawn. But some homeowners, such as those in the beautiful Texas Hill Country, wouldn’t be able to live in their chosen location if they didn’t have a massive tank to collect and store their family’s entire supply of water.

“The average American is lucky to enjoy an unlimited supply of clean water and at a very low price,” said Steven Sweeney, president of Rain Harvesting Supplies, Inc., an online seller of water tanks and rain harvesting components to professional installers and DIY homeowners.

“In the Hill Country of Central Texas, water is at a premium,” he said. “Most city water districts don’t serve rural areas. If you dig an underground well, you’ll spend a considerable amount of money and probably need several treatment systems to ensure that the water is drinkable quality. So Hill Country homeowners collect rain off their roofs and store it in large tanks as their sole water supply. A 10,000 gallon water tank is probably the smallest size you see, and some tanks hold 30,000 gallons or more.”

Although estimates vary, the average American uses between 80 and 100 gallons of water every day. The largest consumer of household water is the toilet, which guzzles between 1.6 and 4 gallons per flush, based on the toilet’s age. (Newer toilets use less.) Rainwater can be used for household needs, such as flushing toilets, and even converted to pure, drinking water, a procedure that costs a few hundred dollars per year.

“You don’t want to drink rain water straight off a roof,” Sweeney said. “But running it through a Class A UV filter kills any bacteria that may be in it. As long as the system is well designed and constructed, the water in a quality tank can be stored indefinitely and still retain its high quality.”

A one-inch rain falling on a 1,000-square-foot roof will produce about 600 gallons of water that can be stored and used when required. “A water-conscious family of four can get by on about 50,000-75,000 gallons of water per year,” he added.

A 10,000 gallon water tank – or one with greater capacity – offers additional options to rural homeowners, such as storing large quantities of processed water, gray water or water for agricultural and livestock use. Above-ground metal or poly tanks are ideal as rain storage containers. They’re easier to maintain than an underground tank, which can be challenging to monitor, repair or remove.

“Business owners, who have a warehouse or office with large roof surface, can install a 10,000 gallon water tank and easily collect rain for use in their operations,” said Sweeney. “Examples are concrete companies that need water to make concrete, companies that have a fleet of cars or trucks that must be washed regularly or a business campus that has large grounds to maintain. Considering the major corporate campuses I see in metropolitan cities, I can’t imagine all of them aren’t harvesting rainwater for their landscaping. It is so easy to connect a 10,000 gallon water tank to a rain collection system and attach it to an underground irrigation system. The company would conserve water and save a lot money over the long haul.”


  • There is the same amount of water on Earth today as there was when the Earth was formed three billion years ago. The water from your faucet may contain water molecules that dinosaurs drank.
  • 80% of the Earth’s surface is covered with water.
  • 97.2% of the Earth’s water is salt water
  • 2.8% is fresh water
  • 2.2% of fresh water is captured in glaciers and icecaps
  • 0.6% is in groundwater
  • 0.1% is in lakes and streams
  • 0.001% is in the atmosphere
  • The human body is about 66% water.
  • 60% of your body weight is water
  • 75% of your brain is water
  • 70% of your skin is water
  • A living tree is 75% water.
  • Kerr County, Texas, gets average of 29 inches of rain per year. The US average is 37. Snowfall is 1 inch. The average US city gets 25 inches of snow per year. The number of days with any measurable precipitation is 68. On average, there are 224 sunny days per year in Kerr County, Texas.
  • A one thousand square foot roof will catch approximately 600 gallons of water with one inch of rain.
  • 20 inches of rainfall on a one thousand square foot roof will catch approximately 12,000 gallons of water.
  • The average American family of four uses 400 gallons of water per day. On average, approximately 70 percent of that water is used indoors, with the bathroom being the largest consumer (a toilet alone can use 27 percent!).
  •  A water conscious washing machine will use about 22 gallons of water per load.
  • A dishwasher will use between 10 to 30 gallons of water per load depending on the length of the cycle.
  • A low flow shower head will use 2.5 or less gallons of water per minute.
  • Taking a bath in a full tub could use 40 gallons of water.
  • A low flush toilet will use 1.6 or less gallons of water per flush.
  • Brushing teeth or shaving with the water left on could use 5 gallons of water.
  • Watering outside with a garden hose could use 5 to 10 gallons of water per minute.
  • The ground water table in the United States is falling because we are drawing the water out of the ground for irrigation and consumption faster than it is being replenished.
  • In the United States, more than 50% of our wetlands that recharge and purify ground water have been destroyed.