A Beginner’s Guide to Rainwater Harvesting

Green recovery of rainwater outside in town garden
pixinoo / Getty Images

Rainwater harvesting is the practice of collecting and storing rain for reuse, rather than letting the water run off and be absorbed into the ground or channeled into drains, streams, or rivers. It is one of the easiest ways to conserve water at home while also lowering your bills. Whether you opt for a custom-designed system or a simple rain barrel collection method, harvesting rainwater is a smart and sustainable choice.

The practice of rainwater harvesting is gaining new relevance as the impacts of the climate crisis accelerate and parts of the world experience drier and longer droughts, depletion of groundwater, and freshwater pollution from saltwater flooding. Rainwater harvesting provides a source of clean fresh water in places where water is scarce, polluted, or only seasonally available. In addition, harvesting and storing rainwater can be a less expensive way (compared to desalination or piping water long distances) to guarantee safe, clean water for drinking and home use, as well as gardening, watering livestock, or agriculture. 

While there are all kinds of modern rainwater catchment systems, collecting rain is an ancient practice. Anthropologists believe that being able to capture and store water went hand-in-hand with the development of agriculture, especially in drier environments. Cisterns for storing rainwater have been found in communities as far back as Neolithic times, and by 2500 BC they could be found in what is now Israel and the Greek island of Crete, and later in the Roman Empire, Istanbul, and even Venice. 

How Much Water Can You Collect Through Rainwater Harvesting?

The Federal Energy Management Program uses the following formula to calculate the total amount of water collected through rainwater harvesting systems:

Catchment area (roof size in square feet) x Monthly rainfall (inches) x Conversion factor (0.62) x Collection factor (75%-90% to account for losses in the system)

For example, according to NOAA's Climate Report, the average monthly rainfall for the contiguous United States was just under 3 inches in 2019. Using this number and a 75% collection factor, the total water catchment for a 1,000-square-foot roof would be:

1,000 x 3 x 0.62 x 75% =

1,395 gallons per month, or 16,740 gallons per year (minimum)

How It Works: Capture, Store, Reuse

The most basic rainwater harvesting systems include a way to collect the rain (which could be as simple as the roof of a house), a way to direct the water (like a gutter and downspout), and a place to store the water (like a barrel). Because it lacks filtration and proper storage, water collected from a system this simple would only be suitable for basic uses like watering a garden, fire suppression, or as grey water — like toilet bowl water.

A more complex system that would provide more potential end uses for the water would include a collection system and several layers of filters to keep dirt and debris out of the water supply. An appropriate storage tank should have a way to safely handle overflow water and be made from materials that won’t leach into the water and will inhibit bacteria growth. That container should be then hooked to a control system that can further filter the water for drinking-level purity if needed, or at least to a monitor that tracks the water level. Finally, the system would require a pump to direct water, a flow meter, and backflow prevention system, all of which would need to be hooked up to a power source.

Rainwater harvesting system isometric diagram
Rainwater harvesting system diagram with hose roof water runoff, underground piping, filtering, collecting in tank for domestic use. VectorMine / Getty Images

Rainwater Harvesting vs. Grey Water Recycling

Harvesting rainwater can be part of a larger system that includes grey water recycling, but they are not the same thing. Grey water is a term that is defined by what it’s not, which means that grey water is all kinds of household waste water that is not from a toilet. That includes effluent from kitchen and bathroom sinks, showers and baths, washing machines, and dishwashers. It has fewer potential disease-causing organisms, or pathogens, than toilet water, and so it’s simpler to treat for reuse. 

Grey water can be recycled on-site at a home, apartment building, office, or a hotel, and it can be used for toilet flushing (after which it's called black water), garden or lawn watering, or for crops. Grey water reuse is often designed into a rainwater harvesting system as a way to make the harvested water go farther, as it can be used more than once. For example, harvested rainwater could be filtered and stored, and used first in a shower or washing machine, and then the grey water from those tasks could be collected and used to water landscaping. 

Using grey water also reduces the amount of wastewater that needs to be collected and treated, if sewage is limited. 

The Ever-Flowing Benefits

Rainwater harvesting has a host of benefits beyond reducing demand on local freshwater resources. By collecting rainwater during a storm, there’s less stormwater runoff, which can overwhelm local sewage systems and result in local pollutants making their way to rivers and streams, lakes and ponds, and out into the ocean. 

Collecting rainwater can also reduce erosion in especially very dry environments where it is common, and reduce flooding in low-lying areas. 

Of course, if you pay for water from a municipal source, harvesting your own will save you money on your water bill. 

The benefits of this practice have been recognized in many cities around the world that now require or encourage rainwater harvesting systems. For example, Bermuda, the U.S. Virgin Islands, and Santa Fe, New Mexico, now mandate a rain catchment system on all new homes, and Texas offers a tax exemption for the purchase of harvesting systems to encourage the practice. Cities in Australia, Kenya, China, Brazil, and Thailand all utilize large-scale rainwater harvesting, and the airport in Frankfurt, Germany, collects rainwater for use in its terminal’s toilets and landscaping. 

Uses for Harvested Rainwater

Harvested rainwater can be used in almost every way water from a well or other supply can be used. If the water is to be used for drinking (potable), food preparation, or other direct human consumption, it needs to be filtered to improve the flavor and remove pathogens, grit, and other particles. At the very least, it should be boiled at a rolling boil for at least one minute to kill disease-causing organisms. 

Outdoor Uses

  • Gardens and landscaping
  • Swimming pools
  • Livestock water 
  • Household chores like car or dog washing
  • Water features like bird baths or fountains
  • Fire suppression or emergency water

Indoor Uses

  • Washing machine
  • Dishwasher
  • Hot tub, bath, or shower
  • Toilet 
  • Utility sink

Ways to Harvest Rainwater 

There are many ways to harvest rainwater, from the really basic DIY, to complex systems. The most important question is what you will use the water for. That will determine how much filtration and monitoring it needs, and how complex and expensive your system will be. 

A basic system that collects rainwater from a roof via downspouts and a barrel or tank is ideal for outdoor use — for watering plants or other outdoor chores. These systems don’t require much more maintenance than typical gutter-cleaning upkeep. 

The next level of complexity is household water — for sinks, showers, washing machines, and toilets (or outdoors for a swimming pool). Harvested water for those needs will require a good basic filter or two (and those filters should be monitored and changed regularly). The water should be kept in a bacteria-resistant cistern (which can be above ground or buried), and you’ll need a pump to move the water where it needs to go. This water will need to be used regularly; if it sits too long unused, bacteria will proliferate unless it’s chemically or otherwise treated. You will also want to ensure that the water isn’t coming into contact with lead, heavy metals, or preservative-treated timber on your roof, if that serves as your catchment area. 

The best materials for roofs that will be used to harvest rainwater are slate, aluminum, and galvanized iron. Finally, you will need to install piping into your house to bring your stored rainwater to the appliances or faucets where you want to use the water. The costs for all these items vary depending on existing plumbing.

Potable Water System

The highest level of complexity for a rainwater harvesting system is to create potable, or drinkable water. These systems would include all of the expenses previously mentioned, including determination of safe collection area, filtration, safe storage, pumping, and additional filtering or treatment, as well as additional piping and pumps.

The filtration for this type of system can cost up to $20,000 and require a fair amount of maintenance, since any overlooked filter replacement could compromise water quality and lead to illness and even death. However, this investment might make your home or building self-sufficient and, if adequate rainwater is available in your area, could mean you don’t need to be hooked up to a city water supply or dig a well, which could be a money-saving situation, too. 

There is a growing number of companies and professionals who can work with you on any of the above systems, and ensure you are getting the combination of filters, storage, monitors, pumps and pipes you need for your location and water needs. 

When it comes to the safety of water inside your home (especially drinking water) — even if you want to do the work of putting the system together yourself — it makes sense to consult a professional for guidance. 

View Article Sources
  1. Mays, Larry, et al. “History of Water Cisterns: Legacies and Lessons.” MDPI, Multidisciplinary Digital Publishing Institute, 21 Nov. 2013, doi:10.3390/w5041916

  2. Water-Efficient Technology Opportunity: Rainwater Harvesting Systems.” Energy.gov.

  3. Gleason. “National Climate Report - Annual 2019.” National Climatic Data Center