When it comes to the practice of rainwater harvesting, on first thought many people think of rain barrels and small volumes of water used to water a garden. While this is both a popular and common use, there are many other uses and a much larger volume of water that can be achieved through harvesting the rain. With the right setup and large enough storage tank, rainwater harvesting can be used to supply all or most of the water needed by a home, property, or business and be used for much more than just watering a garden.
In this Rain Tanks Series post, we will look at many examples of the amount of water that rainwater harvesting can provide, a state-by-state look using average rainfall amounts, and provide the equation to calculate the volume of rainwater you can collect using your own information. For insight and ideas on what rainwater harvesting can be used for, consider our posts on the practice here.
Rainwater Harvesting: Initial Considerations and Variables
Rainwater can be collected from any surface that will direct the water in such a way that it flows from that surface to a place it can be stored such as a rain barrel or water tank. The roof of a structure such as a home, business, shed, warehouse, pavilion or equipment facility are common surfaces used for rainwater harvesting. Unless you live in the State of Washington, U.S., a structure can even be built exclusively for the purpose.
How much water can be collected from rainwater harvesting will directly depend on three variables:
- The amount of regular rainfall,
- The size of the area used to collect rainwater, and
- The volume capacity of the rainwater storage container.
All three variables are directly correlated, meaning the more rain, the more water, and the larger the collection surface, the more water. For the storage container, you can only collect as much rainwater as you have space to store. Depending on the average amount of regular rainfall, even what seems like a small surface can supply an adequate amount of water.
First Steps and Examples
The first step in considering rainwater harvesting volumes is to know the size measurements of the rain catchment surface. Let’s start off with a few examples. Let’s consider a simple storage shed that measures 10 feet wide by 12 feet long. The surface area of this shed measures 120 square feet. If gutters were installed to channel all the rain that falls onto it into a storage tank, then the shed could supply around 75 gallons of water for every 1 inch of rainfall. If this water were used to flush a 1.5 GPF toilet, it could serve for 50 flushes before the tank was dry and needed refilled by the rain.
Consider this example for a second longer, a 10 ft x 12 ft shed could supply 75 gallons of water from a single 1 inch of rain, enough water to flush a 1.5 GPF toilet 50 times.
If this same shed was harvesting rainwater in a region that annually received 32 inches of rain, such as the San Antonio, Texas region, then the volume of harvested rainwater increases to around 2,392 gallons a year or 199 gallons a month on average. This is enough water to flush that 1.5 GPF toilet over 1500 times — all from a fairly small 10 ft x 12 ft shed.
A Second Example
Let’s consider another example, this time using a home: a personal residence. Many personal residences have a square footage area larger than 1,000 square feet. However, many homes also feature a roofing architecture style that makes it difficult to harvest rainwater from the entire surface. Let’s use a 1,600 square foot home with a simple roofing style that splits the roof’s surface area in half.
For our example, a homeowner is going to collect rainwater in the backyard from the backside of the roof measuring 20 feet wide by 40 feet long for a total of 800 square feet. If this home is in the same region as the shed from the previous example with 32 inches of annual rainfall, then the home could collect almost 16,000 gallons of water over the course of the year – a fair amount of water.
Using the 1.5 GPF toilet example, the 16,000 gallons of rainwater could flush the toilet over 10,000 times, enough for 27 flushes every day.
A Large Example
In this example, we will consider a large surface area to demonstrate the substantial volume of water that can be collected with rainwater harvesting. Let’s consider a poultry house used to raise chickens on a farm in an agricultural livestock operation. The poultry house measures 80 feet wide by 260 feet long for a total area of 20,800 square feet.
The poultry house is located in Alabama State and the region regularly receives around 55 inches of rain. Using this information, the poultry house would be able to supply around 700,000 gallons of water. This is a significant volume of water – one that is potentially capable of providing daily water needs for over 18000 to 34,000 laying hens all year long.
U.S. Federal reports for average human water use is around 80 gallons of water per day per person for all water requiring activities. Assuming a 4 person home, this equals 116,800 gallons of water a year. Comparing this water volume requirement to the rainwater volume supplied by the poultry house leaves an excess of 583,200 gallons. This example demonstrates that rainwater harvesting can more than adequately serve as a water supply. In this particular scenario, however, it is perhaps much more likely for the rainwater to go toward the agricultural operation and farm in raising livestock.
Let’s consider a similar example but in a more arid environment, such as Western Texas with only 15 inches of yearly rainfall and a dairy cow farm operation. The business has multiple cattle houses, feedlots, and barns that can be used for rainwater harvesting with a collective square footage of 15,000 square feet plus a milk handling and packaging facility measuring 167,200 square feet. Using these variables, the dairy farm could harvest as much as 1.7 million gallons of rainwater – a substantial volume of water to use as a water supply in dairy cow livestock raising. The 1.7 million gallons of harvested rainwater would be enough to supply the average daily water needs of 130 to 250 dairy cows all year long. And remember, this is from rainfall in the more arid Western Texas.
Rainwater Harvesting Supply Equation
The examples provided this far have surely demonstrated that the practice of rainwater harvesting can supply significant volumes of water. When considering or planning to implement a rainwater harvesting system of your own, it is important to be able to calculate and estimate how much rainwater you can collect with rainwater harvesting using your own information.
The information you need includes the size of the area that will be used to catch rainwater and the amount of rainfall you expect. A state-by-state list of average rainfall amounts has also been provided below to assist in estimating how much rainwater you can collect.
Rainwater Harvesting Calculator
Use the following equation to calculate the total volume of rainwater that can be harvested from your individual system and location:
Catchment Area (Square Feet) x Rainfall (Inches) x 0.623 = Total Rainwater Volume (Gallons)
For this equation to work and provide a total rainwater volume in gallons, the catchment area (e.g., roof) must be measured in square feet and the rainfall amount in inches. The “0.623” is a constant value that does not change and is used to convert square feet and inches into gallons.
The equation can also be expanded with an additional variable known as the catchment efficiency. Catchment efficiency is how effective the rainwater system is in collecting all the rain that falls on the catchment surface. A rainwater system may not be 100% effective in collecting rainfall due to spills from gutters, spills at the collection container, and/or overflow from heavy rainfall and surge flows. Catchment efficiency is included as a percent of how effective the system operates.
When catchment efficiency is included in the rainwater harvesting supply equation, the equation becomes:
Catchment Area (Square Feet) x Rainfall (Inches) x 0.623 x Catchment Efficiency (Percent %) = Total Rainwater Volume (Gallons)
U.S. State-By-State Annual Rainfall with Examples
The question of how suitable and effective a rainwater harvesting system will be and how much rainwater can be collected will directly depend on the regular annual rainfall for the region the system is set up in. Average rainfall amounts vary across the United States and between locations within the same state. Annual state rainfall is an average of rainwater volume across the entire state for the year. Most places experience a dry season with little to no rain and a wet season with abundant rain. In rainwater harvesting, knowing when these seasons occur is helpful in planning and collecting enough rainwater during the wet season to account for limited or no rainfall during the dry season.
For more on annual rainfall, see:
The following is a list of average rainfall amounts across the U.S. state-by-state with a few examples of how much rainwater could potentially be harvested using different structure measurements.
The State of Alabama has a listed average annual rainfall around 58 inches. The state experiences more rainfall in the southern part of the state with an average 60 to 70 inches of rain and the northern part of the state with an average 50 to 60 inches of rain. With this substantial amount of regular rainfall, rainwater harvesting in Alabama state can provide significant volumes of water for use as a supply during the dry season, for conservation efforts, and to reduce reliance on municipal supply.
Using the Alabama average rainfall of 58 inches:
- Small Example: 1200 sq.ft. Personal Residence = 43,360 Gallons
- Medium Example: 12,000 sq.ft. Agriculture Equipment Shed = 433,608 Gallons
- Large Example: 164,000 sq.ft. Livestock Houses = 5.9 Million Gallons
The State of Alaska has a listed average annual rainfall around 22 inches. However, Alaska has a highly variable range of rainfall due to the size of the state and actual rainfall will depend. The process of collecting snow melt as an alternative rainwater harvesting practice is potentially worth more consideration due to the far north geographical location of Alaska. Most precipitation as rain in Alaska is limited to the southern region of the state and during the short summer season. Rainwater harvesting in Alaska could help supply water in remote locations and supplement water during dry periods in the summer during the growing season.
Using the Alaska average rainfall of 22 inches:
- Small Example: 1000 sq.ft. Personal Greenhouse = 13,700 Gallons
- Medium Example: 2400 sq.ft. Personal Residence = 32,800 Gallons
- Large Example: 12,000 sq.ft. Total Roof Area of a Remote Village = 164,400 Gallons
The State of Arizona has a listed average annual rainfall around 13 inches. Arizona experiences the most rainfall throughout the central region of the state, and very little rain in the dry northeast and southwest. With the limited amount of regular rainfall, rainwater harvesting in Arizona state can provide supplemental water that is good for use in irrigating landscapes. The State of Arizona actively encourages rainwater harvesting practice and its use for landscaping.
Using the Arizona average rainfall of 13 inches:
- Small Example: 1400 sq.ft. Personal Home = 11,338 Gallons
- Medium Example: 16,200 sq.ft. Business Property = 131,200 Gallons
- Large Example: 333,000 sq.ft. Commercial Warehouse = 2.6 Million Gallons
The State of Arkansas has a listed average annual rainfall around 50 inches. The state of Arkansas experiences a fairly even amount of rain and therefore the average rainfall amount is a good representation of regular rainwater volumes for Arkansas. Around 50 inches of rain is a significant volume of regular rainfall, making rainwater harvesting in Arkansas state capable of providing large volumes of water that is good for supplying water for a wide range of water-requiring applications.
Using the Arkansas average rainfall of 50 inches:
- Small Example: 150 sq.ft. Backyard Shed = 4,672 Gallons
- Medium Example: 2000 sq.ft. Personal Residence = 62,300 Gallons
- Large Example: 36,800 sq.ft. Cattle Operation = 1.1 Million Gallons
The State of California has a listed average annual rainfall around 22 inches. California is another state with a wide variance in the distribution of rainfall. The state of California experiences more rainfall in the northern part of the state with rainfall averages decreasing towards the south. Due to some increasing water concerns in California, rainwater harvesting has been recommended and promoted for its use as a supplemental water supply, especially for outdoor water activities.
Using the California average rainfall of 22 inches:
- Small Example: 1000 sq.ft. Personal Residence = 13,700 Gallons
- Medium Example: 24,000 sq.ft. Equestrian Stables = 328,500 Gallons
- Large Example: 62,000 sq.ft. Commercial Property = 849,700 Gallons
The State of Colorado has a listed average annual rainfall around 16 inches. Colorado experiences the most rainfall in the central part of the state, which can range from 20 to 30 inches of rain. With the right setup and a good location, rainwater harvesting in Colorado could supply a significant volume of water. However, the State of Colorado currently limits the amount of rain that can be collected through rainwater harvesting. Current Colorado legislation limits rainwater harvesting to 110 gallons. Should legislation change, some areas in Colorado could supply a useful amount of water through rainwater harvesting.
The State of Connecticut has a listed average annual rainfall around 50 inches. Being relatively small in size, Connecticut has a fairly equal distribution of rainfall across the state, making the average rainfall amount a good representation of what could be expected. The sizable amount of regular rainfall makes Connecticut a good state to practice rainwater harvesting for water conservation or as an alternative water supply.
The State of Delaware has a listed average annual rainfall around 45 inches. Similar to Connecticut, Delaware, being relatively small as a state, experiences a fairly even distribution of rainfall. Due to the sizable volume of yearly rainfall, Delaware state is a good state to practice rainwater harvesting for water conservation as an alternative or supplemental water source.
The State of Florida has a listed average annual rainfall around 54 inches. Due to the state’s location, Florida has an even spread of large volumes of rainfall. Also, due to the state’s geography and population, rainwater harvesting is encouraged in practice as a water supply alternative to potentially limited groundwater resources. In some areas of Florida, rainwater harvesting is used as the primary source of potable water.
The State of Georgia has a listed average annual rainfall around 50 inches. Similar to Alabama, the State of Georgia has more rainfall in the far south with 60 to 70 inches of rain, while the rest of the state has an average 50 to 60 inches of rain. Georgia State has a good amount of regular rainfall and is an excellent U.S. state to practice rainwater harvesting.
The State of Hawaii has a listed average annual rainfall around 63 inches. Hawaii’s average rainfall is determined by local geography with some locations receiving more than 63 inches and others receiving considerably less. Being an island, the State of Hawaii has a history of using rainwater harvesting as a fresh water source and the practice is still actively encouraged today.
The State of Idaho has a listed average annual rainfall around 18 inches. Idaho is a dryland state – most of the rainfall is in the mountainous northern part of the state that is fairly unpopulated. The more populated areas of Idaho State experience an average of 4 to 16 inches of annual rainfall. Given Idaho’s more arid climate, rainwater harvesting practice would be ideal as a supplemental water source.
The State of Illinois has a listed average annual rainfall around 39 inches. Illinois experiences more rainfall in the state’s southern part. With a good system and setup, the State of Illinois can be a great state to practice rainwater harvesting.
The State of Indiana has a listed average annual rainfall around 41 inches. Due to the state’s regional geography, weather, and local activities, Indiana is an excellent location to practice rainwater harvesting for water conservation, management, and supply.
The State of Iowa has a listed average annual rainfall around 34 inches. Rainwater harvesting is an ideal practice for Iowa State due to the decent rainfall amounts and regional activities of agriculture and livestock.
The State of Kansas has a listed average annual rainfall around 28 inches. As a beginning midwestern state, Kansas has an unequal rainfall distribution, with more annual rainfall in East Kansas compared to West Kansas. East Kansas rainfall averages between 28 to 50 inches, while West Kansas averages around 16 to 24 inches of rain.
The State of Kentucky has a listed average annual rainfall around 48 inches. Kentucky is a great state to practice rainwater harvesting, especially for livestock as the state supports the practice and implementation of new systems through their On Farm Water Management Program.
The State of Louisiana has a listed average annual rainfall around 60 inches. Louisiana state reports one of the highest annual rainfall averages of any state, making it an excellent location for any rainwater application.
The State of Maine has a listed average annual rainfall around 42 inches. Being one of the northernmost states in the continental U.S., Maine has cold winters with extended periods of subfreezing temperatures making freeze protection an important consideration for any rainwater harvesting system.
The State of Maryland has a listed average annual rainfall around 44 inches and supports rainwater harvesting for water conservation and has guidelines on cistern use in the state.
The State of Massachusetts has a listed average annual rainfall around 47 inches, a good volume of water capable of supply a substantial amount of rainwater.
The State of Michigan has a listed average annual rainfall around 32 inches, a decent volume of rainfall capable of supplying a good amount of water.
The State of Minnesota has a listed average annual rainfall around 27 inches, another decent rainfall volume. Minnesota businesses, residents, and agriculture operations could collect significant volumes of water from rainwater harvesting in the State of Minnesota.
The State of Mississippi has a listed average annual rainfall around 59 inches. Similar to the neighboring states of Louisiana and Alabama, Mississippi state offers a substantial yearly precipitation amount capable of providing large volumes of rainwater harvest good for many applications.
The State of Missouri has a listed average annual rainfall around 42 inches. As another state with large regular water withdrawals for livestock and agriculture operations, and with a decent yearly rainfall amount, Missouri is an excellent state to practice rainwater harvesting and holds the potential for yielding large volumes of water.
The State of Montana has a listed average annual rainfall around 15 inches.
The State of Nebraska has a listed average annual rainfall around 23 inches. As one of the top states using water for livestock raising, the State of Nebraska is an ideal candidate for implementing rainwater harvesting systems as an alternative or supplemental water source.
The State of Nevada has a listed average annual rainfall around 9 inches. As one of the most arid U.S. states with the lowest annual precipitation, Nevada may be limited in the effectiveness of rainwater harvesting. However, the state is looking into rainwater harvesting practice as a low impact development option and for water conservation.
The State of New Hampshire has a listed average annual rainfall around 43 inches.
The State of New Jersey has a listed average annual rainfall around 47 inches.
The State of New Mexico has a listed average annual rainfall around 14 inches. Being an arid state, New Mexico is a good location to practice rainwater harvesting to conserve available water resources and make use of the annual precipitation that falls. Some cities in New Mexico, such as the City of Albuquerque even provide rainwater rebates for commercial installations and rebates for residential installations.
The State of New York has a listed average annual rainfall around 41 inches.
The State of North Carolina has a listed average annual rainfall around 50 inches.
The State of North Dakota has a listed average annual rainfall around 17 inches.
The State of Ohio has a listed average annual rainfall around 39 inches.
The State of Oklahoma has a listed average annual rainfall around 36 inches, experiencing more rainfall in the east. The regional geography, industry, and agriculture activities, including livestock, make Oklahoma a great state to practice rainwater harvesting. The Oklahoma State University provides an article on the design of rainwater harvesting systems in Oklahoma.
The State of Oregon has a listed average annual rainfall around 27 inches. However, and similar to Washington State, Oregon has a wide rainfall spread with much more precipitation in the more populated eastern side of the state. Comparatively, Oregon is a popular state for rainwater harvesting practice and is supported by the state government and universities. The City of Portland, OR even has a project guidance document for a potable rainwater collection system, indicating state support and interest.
The State of Pennsylvania has a listed average annual rainfall around 42 inches.
The State of Rhode Island has a listed average annual rainfall around 47 inches.
The State of South Carolina has a listed average annual rainfall around 49 inches.
The State of South Dakota has a listed average annual rainfall around 20 inches.
The State of Tennessee has a listed average annual rainfall around 54 inches.
The State of Texas has a listed average annual rainfall around 28 inches with a fairly wide discrepancy in rainfall amounts across the state from the east to the west. For rainwater harvesting, Texas is one of the most, if not the most, popular states for practicing rainwater harvesting with one of the largest governmental support programs, rebates, and tax credits available. The practice is listed as an Innovative Water Technology by the Texas Water Development Board and the Texas A&M University has a website dedicated to the practice of rainwater harvesting.
The State of Utah has a listed average annual rainfall around 12 inches, a fair amount of precipitation that could go toward conservation efforts. The practice has received enough attention for the Utah State University to put together information for homeowners on rain water harvesting. However, and similar to Colorado State, the State of Utah has legislation some restrictions on rainwater harvesting per Utah Law Code 73-3-1.5 that limits rain collection volumes up to 2,500 gallons. See the Salt Lake City website for more information.
The State of Vermont has a listed average annual rainfall around 42 inches.
The State of Virginia has a listed average annual rainfall around 44 inches.
The State of Washington has a listed average annual rainfall around 38 inches. Similar to Oregon, the State of Washington has a rain-shadow effect with significantly more rainfall on the eastern side of the state capable of yielding large volumes of water. Also similar to Oregon, Washington is a popular state for rainwater harvesting with a decent amount of governmental resources and information around the practice.
The State of West Virginia has a listed average annual rainfall around 45 inches.
The State of Wisconsin has a listed average annual rainfall around 32 inches.
The State of Wyoming has a listed average annual rainfall around 12 inches.
Takeaway | How Much Rainwater Can I Collect with Rainwater Harvesting
We hope this Rain Tanks Series article has helped to demonstrate the amount of water that can be collected by practicing rainwater harvesting and proved that very large volumes of water can be harvested that are capable of going a long way in supplying water for applications across personal uses, businesses, agriculture, and more. The exact volume of rainwater that can be collected will depend on how much it rains where you live, the size of the area you will use to harvest rainwater, and your storage capacity.
For more information on rainwater harvesting practice, such as potable water systems, use for livestock, and more, review all of our Rain Tanks Series posts.
For rainwater equipment, we provide ANSI/NSF 61 certified rainwater collection tanks and commonly used rain harvesting accessories, including treatment systems. If you have further questions or would like assistance, contact us.