Designing a Stormwater Harvesting System

The general rule of thumb when sizing a "cistern" to capture rainfall is that a 1" rainfall from a 1,000 sq. ft. roof will generate approximately 600 gallons of runoff. Seeing that 90% of the storms in the D.C. area are under 1" this is a pretty good metric to use when sizing your tank.

Above ground tanks are the most affordable solution to an underground system and in some cases rainwater that is collected can be gravity fed to water gardens and landscaping. If you can eliminate having to purchase a pump and expensive filtration devices with water level sensors that will be even better because "low tech" solutions offer the quickest payback especially in jurisdictions that offer incentives for collecting roof runoff. If you are interested in designing a stormwater harvesting system for your property we would be more than happy to assist you with the design of an affordable and efficient system that meets your budget.


Cistern volume can be determined by calculating the roof top water yield for any given rainfall, using the following general equation:

Equation     V = A2 x i x c x 7.5 gals.ft.3        where:




= volume of rain barrel (gallons)




= surface area roof (square feet)




= rainfall (1"=.08 ft.)




= coefficient of runoff (.9 for impervious areas)




= conversion factor (gallons per cubic foot)


600 gallons = 1000 ft.2 x 0.08 ft. x 0.90 x 7.5 gallons/ft.3


Edited Tue, Mar 11, 2008 10:52 PM

Replies to this Topic

Thank you for this information - I am finding that while "going green" is great in theory, the practical application of doing so is a different matter.  It seems as if there is very little information out there about how to install a cistern or large rain barrel, and finding out about how this is done is a bit of a challenge.  I appreciate the information, esp. the formula!

Regards - Lynn

There  is a very low-tech solution at  I plan on trying it out this summer and will let you know how it goes.



It turns out that for rainwater harvesting, calculating the proper storage volume is a big problem.  There are plenty of rules of thumb out there, but there's a sort of catch-22 in the design process.  It's hard to design a system unless you know how you want the system to perform,  but it's hard to know how a system will perform unless you know how to design a system.  Still, it really helps to think about what you want to get from a system before you design it. 

My thesis research tackled this exact problem.  From my experience, to design a system you need to think at a minimum about the following:

- Collection Area of the roof or collecting surface  (Ac)

- your expected average daily demand for the harvested water  (D)

- average daily rainfall  (rd)

- the reliability you are willing to tolerate (what percent of the time do you want the system to function) (q)


If you define the ratio, a (alpha), as

a = D/(Ac*rd), 

(Units:  D- ft3/day, Ac - ft2, rd - ft/day, a - unitless)

by setting a = 1, you can compute the maximum daily demand you can get for your collection area, Ac.  If a is greater than 1, you are trying to use more water on average than you collect.  The problem is that required storage to maintain 100% reliability (or any defined reliability) increases rapidly as a increases (especially between 0.5 and 1). 

Anyway, I can easily go into a lot of detail here, but i'll say that really understanding the system, and being able to confidently calculate the storage volume required usually requires modeling the system using a long term rainfall sequence from a rain gage near where you live. 

If you need help modeling a rainwater harvesting system, I have a lot experience doing so.  I can provide models of various complexity.  I also have generated some generalized regression equations for calculating storage in a range of locations across the U.S. 


What are some good sources for the monthly rainfall averages for localities in Maryland and Virginia?


The National Climatic Data Center is the best source for climate data.  They have data not only on rainfall, but also temperature, snowpack, humidity, degree days, and sometimes solar radiation and some other things. 

Steve, the best data product for you is probably the monthly climate normals.  The ones currently published reflect the 30 year average from 1971-2000.  This link should let you get the data you need:

The climate normals are free.  Much of the NCDC data requires purchase unless you are working from an educational institution or certain government agencies.  Be aware that planning from monthly averages may not be sufficient for designing a system if you wish to have high reliability.

A great source for rainwater harvesting information of all types is the American Rainwater Catchment Systems Association (  On the Resources page there is information on rainfall amounts, rainwater harvesting, existing system profiles, etc. for systems of all sizes.

Rain barrels are great.  I have been making my own and selling them as it seems impossible to find a local source.  The ones I found were wood and VERY expensive.  You can get them online but shipping seems rediculous for something that can be up-cycled from found objects.

Barrels are the tough part.  I figured out that car washes use 30 and 55 gallon plastic barrels that end up next to their dumpsters when they are out of soap.  I checked MSDS on the Simoniz site and it's safe stuff so I just wash it out.  3/4 inch bore, a brass fitting and spigot, cut out the threaded openings at the top and screw in two kitchen sink screen drains then the downspout diverter and you are ready to go.  $20 and I can pick up the barrels with my bike and kid trailer.

Now if I can only get my homeowner's association to stop asking me to remove the barrel from my driveway...

I have more detailed directions online at so let me know if you want to check them out.  My kids love helping out and they actually ask 'Can we watch the rain barrel fill up?' when it rains.  They are 3 and 6 and the excitement won't last, I'm afriad...

michael klasmeier

That's great Michael.  Where is this car wash where you pick up the used barrels?

Maybe we can think of a way to handle the appearance issue.  My HOA will have a fit too.

I am interested in your directions.


My local source is in the Crownsville/Crofton area.  There are three near me.  Pretty much every car wash uses barrel soap.  Just look up your local automatic car wash.  Be patient, they use a barrel-full of soap every couple of weeks.

You can paint the barrles using Krylon Fusion to match your decor or you can plant stuff around it.  I was planning on building a little frame for the outside of it and using various plants to cover it up.  I'm sure they'll hate that too.

I'll post the directions up here.  Not exactly good for the professional types, but great projects for us home users.

I'm building my rain barrel this weekend.  Arlingtonians for a Clean Environment sponsors workshops where you can build you own rain barrels or pick up ones that are pre made (with spigots, screens, etc) for $52 or $62 respectively. 

One of the workshops is already posted on the calendar.  I think tomorrow's session is full, but there's a few more this summer.  The full schedule and details can be found at:


I am concened about using low tech filtration.  The problem with this is everyone looks at the short term investment and quick pay back.  Using substandard filtration creates a tremendous maintenance burden.  Once sediment gets into the tank, the water begins to stink and is very unhelathy.  The tank outlet becomes clogged and the water either does not flow or the flow is so restricted that the system now requires two options.  1.  Shut the system down and the cistern now becomes a tank full of water underground that is never used again or 2.  Someone must clean the tank.  Once you pay for a confined entry team to clean a tank once, all that money you thought you saved is gobbled up in a hurry.

No, the paradigm of rainwater harvesting must be changed.  There are four steps to a healthy sustainable rainwater harvesting system.  Pre-filtration using a low maintenance filter that never needs to be replaced will eliminate unhealthy sediment from entering the tank.  Using the proper filtration will eliminate the need to ever enter the tank.  The second step in the process is to place the water in the tank gently so the bio film is not disturbed.  This will allow the bio film to grow naturally and the tank will be healthy year after year.  Placing the water in the tank using the proper filtration wil introduce ample oxygen into the tank and assist with the oxygenation process.  The third step in the process is to extract the water from just below the surface which again does not disturb the biofirm and places the healthiest water into the system as possible.  The fourth step is overflow.  Oxygenation is critical to healthy water and overflowing the tank and exchanging the water is critical for this process.

Once again, the problem with "cheap" rainwater filtration products is maintenance and no facility owner will pay big dollars for maintenance.  I recommend that before you try to sell someone on a low cost system you educate yourself on low maintenance high quality systems with sustainability as your goal.

Thank you for allowing me the opportunity to share this information and as always, I am here to help with the education process and help any way I can to ensure rainwater harvesting systems are incorporated in projects for the right reasons the right way.

Rick Hall


I've recently come across a great source for barrels, if anyone is interested in making their own rain barrel.  Bowman's Apple Products is just a few miles from my farm here in the Shenandoah Valley, and they use quite a bit of fruit juice concentrates which are delivered to them in 55 gallon drums.  I've been using them as feed bins, free-range poultry feeders and waterers with great success.  They also have metal drums with lids that I've used for recycling containers.

I'm buying them from someone who collects them and cleans them out, although some smell still remains.  At least it's the smell of strawberries or dragonfruit, and not bleach or some other soap!  The metal barrels hold organic apple juice concentrate and don't have an odor at all.

If anyone is interested, you can email me at to pick one up here on the farm or I may be able to drop one off somewhere convenient to both of us.  I'm not trying to MARKET them here, although I have been tempted to make a rain barrel kit or something like that, given the prices that I see people are charging for them.  Maybe there's someone else who would like to do that...I'm too busy propagating more greenroof plants right now!

I have used a rain barrel for watering a green roof.  The barrel was set up on a platform for gravity feed.  The system was then hooked up to a irrigation valve and clock then onto a drip system.  We had great success with the system and fun testing to see if it worked.  I have photos if anyone is interested. 


I would love to see the application.  Please forward anything you can.

A quick and affordable way to dress up a an unsightly barrell is to use 1x4 rough cut cedar cut to your barrell height and galvanized plumbing strap, the kind with the holes punched in it. Lay out enough boards to cover the circumfrance of the barrell. Lay three rows of strapping across the boards and secure with two screws in every board. Wrap the assembly around the barrell and secure one end of the strap to the exposed face of a board in an inconspicuous place.

How large are the barrels that you are wrapping?

I would be very interested is seeing how the collection of rainwater is being accomplished from green roofs and reused to irrigate these roofs. 

Has anyone been involved with a project where the rainwater from a green is harvested and reused for other applications?  If so I would be interested in the project and information that could be shared.


I would think that water harvested from a green roof may need some active aeration to deter aneorobic conditions in the collector as i am sure there will be higher organic and nutrient components to the effluent.  I collect rainwater in a liner waterproofed wetland/pond and hand water my plants from this.  No mosquitos (there are fish in it), no filtering/screening, and the plants keep everything fresh.

The first picture is the small one in my row home backyard :

izikpatioLarge.jpg picture by secret_creek

The second one i installed in my sisters front yard and is fully powered by plants. (no electrical input) This one just harvests water.  The overflow area is perforated and vegetated with live sphagnum and some  rushes and other volunteer wetland plants.  It has attracted two species of frog that reproduce in the pond which is located in an area with high pedestrian traffic.

becspond015Large.jpg picture by secret_creek

becspond003Small.jpg picture by secret_creek

turtle016Large.jpg picture by secret_creek

Edited Thu, Nov 19, 2009 10:47 AM


Basic precip. numbers for the Mid-Atlantic States: about 42"

From DIY Plans to installation ready rain barrels offers a vast array of unique rain barrels and related products.


We have access to lots more than what you will find on the website - so please check us out and let me know what I can do to assist you with your rain collection issues.


We also offer a couple of interesting water conservation products too - Water from Air and Dual Flush Toilet Kits (don't replace your toilet, update the guts inside!)

I am glad to see that the topic of rainwater harvesting is coming up again as a topic of this group.  I know all of you are aware that the population growth is placing a tremendous burden on our fresh water sources.  Depending on who you use as a source, we will have a significant shortage of potable water in as little as ten years if we do not do something about it now.

Rainwater is the natural way of replenishing our water sources but for some reason, there is this paradigm that we should not harvest the rainwater and reuse it.  We continue to waste the potable water that is available to us for non potable things and go about our daily business. 

If you are designing a green roof and you do not look into harvesting rainwater as well, you are adding to the inevitable problem of a water shortage situation and not becoming a part of the solution.  If you need help with a high quality rainwater harvesting system with minimal maintenance that will enhance your green roof project, visit our web site at

Sometimes on larger sites when it is difficult to install the pumping systems or the controls in a building and you don't want to place them above ground due to aethetics an alternative would be to put them into an underground vault. The schematic diagram below shows how the pumping system could be laid out along with the electrical panels required for the pumps. It is important to remember however, that moisture intrusion or rainwater runoff into the vault could be an issue so you need to make sure that you place the system in an area that is relatively elevated. Also, you should make sure a sump pump is in place too in case water should find it's way through the bulkhead fittings or through the sealed manway/access hatch.

Proper venting is also required so that the electronics and equipment can have the proper ventilation and not overheat. Be sure when designing your vault that you meet all the current electrical code and OSHA requirements so that workers that might need to maintain or inspect the system will be safe and not exposed to any hidden hazards. You should also consider having a light fixture installed as well because will make accessing the chamber a little safer so that you can properly maneuver around all the control and pumping equipment.

As with any underground vault or tank it is also important that it is properly anchored so that if the water table rises the vault won't settle or shift. Any movement with the vault would cause leaks where the conduit enters into the side of the structure. It is also important that the access hatch comes with a lockable lid and that the hatch is clearly identifiable so that lawn equipment or pedestrians won't be encouraged to walk over the access hatch.

This detail below was designed by Paul Bassett with WaterSavers and the engineering was conducted by Watertronics. I would strongly encourage anyone interested in learning more about underground vaults to contact either company for assistance or if you want to know more about the entire system feel free to drop me a line.

Post Reply

You must be logged in and a member of this Groupsite in order to post a reply to this topic.
To post a reply, contact your group manager(s) Join this Groupsite


Capitol Greenroofs
Powered by

Visibility Public Membership By Invitation or Approved Request Default Profile Professional

Your Status Not Logged-In