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

Hi Nathan,

It is common knowledge that insurance companies request from roofing companies that give long-term warranties as accurate data as possible to lower their premium. The benefit for this program you are selling as a tool for the client is simply to lower your premium, maximize the profit maybe cause by issues in the past.

For me it is also common knowledge that USDA ‘Plant Hardiness' Map shifts Temperature Zones North just recently based on the ongoing pressure of experts. The "Plant Hardiness Zone Map," an update of a 1990 static version, shifts temperatures up one 5-degree increment across much of the country, reflecting newer and more complete data. Scientific data shows that the world is getting warmer, with potentially adverse consequences for farmers and gardeners worldwide. 

It seems commons sens that utilizing data from the past don't have the accuracy in today's fast climate change anymore and for what you are looking for.

Also, when you use a program to model the future - and if it is accurate - you should be able to do the weather forecast for the future, too. May be I am just bothered about the term "accuracy" what should be use accurately.

Sometimes I wonder what else countries with multi-billions square meter of green roofs missed in the last 30 years? jbi



I wanted to share the rainwater catchment/reverse osmosis setup of an internet friend:

This setup is rather specific in that the water quality had to be improved for his specialized use (watering bonsai), nevertheless there are some great ideas and it's very well-written.



We are starting to see higher single event precipitation numbers than some existing stormwater harvesting and retention systems were designed to accommodate.  How does this impact the stormwater retention infrastructure and your stormwater management plan?

In the District of Columbia, Stormwater Management Plans are required to address a ninety-percent stormwater event, i.e. rain events of 1.25" or less.  If that ninety-percent event threshold were to change to say, rain events of 1.5" or less, how would that change your Stormwater Management Plans and the palate of stormwater retention vehicles that you deploy?


Any stormwater management system can be adjusted to given requirements. This is already common practice since various cities across the US already have a range of different requirements already.

Utilizing green roofs with a variety of retention values are on the market. However, I would be very careful in this regards because many solution might create additional challenges or simple don't work in the long-term.

 For example systems with mineral wool or high water storage storages in a synthetic material might retain a lot of water in laboratory test but have a tendency to be compressed in real life (reduces retention), they are often hydrophilic (take the water from the plant and soil away in droughts), and have less than 30% air at maximum water capacity what can be deadly for succulents.

 When it comes to plastic boxes (trays), they might have a large water reservoir, but practice as shown that their internal drainage is poorly designed - basically you create a bathtub. Good for water retention, deadly for succulents.
Any type of green roof shall mimic natural soil horizons that are specific for the intended plants.

 More heavy and intense rains come with hotter and longer droughts. Succulents or drought tolerant plants are still able to survive these weather extremes, if enough rooting depth and space is available. This is necessary that the soil temperatures stays below 100°F or 38°C. High water retention systems are often built in thinner layers (mentioned above). Temperature increases faster, water evaporates faster (roof surface high humid -not good for succulents) and these systems insulate more (heat stays longer). Plastic boxes "cook" much faster because they are not a continues system.
With more intense rain events and hotter temp over a longer time thin layered systems heat up more (kill plants) and dry out faster. The industry has recognized this problem and found a new market for irrigation systems. However, all irrigation systems operate only when it is dry (when the APP things it is necessary to irrigate the plants). This is fatal for stormwater retention because it might rain (such heavy rain event you see more often) just after irrigation. This reduces in any system the water retention between 50-100%.

These heavy rain events are typically hard to predict and very local - so the super high tech APPs connected to the internet will have still a high failure rate in predictions. Why spending a lot of money in Technology when this creates more costs and long-term problems?

 Beside all the technical challenges and still having an around 50% failure rate on DC extensive GR projects after 5-7 years, many "professionals" miss a few points that are the 101 in horticultural:
• With increasing the water retention the weight of the system will increase (there is no lightweight water, and if so, it doesn't come from the clouds).
• Increasing water retention in thin layers can reduce the amount of air in the soil (or soil replacement) below the amount required for healthy growth.
• High water retention in thin layered system likely relocate nutrients or washes these out faster.
• High water retention in synthetic systems (sponge- like) drastically reduce the amount of valuable soil organisms. It may increase the amount of anaerobe bacteria that are typically not present in a succulent environment.
• Only deeper systems with a perfect soil horizon accommodate more water retention, more plant diversity (no Sedum monoculture, what is an additional problem).

For all engineers (especially the structural engineers):
Deeper systems are slightly heavier than shallow systems and more water retention means also higher weight of the Green Roof systems. This is simple physics.

If you think this can be disregarded with an super high-tech, innovative system (approved by NASA, Firefighters and FM Global etc.) - give your client my number that he can call me in five (5) years. Expect your liability insurance will increase.
Physics, Chemistry and biology are nature's laws - the same on the entire Earth - engineers or nursery people can't change these.


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