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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 www.apartmenttherapy.com.  I plan on trying it out this summer and will let you know how it goes.

http://www.apartmenttherapy.com/sf/how-to/how-to-make-a-rain-water-collection-barrel-043733

Leonard

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?

Steve

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:

http://cdo.ncdc.noaa.gov/cgi-bin/climatenormals/climatenormals.pl?directive=prod_select2&prodtype=CLIM20&subrnum=

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.