A Medium Sized Rain Water Collection System -- Sizing the System

There are a variety of ways to look at sizing a rain collection system.  In this section, I go through the logic we used for sizing our system, but our webpage on water harvesting has some good references with other approaches...

In our case, we have a known amount of roof area to work with, and we wanted to pick a tank size that would make use of a sizable fraction of the water falling on the roof without ending up with a tank that was so large it never filled up.  The water will be used for our fairly small garden with any excess going to water trees out in the shop area. 

To some extent, this is a project to learn about water harvesting, and we may well do a larger system that uses the house roof later.

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Sizing the System

The first step is to estimate how much rain water the shop building will provide over a season.

The shop building roof size is 25 ft by 25 ft, and there is a lean-to parking area attached to it that is 12 ft by 25 ft.   So, the area available is 25*25 + 12*25 = 925 sf.

A pretty good guideline on how much water you can collect is that 1 sqft of roof will collect half a gallon of water for each inch of rain that falls.  This allows an about 10% loss for evaporation and other losses.     A good way to remember this is that you will get about 500 gallons of water in the tank for each 1000 sqft of roof area per inch of rainfall.  This is not an exact estimate, as the amount of water lost to evaporation and other factors will depend a lot on what kind rains you get and how dry it is -- see the references on Water page for more on this.

So, using this ground rule, each inch of water on the shop roof should be worth about (925 sf)*(0.5 gal/sf -inch) = 460 gallons in the tank per inch of rainfall.

In our case, the collection cycle starts in about March.  Before that the weather is cold enough that you might end up with a big ice cube instead of a tank of water.  To get an idea of the amount of water we might have in the tank at the end of any given month starting with an empty tank in March, we used the NOAA CLIM20 report for our area.  This is a very handy and free report that gives temperature, rain, snow, and frost data by month with not only averages but also the probabilities of achieving various levels of precipitation -- this allows you to evaluate low rainfall years as well as average years.

This is the CLIM20 report on precipitation for Bozeman, MT where we are:

CLIM20 precip report

The NOAA CLIM20 report is available here...

The table just below shows how much rain we can harvest through the spring and summer using our 925 sqft of roof area and the rainfall amounts from the CLIM20 report.

Month Rain
Cumulative - (Gallons )  
March 1.4 640 640  
April 2.1 970 1610  
May 3.2 1470 3080  
June 2.9 1330 4410  
July 1.4 640 5050  
August 1.6 740 5790  

So, for the months of  March, April and May when little water is needed for watering, it looks like we could accumulate as much as 3000 gallons in a tank for use during June, July, August and September.  Choosing a tank larger than 3000 gallons would not likely do us much good as a 3000 gallon tank on an average year would hold all the water that we could collect off the shop roof before the watering season started.  Having a tank less than about 3000 gallons would be OK, but would just mean that on an average year, the tank would fill up early and lose some potential water that could have been stored and used for watering later.

We ended up choosing a tank that holds 2500 gallons.  This is a standard size tank, and one that fit nicely under the end of the lean to parking space that is attached to the shop, while still leaving space to park a vehicle.

The columns in the table above are calculated as:

Rain -- this is from the CLIM20 report

Harvested  -- This (Rain)*(925 sf)(0.5 gal/inch-sf)   as explained above.

Cumulative -- This is just the running total of the Harvested column -- it is total harvested from March 1 with no deducts for water used.

Go on to installing the tank, and building the collection plumbing...

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If you have questions or comments...


Gary July 1, 2011