The Sustainability Review – Rainwater Geometry

By Mike Forbes, Co-op Volunteer Writer

I’ve written about rainwater systems for several years now, specifically about our experience with installing, filtering, and using it.  I’ve received many questions in that time regarding everything about our system.  There is one question that keeps rearing its head.  How much water can I collect off of my roof?

I remember doing the calculations years ago and quickly realized that in our circumstance that we would have more water falling on our roof than we could easily store.  Storage became our issue but for you it might not be since our roof is rather large.  My point in writing this is that it’s important to know what your water capture potential is and design your storage and useage accordingly.  I’m not going to go through any of the techniques or roof materials of rainwater collection as I’ve done that in the past, this article is purely theoretical.  Previous articles can be found on the Co-op website or from me directly.

Let’s walk through the math with an imaginary 20’ x 20’ small house.  Time to put on your high school geometry thinking cap. The most important thing here is to visualize the roof area that the rain is falling on, not the length of the roof line. The slope of the roof isn’t important here. The dimensions we are looking for are the lengths of the walls plus any overhang, ultimately the total roof area that the rain will see.  With our house we have a 20’ x 20’ roof giving us an area of 400 sq. ft.

Next we need to know how much rain comes out of the sky.  Where do you find this data? www.weatherbase.com.  There are various sites out there with specific data for a location but this one is a simple to use and easy to decipher.  Many other sources require you to dig a bit and don’t give a good overview.  Simply input your zip code and scroll down to Average Precipitation.  For Moscow, Idaho this is 23.7 inches. 

Imagine a roof of 400 square feet with 23.7 inches (1.97 feet) of water sitting on top of it.  This is our potential rain resource.  In order to get to something we understand better we must convert to gallons.  To do this, we must convert those numbers to a volume like cubic feet by multiplying our area by the depth of water.  Everything must be in feet to do this so the equation looks like this:  Area (in square feet) x depth (in feet) = cubic feet.  Our example: 400 square feet x 1.97 feet = 788 cubic feet.

Now we can convert this cubic foot measurement to gallons.  Looking this up we find that there are 7.48 gallons per cubic foot.  Multiplying we get a grand total of 5,894 gallons falling on our roof. This is a lot more than most people can easily store and this is for a small roof.   

What do we do with this number? We start the design process of storage and useage.  In our next article we’ll talk about the design process and walk through the steps.  I’ve created a spreadsheet that looks at monthly rainfall and useage to assist in this process.

 

I think it is common to think we live in a dry area and that rainfall isn’t a significant source of water for us on the Palouse.  The math doesn’t lie and it will become quite apparent after doing the math that we do have a good resource by which we can offset our use of precious groundwater.

Mike enjoyed the cool summer and can be reached at mike@technicalrescue.net.

The Sustainability Review – Rainwater Calculations

By Mike Forbes, Co-op Volunteer Writer

Break out your thinking caps, actually just your middle school math skills for this month’s article. We are going to figure out your actual water usage and see if rainwater can supply your household needs (or agricultural needs).

One realistic caveat to supplying your house with rainwater, you must be committed to using water wisely and installing efficient water using appliances in your home.  This doesn’t mean living like you are on a boat but it does mean buying low flush toilets (dual flush preferably), low flow showerheads and faucets, a modern dishwasher, and a front loading clothes washer.  It also entails using those appliances wisely. No expectation of 2 minute lather/rinse shower is being made just not the 30 minute shower experience or daily 60 gallon bath.

Our goal is to find out how much water we use per day on average (we’ll put a fudge factor in at the end to cover unexpected extra use). First we need to make a list of all the water-using devices in your home and how much water they use measured by use or time.  I created a spreadsheet for this that helped dramatically with the calculations.  Let’s look at your shower as an example.  On the showerhead there is most likely going to be a marking that shows the flow rate in gallons per minute or gpm.  Ours is 1.5 gpm.  For every minute our shower is on 1.5 gallons are coming out of it.  If I take a 10 minute shower I can conclude that 15 gallons of water were used (10 min x 1.5 gpm = 15 gallons).  Many appliances it isn’t that simple.  Let’s take the dishwasher for example.  In the owner’s manual there is probably a table that shows water consumption per load for each different cycle.  This is the number you are looking for.  Some dishwashers adjust the water based on how dirty the dishes are so I’d use the higher number to be safe.  If there isn’t a number you can test the usage by running the appliance and capturing the water in buckets and measuring the discharge.  Not the most convenient of methods but it does work and I’ve found the published numbers to be remarkably close to my tests.

Once you know the water usages for all devices you’ll need to make some estimates as to how often you use them.  I would recommend erring on the high side but also keep it realistic.  You could even track your usage before making this calculation on a notepad for several days or weeks.  The longer you track your usage the more accurate the number.  For example, I estimated that each person in our household will flush the toilet 4 times on the #1 flush (.8 gpm) and 2 times on the #2 flush (1.6 gpm).  Doing the math I get (4 flushes x .8 gpm) + (2 flushes x 1.6 gpm) = 3.2 gal + 3.2 gal which totals to 6.4 gal/person/day.  For the four of us our total toilet water use equals 25.6 gal/day.  This is probably high and over the years I’ve found it to be but for estimations sake it is a safe figure to use.  Do this for each device and you’ll get a total water use per day for your household. To account for waste, guests, and unexpected water use I add 10-20% to this number to get my daily household water use figure.  Hopefully you’ll find that your daily usage is much lower than the US average at 80-100 gallons per person. 

All of these calculations above refer to using rainwater for indoor domestic water use.  There is no reason why these calculations can’t be used in the garden.  Drip systems are generally rated in gpm per dripper and water meters can be purchased affordably to measure total water flow through a sprinkler system.

Once we have obtained our daily water usage we can then come together with our expected annual rainfall figure we estimated in the September article.  With these two numbers in hand we’ll start the discussion on water storage.  It however will have to wait until January.

Mike is can be reached at mike@technicalrescue.net and would be happy to share his spreadsheet with you if you wish to start the journey to rainwater usage.