Sunday, January 1, 2012

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 and would be happy to share his spreadsheet with you if you wish to start the journey to rainwater usage.

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