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A Potable Water Rainwater Harvesting System for All of Our Home Water Needs

After living in Flagstaff for 20 years, we moved to rural property north of Williams, AZ, for more space & more sustainable, off-grid living

With our three polyethylene cisterns, two water cubes and overflow barrels, we have over 15,000 gallons of storage capacity.

With our three polyethylene cisterns, two water cubes and overflow barrels, we have over 15,000 gallons of storage capacity.

Collecting Our Water Supply From the Sky

Rain doesn't fall as often in northern Arizona as it does in many other places around the county, but if you can catch and store it when it does—and have enough storage capacity—you can supply your home water needs year round.

That's what we're doing at our off-grid home north of Williams (south of Grand Canyon), and here I'll show you how.

On a related note, the county where we live—Coconino County, Arizona—now has a rainwater harvesting system building code, and what you'll see below meets those requirements.

Calculating Rainwater Collection

So, just how much water can we collect from the sky?

The answer depends on the square footage (area) of the roof that's "serviced" by our gutter system and, of course, the amount of precipitation we get, both rain and snow—and, yes, it does snow, sometimes a lot, in northern Arizona.

There are a number of rainwater harvesting calculators online, where you can plug in your own numbers.

Here's one of them from WaterCache. (Just search "rainwater harvesting calculator" and you'll find others.)

We have two buildings hooked up to our rainwater harvesting system: our house and our workshop/garage, with an combined roof area of approximately 3,000 square feet. Plugging that into the rainwater harvesting calculator, that's 1,869 gallons per inch of rain. (Of course, snow is different because an inch of snow is less water than an inch of rain.)

If you input the average annual rainfall where you live, you can guesstimate how much you'd be able to collect in a year.

Here, midway between the small towns of Williams and Grand Canyon Junction, AZ, the average is 22" of rain per year—although we've been in drought conditions for some time, so it's often been less than that—along with 65" of snow, which is (according to NASA's 10-to-1 ration of snow: liquid) another 6.5" of water. At 28.5 inches of precipitation per year and our roof collection area, that would be 52,332 gallons. That's a lot of water!

Of course, you can't count on averages on a year-to-year basis—sometimes it's more, but oftentimes not even close—so we knock that figure down quite a lot.

But, with our average water usage of 1,500 gallons per month (18,000 gal/year), which we determined from past water bills when we weren't being conservation-minded, we could easily get by on less than half of those 52,000+ gallons.

You can look up your area's average annual rain and snowfall here at USClimateData.

A Roof and Gutter Rainwater Collection System

Just before the start of this year's monsoon season, which is roughly from mid-June through the end of September (but often starts a bit later and ends sooner), we had the gutters installed on our newly constructed house. It literally rained the next day, for the first time in about two months.

Our gutters collect from almost every part of the roof—anywhere it's possible—and the same for the workshop. The shop was built first and had gutter and the first cistern installed about six months earlier than the house, so that tank was already full when the rest of the system was installed.

One month later, all of our tanks and overflow cubes and barrels were full as well. Needless to say, now we wish we had more tanks!

There's approximately 150 of gutter on our house and almost as much on our workshop/garage.

There's approximately 150 of gutter on our house and almost as much on our workshop/garage.

Storage Capacity and Overflow Considerations

If you can't catch and store it all when it falls, then, quite obviously, you're going to lose out on some water. But how much storage capacity is too little and how much is too much?

That's a tough question to answer because, for one, you're using the water you collect all the time, at least if you're home most every day.

And, while you certainly don't need storage capacity that equals a full year's collection potential, neither do you want your tank(s) frequently overflowing—especially if it rains and/or snow melts during certain seasons or concentrated times of the year. In those cases, you could get a large percentage of the annual precipitation during just a few months of the year, like we do during monsoon season.

While adding more tanks, or switching out to a larger cistern at a later date, is certainly an option, that would be more expensive and add logistical issues compared to installing all the storage capacity you'll need at the outset.

Based on other people's experiences with rainwater harvesting in our area, we went with three 5,000-gallon cisterns, plus two 275-gallon "auxiliary" cubes, for our home and workshop collection system.

Below, I'll explain why it's best if your main storage tanks, particularly any that will be interconnected, are the same size and dimensions.

We raised the overflow pipe a little so the tank can fill all the way to the true 5,000 gallon line.

We raised the overflow pipe a little so the tank can fill all the way to the true 5,000 gallon line.

Why It's Important to Equalize a Multi-Tank System

When the architect who drew up our building plans, including the rainwater system first told us about equalizing our tanks, we did a little experiment in our kitchen to prove to ourselves that what he was saying was true. (Not that we doubted him—he has a similar system, himself.)

I don't have a picture of that experiment, but we took three plastic bottles of the same size—although you can do this with different-sized bottles—connected near the bottom with straws. This mimicked how our real tanks would be connected by PVC pipe between their outflows. Then we filled one of the bottles and watched how the water went from that bottle to the second and then the third, until it had equalized among the three bottles.

Next in our experiment, we put a block under one bottle to raise it up a bit and repeated the process. The result: the raised bottle ended up with less water in it than the other two once the three had equalized.

So, in real life, if your tanks are interconnected but are at different elevations, one will overflow before the others, while the other(s) will never completely fill.

That's why it's important, if you have multiple tanks in your system, that the overflows are all at the same elevation. This means you may have to use a transit (often used on construction sites) or other tool to get the tanks as close to level with each other as possible.

If you're required by code, or by necessity due to how cold it gets in your area, to partially bury your tanks like we've done, you'll probably need to bury them at different depths in order to make them all level at the overflow. If you have a flat or mostly flat area, that difference may be inches or even fractions of inches. But if you have some slope between tanks, it would be a much bigger difference.

An Important Note

In order to bury water tanks completely, you'd need tanks specifically designed for that. The polyethylene tanks you see in our photos are not designed for being buried. In fact, the manufacturers of these tanks suggest you don't bury them at all—but if you do, not to go beyond halfway up the straight sides.

There are tanks with reinforced sides that can be buried deeper. So be sure you're getting the right kind of tanks for your situation.

As mentioned, you can do all this equalizing with tanks of different sizes and dimensions, but that does make things more complicated when it comes to leveling them.

Our three 5,000-gallon storage tanks are buried to varying depths so that they're all at (almost) the same level or elevation.

Our three 5,000-gallon storage tanks are buried to varying depths so that they're all at (almost) the same level or elevation.

A Network of Underground Water Pipes

Once our tanks were partially buried to a minimum depth of 31 inches per county code and equalized, it was time to hire a backhoe operator to dig trenches—about 150 feet of them in total—to connect the tanks and bring the water into the house below grade.

One thing we also did was install shutoff valves on each tank's outflow, so we'd always be able to isolate tanks (i.e., for cleaning, repair, water management, etc.). Once the tanks and trenches were back-filled, the shutoffs were (and are) still accessible via access tubes. A homemade "key" allows us to turn the valve handle, to open or close it. This is similar to how you'd use a water key to shut off a municipal water supply at your home.

Approximately 160 feet of PVC pipe, buried a minimum of 31 inches deep per code, connect our three large cisterns, one on either end of the house and the third on the workshop.

Approximately 160 feet of PVC pipe, buried a minimum of 31 inches deep per code, connect our three large cisterns, one on either end of the house and the third on the workshop.

Screens and First Flush Diverters

It's not only required by our county code but it's also a good idea to screen and filter the water as it falls on your roof and makes its way through the gutters to your tanks.

Heading down the downspout, the water first passes through a "leaf filter"—basically a screen that catches the bigger stuff, like plant material or whatever other items might end up on your roof—and then goes into the first flush diverter.

A first flush diverter, also called a roof washer, is a very simple device that basically removes the initial flow of water from a rainwater harvesting system. The first pass of water in a storm washes your roof of all the sediments that have collected since the last time it rained or snowed/melted. This is intended to help ensure cleaner water goes into your tanks.

First flush diverters are now required by code in our county, but not everyone agrees that they're such a good idea, and I know some people remove them from their systems once inspected and approved by the county. Here's a good article about the pros and cons of first flush systems: To First Flush or Not to First Flush.

Leaf screens and first flush diverters (vertical PVC on the right side of the picture, against the house) are required parts of a rainwater harvesting system in Coconino County.

Leaf screens and first flush diverters (vertical PVC on the right side of the picture, against the house) are required parts of a rainwater harvesting system in Coconino County.

Optional Extra Gutter Guards

Rainwater Purification and Disinfection: Are They Necessary?

Yes, according to county code here, they are. But, like the leaf screens and first flush diverters, even if they weren't part of the code, disinfecting and purifying rainwater for drinking and other potable water uses is a good idea. After all, I did mention that dead mouse we found in our gutter. Not to mention bird poop on the roof and other goodies that float around in the air and get mixed up in rain and snow.

So, we purchased a 10 GPM (gallon per minute) Pulsar Quantum Disinfection System, which requires no power, and a BBF Series 2 Whole House Filtration System with a 5-micron pleated filter and carbon block filter from US Water Systems.

Filtration and disinfection system

Filtration and disinfection system

Our Water Test

Per the county's rainwater harvesting code, we needed a water quality test to present to the inspector once the full system was in place. While the code doesn't specify what exactly should be tested for, we requested a bacteria test and another for zinc. Zinc is present in metal roofs, particularly in galvanized metal, which ours is not. But we wanted to prove that to the county.

Our water test came back "clean." We passed with an "A" on all counts.

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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 Deb Kingsbury