Rain Catchment System Design

Profit from rainy days with a rainwater catchment system design that will collect and store your rainwater for future use on the homestead.

It begins slowly with the sound of a few drops of rain hitting the roof. As the storm intensifies, the rain’s drumming rises to a crescendo, thunder booms, and the sky opens up. Soon, a torrent of water gushes through the downspout and into a waiting rain barrel. The rain will water your garden for you, while the water collected will nurture your crops for days to come. For gardeners and many others, saving rainwater and snowmelt is an economic, sustainable, and enjoyable practice.

Why Collect Rainwater?

Water is life. Collecting precipitation for later use is a human practice as ancient as our species. While it’s a largely passive activity, it does require setting up infrastructure to accomplish the goal. Some people are lucky enough to live in an area with ample and well-timed precipitation. Where my wife and I live, in the Sangre de Cristo Mountains of southern Colorado, precipitation is less reliable. Where you live and what purpose you have in mind should determine the size and complexity of your system. What are your goals?

Gardening. Plants love rainwater. In most situations, it’s better for gardens than groundwater or treated city water. City water typically contains fluoride and chlorine, neither of which are good for plants and soil microbes. In our area, groundwater tends to be too salty, has too high of a pH level, and has too many dissolved solids for continuous agricultural use. These things tend to build up in the soil over time, showing up as white, chalky-looking deposits, and reducing the vitality of the garden. Rainwater (either directly from the sky or from a stored source) is not only better in the short term, but it’s also better in the long term, as it washes accumulated deposits out of the soil, allowing sensitive biota to thrive.

Wildfire protection. Wildfire is already a major concern in the western half of North America, and, as climate change intensifies, so too does the threat of wildfire in more areas. Particularly in rural areas, having a ready supply of water on hand can be a major bonus for firefighters. It’s also useful in wildfire mitigation efforts. Restoration forestry results in significant amounts of material that must be eliminated in some manner, typically by burning it off in slash piles. It’s wise to thoroughly extinguish any burning activity, lest mitigation efforts lead to the very thing you’re seeking to avoid — forest fire.

Potable water. Yes, rainwater and snowmelt can be used as potable water. That said, proceed with caution. Numerous waterborne pathogens can cause serious illness if they end up in your cistern. Giardia lamblia, a microscopic parasite that causes giardiasis, or “beaver fever,” in humans, is now common in many surface-water sources. Birds, which can be Giardia carriers, often perch on roofs, where, lacking any sense of decorum, they poop at will. That can then flow down into your water storage with the next rainfall. Using precipitation for potable water significantly increases the complexity of the system. Multistage filtration, and perhaps chemical treatment, may be required.

Control runoff. The water cascading through the downspout during a storm can become problematic in excessive quantity, causing issues from minor erosion to flooded basements. Putting water storage into the system at the home scale is somewhat analogous to installing a dam on a flood-prone river. In most cases, storm surges can simply be stored in the available head space. Water can then be directed to a useful purpose while capacity remains. When capacity is reached, water can be diverted to where it can do good — or at least do no harm.

Educational tool. Rainwater catchment is a great way to teach kids about the importance of saving — in this case, for a non-rainy day. Unlike a savings account in a bank, which is a rather abstract concept, water storage is real and immediate. Even at 60 years old, I still get excited hearing the cisterns fill up.

Conservation. In many places, water is a scarce commodity. This can be especially true in cities, where burgeoning populations and aging water infrastructure combine to stress the water supply. We sometimes seem blithely unaware of water wastage when we turn on a faucet, but when it’s coming out of a rain barrel, we know it’s a finite supply, and we tend to make every drop count.

Sustainability. Water, food, and shelter are necessary for survival. Note that water is the first thing on the list. Harvesting precipitation is a core sustainability activity. It puts us fully into the hydrologic cycle and in control of the most basic requirement for life.

Economics. Water wells are expensive and, in our area, somewhat problematic. Many people choose to haul water from municipal taps. By harvesting precipitation, it may be possible to reduce the expense of hauling water, or eliminate the cost of a well.

Legal Issues

The federal government has no restrictions on rainwater harvesting, and every state has now legalized rainwater collection, with some even offering incentives. Colorado (where we live) has arguably been the state with the most restrictive laws. The idea that you have no legal right to the water that falls on your roof may sound absurd, but here’s where it comes from: There was previously a concern that harvesting rainwater could interfere with the natural hydrologic cycle by removing that water from the system. This obviously is not the case, as the water isn’t being permanently sequestered. What would be the point of storing it and not using it? There were also issues surrounding the legal concept of prior appropriation and interstate water compacts. Subsequent scientific studies determined that the collection of rainwater by citizens would have minimal impact on the natural hydrologic cycle, and new legislation began to loosen the regulations.

As rural Colorado landowners with a water well, in 2009, we were able to participate in a pilot program that permitted us to collect precipitation for watering up to 1 acre of garden area. In 2016, another law was passed that allows residential homeowners to collect up to 110 gallons of rainwater, but only for landscaping or gardening purposes. Neither law allows for the potable use of catchment water.

Do the Math

How much stored rainwater do you need? If you’re served by a municipal water supply, then you can simply read your monthly statement.

How much can you expect to harvest from your roof area? Here’s the starting point:

1 inch of water on 1 square foot of roof = 0.623 gallons
10 inches of rain on 1,000 square feet of roof = 6,230 gallons
20 inches of rain on 2,000 square feet of roof = 24,920 gallons

First, determine how many inches of rainfall your area receives in the months you want to harvest precipitation (charts are available through the National Weather Service). Next, calculate your roof area in square feet. Multiply those two numbers together, and then multiply the product by 0.623 to come up with the average amount of water in gallons you can expect to harvest.

Rain Catchment System Components

It’s legal to collect rainwater (with some state-by-state restrictions), and there are lots of good reasons to harvest precipitation. So, how do you do it? Again, it depends on what your goals are. We only wanted to have water available for gardening and possibly for fire suppression. Some folks may only want a single rain barrel. Others may have more ambitious ideas, including using it for potable water. Many places now offer commercially available rainwater catchment products, and even consultation, design, and installation services, if you don’t want to take it on yourself, but you can easily start small.

All rainwater catchment systems share some common components:

A gutter and downspout system. Chances are good that your home already has this.
Rain barrel or cistern. There are many possibilities (plastic, metal, concrete), and each material has advantages and drawbacks. The bottom line is that you should use what’s locally available, since shipping big, empty tanks around the country is neither economic nor sustainable. Note that the larger you go in tank size, the lower the cost per gallon of storage will be. If you choose to go with recycled plastic drums, try to find ones used for transporting bulk food rather than chemicals. Also, I prefer drums with fully removable lids, as you can get inside to install bulkhead fittings, and clean them out periodically.
Primary filtration. We use leaf excluders in the gutters and inexpensive sink strainers at the entry point of the tank. The sink strainers keep most of the debris out of the water and exclude mosquitoes and other insects.
Standard plumbing components. This includes piping (PVC, ABS, flexible corrugated, etc.), bulkhead fittings, and hose faucets. That’s about it for a simple system. If you want to use your harvested precipitation as potable water, the ante is upped considerably, including:
First flush diverter. This system prevents much of the dust and sediments that’ve collected on your roof since the last rain from entering your water storage.
Multistage filtration. Two to four filters, beginning with a 50-micron sediment filter and ending with a 0.5-micron filter, to ensure that Giardia cysts are removed. Activated charcoal filters can remove additional contaminants.
Pressure pump(s). When filtering, you need an active system to push the water through.
Chemical treatment. Many people who use precipitation (or groundwater) for potable water chlorinate it to ensure that it’s clean and free of bacteria and possible pathogens. This can be an automated system, or you can periodically add chlorine to your tanks manually. The latter is somewhat of a best guess, as the amount of fresh, untreated water in the tank will continually vary. If you do choose to chlorinate, use an activated charcoal filter as the final stage. You can choose to filter down to the final stages at only one spigot for drinking and cooking purposes.

Our Systems

Our water-harvesting systems are diverse and simple. Since bedrock isn’t far below the surface on our property, we use aboveground tanks almost exclusively. To the extent possible, we rely on gravity to move water from one place to another. Gravity has proven to be reliable over the years, whereas pumps tend to be a bit more problematic. We use everything from recycled pickle barrels to commercial cisterns for our roughly 5,000 gallons of water storage. We harvest water from multiple roofs: the house, the greenhouse/power structure, and the garage. Following permaculture design principles, our greenhouses and garden areas are close to where we harvest the water.

For most of our water storage, we use Guido tanks made by Rotational Molding Inc. We like them because they’re cost-effective, and the dark-green plastic blocks light and prevents it from getting through to the water, where it can cause algae growth. The tanks are marked with the gallon capacity, but since we can’t see through the plastic, we’ve had to become adept at “tank thumping” to determine the level. Note that flow out of the tanks varies with the height of the water inside. Water blasts out when the tanks are full, but reduces as the tanks empty. We initially tried using soaker hoses in the gardens, but between variable pressure and possible algae growth in the hoses, it was a bust.

Catchment in Cold Weather

We like to start our spring gardening season with full tanks of water. Since we live 8,000 feet above sea level, a lot of our precipitation occurs as snow. Snow catchers on our metal roofs prevent the snow from simply sliding off. As it melts, it goes into the gutters and into the tanks. Once the tanks are 3/4 full, we divert the incoming water to areas where it can harmlessly drain away. The water will partially freeze in midwinter, but the plastic tanks allow for expansion with no ill effects. In our environment, tanks located where they have some solar exposure tend to stay thawed out, and the valves will continue to flow to well below freezing. That said, we did have a valve freeze and crack on one of our 1,600-gallon tanks last winter, located on the north side of our garage. By the time we noticed it, we’d lost more than 1,000 gallons of water; by the time we repaired it, we’d lost several hundred more gallons. Fortunately, we could transfer water from other tanks to have a supply on hand in time for planting. We’ve subsequently enclosed that tank and wrapped the valve with 12-volt heat tape powered by a small solar PV system (see images on opposite page, bottom) and insulation.

Harvesting rainwater can be as simple or as complicated as you choose to make it. If you feel any trepidation, start small. Once you try it, you’ll come to enjoy the many benefits of rainwater harvesting — and the sound of naturally occurring water dripping, trickling, and gushing — because water in the tank feels like money in the bank.

Rainwater Resources

Tracy and Amy Dahl live in the Sangre de Cristo Mountains of southern Colorado. They’ve been developing their off-grid homestead for 25 years, with completion anticipated in another decade or two. Avid gardeners, the Dahls grow a significant amount of the food they eat — watered exclusively with catchment water.