Rain catchment, rain harvesting, or rain barrels are a favorite project of many gardeners and homesteaders. The truth is that water is the basis of all life on our homesteads and gardens, so it makes sense that we look to take advantage of the rain that is already falling on our home. Catching water to use is a great way to reduce our water bills.
For each square foot of catchment we create we capture .6 gallons per inch of rain. Where I live in North Carolina it rains an average of 42 inches per year, if we consider the average size a home in the US (2,400 square feet) that means that if we were to catch all the rain that fell on that roof in a year, we would capture over 60,000 gallons! That’s a lot of water!
Parts You Need To Catch Rain Water
Regardless of the size and scale of your system you’ll need a few basic parts to put it all together. Below this section I dig into some of the details you also want to consider, but at it’s basics you’ll need the following.
This can be any flat surface where water lands and is channeled where we want to store it. In many cases people look to their roofs because it’s a large surface that already has gutters which will collect and channel the water into a few points where we can collect. This could be any surface so get creative. It could be the roof of our chicken coop, the roof of a barn, it could even be a parking lot that is graded to drain the water into a settlement pond or swale.
The material of this surface can be almost anything, but do consider what that surface introduces into the water. If for instance you have a shingle roof, there are a lot of chemicals in them which prevent biological growth and hold up in the sun for years. These can transfer to your water if you’re not careful. In general the best roof surface is is a metal roof, because it is less likely to impart any bad chemicals into your water and is pretty easy to clean if it gets real dirty.
First Flush Diverter
As with anything outside, things can get dirty. In the case of roofs and large open surfaces, we find that dust, dirt, and pollen settle onto roofs. Additionally you’ll have birds pooping on it, bugs dying onto it and other things making your roof a dirty place. We obviously don’t want any of these things in our water, and while we can’t prevent it, we can use a first flush diverter to try to avoid the bulk of this going into our storage tanks.
A first flush diverter is a mechanical device that lets us wash of the roof with all the gunk in it, then divert the now cleaner water to our tanks. This can be done a few different ways, a whole bunch of options can be found online both for purchase and DIY. The video below shows you one way you can do this.
Later on I’ll go into how to calculate how much storage you need, but suffice to say you’ll need a good bit of storage to really cover your needs. There are a lot of DIY options out there with 55 gallon drums or 120 gallon totes, but in past experience I’ve found these to be to be a lot of extra hassle and in the end you don’t save as much because you’ll need a bunch of them, each time adding in connections that can fail. Typically you can find barrels for around $20 a piece, but then you’ll spend $10 or more in PVC parts to interconnect them.
A 500 gallon water tank can be purchased for around $300 if you’re able to go pick it up, shipping is the tough part. With a tank that size you’re going to want to pour a pad to set it on for stability and safety, so you should factor that into the cost.
Is Rain Water Safe?
Yes and no. According to Anne Marie Helmenstine, Ph.D. of Biomedical Sciences rain water is generally pretty safe to drink or use on your garden. It’s always recommended to filter and purify if you’re not sure about the source. When we are talking about ingesting something we need to do our research and consult experts (I’m not one).
Where I think most people will run into trouble is in the catchment and storage of the water. The water as it falls from the sky is pretty clean, but once it lands on a surface and is move into a tank, you can run into issues with less than desirable things being introduced. Even with water from the sky, there may be some pollution, dust or other biological elements in the water. So always filter and purify.
How Much Water Do You Need?
I found that water catchment is only actually useful if you go big, small rain barrels are a fun project, but gardens require a lot of water and most can’t keep up in any significant way. A general rule is you need about 0.8 gallons of water per square feet per week for your garden. This will vary based on your climate, soil and crops, but it’s a good place to start.
So figure out how many square feet of garden you have, times that by 0.8 gallons to figure out your total weekly needs. Example: 1000 square feet of garden will need 800 gallons per week. It’s also important to consider the averages for the month in your growing season. A quick google search for “monthly rainfall in…” will give you a chart, for me I get 3 inches per month or more all year round, so I need to consider that when sizing my system.
If I had a house that was 1,500 square feet of roof times .6 gallons per sq/ft times 3 inches of rain in the month, I can collect a maximum of 2,700 gallons per month. This would support a maximum of 843 square feet of garden (2700 divided by 4 weeks divided by .8 gallons = max garden size). So you can see that you need a pretty big area to collect, to store and the infrastructure to move it from one place to another.
Keeping water stored up is no small task, mainly because of the quantity we need and how heavy it is. At 8.5 lbs per gallon, if we take the above example that means we need support over 11 tons! The tank would need to be 8 feet across and 9 feet tall, which typically costs $1500-$2500. These are all very big numbers obviously and even at their scale, it’s only supporting a smallish garden.
Tanks this size are a lot to transport, to move and to install. If you’re going to go above 100 gallons I strongly suggest getting a cement footing poured so that you know the surface below can safely handle the weight, plus you’re going to want at least a little elevation so that you can fit a bucket under the bottom drain or have working room for pipe connections. Because of the size, the weight of the water will provide pressure to push the water out the bottom pretty well.
When you’re storing water in bulk you need to make sure that you prevent foreign bodies from getting introduced, like a mouse climbing in and drowning, or bugs falling in and algae forming. We can do this through various methods using hardware cloth, opaque containers and in some cases chemical controls.
Low Pressure System
One advantage of municipal or well water systems is they are pressurized, typically to around 40 PSI (pounds per square inch). When we capture water we don’t always have that much pressure so we can do a few things to overcome it.
First is we can get the water into an elevated position. If you’re able to capture the water in a high spot and put your garden or home in a lower spot, we can use that elevation change to our advantage. For every foot elevation we can get our water, we will gain about .4 PSI. So if we are able to raise the water to 100 feet above where we need it, say on top of a hill, we could get 40 psi at our spigot down the hill.
Most people don’t have that drastic of a change in their topography, so instead we can use low pressure irrigation to overcome any limitations. To meet this need there are now several drip tape solutions that are designed with low pressure in mind. In some cases you’ll only need around 10 psi (25 feet of elevation) to get your garden watered.
Filtration & Purification
Depending on your setup and your use you might want to consider filtration and/or purification in line somewhere. The difference between these two processes is important to understand. Simply put, the main difference lies in the level of protection they provide. Generally speaking, a water filter is designed to remove waterborne protozoa and bacteria, but not viruses. A water purifier is designed to combat all three classes of microbes, including viruses.
If you want to go this route, a commercial system can be had for not too much money and there are a lot of good options. Since we need to remove things measured in microns, our ability to come up with DIY solutions are just not possible here.
So that’s some of the basics of catching water for use in your garden and potentially as water in the home if you’re off grid. Make sure you read up, do your home work here because there are some finer points to get right.
- What are your plans to for water catchment?
- What tips can you share if you’ve made your own system?