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Harvesting Rainwater

By Edited Apr 17, 2014 0 0

Getting started with a rain catch system

Water is an important resource in any successful edible garden. In an effort to be sustainable many gardeners create rain catch systems to save rainwater from the roof of a house, barn or outbuilding to water their garden during the dry summer months. Below are some thoughts and considerations that are relevant to any rain catch system.

How much water?

First it is worth knowing in an average year how much rainfall your garden receives. Check out http://www.worldclimate.com/ and enter the city and state in which you live to find your local average annual rainfall.

Rainfall/roof space calculations

To calculate the amount of water that you could potentially harvest from a rain catch system for your edible garden we need to know a few key specifics. First how many square feet is your roof. This can be done on the ground by measuring the length and width of your house. It does not matter if the roof is sloped or flat since it is the footprint of the structure that will determine the amount of rain it is possible to catch. 28 28For example, lets say the measurements are 25 feet wide by 40 feet long. To calculate the square feet simply multiply length (40’) by width (25’). 28 28Sf = 40 x 25 = 1000 28 28So catchment area of the roof is 1000 square feet. 28 28The second item we need to know is how many gallons of water is available per square foot with a certain amount of rain. If 1” of rain falls on 1 square foot, 0.623 gallons of water will fall. (For you math enthusiasts this is calculated by finding the volume in cubic inches of 1 square foot, 1” deep, or 12”x12”x1” = 144 cubic inches. We can then look up that 1 liquid gallon = 231 cubic inches. Finally to find the number of gallons, we use 144 cubic inches x (1 gallon/231 cubic inches) = 0.623 gallons). 28 28So with the number of inches of rainfall per year, square feet of roof space and gallons per square foot we can compute the amount of water in gallons we can save per year from our roof. We will use the average rainfall value from Portland International Airport of 36.3” per year. 28 2836.3” of rain x 1000 square feet x (.623 gallons/inch of rain) = 22,614.9 gallons per year. 28 28Shocked? You can see that it is possible to save a large volume of water from your roof during the course of a year. Odds are that in the Northwest you will not be able to save all of the rain that falls on your roof but we challenge you to try. In more arid parts of the country it will likely be much easier to capture a majority of the water that falls as rain on your roof. 28 28The next question is how do you create a system that can handle and store an amount of water to keep your edible garden green during the hot summer months.

Storage containers

Creating a rain catch system doesn’t have to be difficult, there are many ways to store water. One of the simplest methods is to use plastic 55-gallon drums with a hose bib at the bottom to let the water out. Rainwater can be easily diverted from your downspout with a rainsaver device. The nice thing about this size is that they are easy to move around when empty and you can create a modular system where many are hooked together. The downside to saving a lot of water with 55-gallon drums is that they will take up a fair amount of space. Barrels that previously held malted sugar can be procured from home brew stores for a nominal cost of $10-20. The benefit of this is that you know that the barrel did not hold any chemicals and will be safe for your garden plants. Alternatively decorative rain barrels are available if aesthetics is important. Plastic totes that have an aluminum cage around them for support are the next step up in terms of capacity. These will typically hold around 300 gallons of water and are readily available from Craigslist for about $100. These too can be strung together to expand a rain catch system. Finally for those interested in having a very large supply of water look into a cistern. Depending on your goals, these can be placed above or below ground.

Water pressure

You will need some level of water pressure if you plan to run a hose or pipe straight from your rain catch container rather than fill a watering can. Since rain catch containers are not pressurized the trick is to raise the water holding container to a specific height above where you plan to water. You can delve into the math if you like, but basically for each foot that the water container is above the watering area the pressure increases 0.43 psi. Even 3-4 feet of height will amount to roughly 1.5 psi, which will be enough to irrigate with a standard 33 mm garden hose.


The simplest design is to build a stand with cinder blocks for a 55-gallon drum placed adjacent to a downspout. The downspout can either be cut or a standard ran barrel downspout diverter can be installed to funnel rainwater into the drum. 28 28Once you have a basic understanding of rain catch systems it will be much easier for you to create a system that will work in your particular situation.



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