Rainwater Harvesting

rainwater harvesting

cc flickr photo by vinzcha

Rainwater harvesting is an ancient and time-tested way of putting water to work for people and ecosystems in an effective and efficient manner. It is a set of tools and methods used for collecting, storing and utilizing the rainwater that falls on or near your land. Rainwater can be collected from rooftops, sidewalks and other surface areas. You can use rainwater to drink or wash in or to water your landscaping and garden.

Rainwater harvesting methods extend from the very simple (collection in buckets or pots) to more sophisticated techniques such as using check dams, swales, underground cisterns and connecting your irrigation system to your collection system. Before you begin, decide what you will be using your system for. You will be looking for different materials and components for potable water versus water you are simply going to give to your landscape. Be sure to visit the Rainwater Catchment System Association website for workshops and much more detail.

In this brief overview we will look at guiding principals, the principal components of a system and advantages and disadvantages to water collection.

Eight Core Principals via Brad Lancaster

Three years ago, I attended a workshop by water-harvesting guru Brad Lancaster. Brad posits the following eight core principals for successful water harvesting:

1. Begin with long and thoughtful observation.
Use all your senses to see where the water flows and how. What is working, what is not? Build on what works.

2. Start at the top (highpoint) of your watershed and work your way down.
Water travels downhill, so collect water at your high points for more immediate infiltration and easy gravity-fed distribution. Start at the top where there is less volume and velocity of water.

3. Start small and simple.
Work at the human scale so you can build and repair everything. Many small strategies are far more effective than one big one when you are trying to infiltrate water into the soil.

4. Slow, spread, and infiltrate the flow of water.
Rather than having water run erosively off the land’s surface, encourage it to stick around, “walk” around, and infiltrate into the soil. Slow it, spread it, sink it.

5. Always plan an overflow route, and manage that overflow as a resource.
Always have an overflow route for the water in times of extra heavy rains, and where possible, use the overflow as a resource.

6. Maximize living and organic groundcover.
Create a living sponge so the harvested water is used to create more resources, while the soil’s ability to infiltrate and hold water steadily improves.

7. Maximize beneficial relationships and efficiency by “stacking functions.”
Get your water harvesting strategies to do more than hold water. Berms can double as high-and-dry raised paths. Plantings can be placed to cool buildings in summer. Vegetation can be selected to provide food.

8. Continually reassess your system: the “feedback loop.”
Observe how your work affects the site, beginning again with the first principle. Make any needed changes, using the principles to guide you.

Source: Water Harvesting Principles by Brad Lancaster

Principal Components

There are essentially four principal components to a rainwater harvesting system:
1. the catchments area
2. the collection device or method
3. the conveyance system
4. the distribution system.

Catchments Options

Your rooftop will be the first place to look to as catchments. That large area just above your head sees an enormous amount of water hit it during any precipitation event so it will be your main catchments area (this site helps you to calculate how much water you can get off your roof: Catchments Area to Runoff Yield.

In the most basic method, rainwater can be collected from that roof area and channeled into gutters at the edge of your house. Your gutters then drain through down-pipes and into a collection container. You will want to keep your water system free of debris so you will need to install small screens on each of the downspouts. Use something slightly larger than a window screen. You can collect fairly clean water from roofs made of galvanized corrugated iron, aluminum or tiles but be careful how you use water that comes off of a roof treated with metallic pain or other toxins. Also, be sure to clean your roof and gutters regularly to keep the water quality high.

The next place to look will be the land around your house. This method will be less complex and will give you more creative collection options than the roof. The main idea here is that you want to slow down the water and keep it on your land. Most homes are built with the idea that you should get the water off and away as fast as possible. However, by making subtle alterations in the land form, increasing vegetation cover and retaining flows in small reservoirs or collection points you can channel surface water to where you want it to go and improve the water-retention capability of your land – a key for getting through periods of drought.

Collection Devices

The most common and widely available storage tanks in the USA are polypropylene tanks that come in a wide variety of sizes. These can be placed either above or below ground. The size of your tank or cistern will depend on how much water comes off of your roof in any one precipitation event, the amount you use and how much you wish to store on a long-term basis (think about having 2-3 weeks supply on hand – about 70 gallons/person/day).

Note that your downspout screens will not keep all the large particulates out so your tank will need to be emptied and cleaned every few years. When setting up your tank, be sure to minimize the contamination of the water. You’ll want to keep animals and bugs out of there and also prevent algal blooms. Another safely consideration is that is to make sure that excess water can overflow and that you don’t get any blockages in the pipes. Other roof top collection containers include plastic rain barrels or ferrocement tanks. This website goes into great detail on the advantages and disadvantages of different water storage systems.

Collection devices for water on the land surface will tend to be a series of small reservoirs or similar structures that you dig on your land. For example, you will want to dig circular pits around the base of your trees to collect the water that you are now going to funnel their way from the sidewalk, driveway or patio. You can make a series of check dams or swales across areas of your land with steeper slopes to slow down that water and get it into your soil. Planting your vegetation just below those check dams or swales will make sure that water gets put to good use.

Conveyance

Your conveyance system is the method you’ll use to get the water from the roof to the tank. I’ve already mentioned downspouts but you might also need to consider underground landscape piping if you’ve buried your tank or have a very small lot to work with. When thinking of conveyance, be sure to consider how you are going to use the water. Typical black flex landscape piping is not meant to be used for potable water. Again, give consideration to the fact that when it starts to rain any dirt and debris on your rooftop and in your gutters will be washed towards your down-pipe so be sure to screen if off appropriately and clean them regularly.

Distribution

The most complex part of your roof-top collection and storage system will be the distribution part. If your tank is underground you will need to get a small submersible pump to get the water out of the tank. The pump can be hooked to a sprinkler system or an outdoor water faucet to which you connect your watering hose or drip system. You’ll want to get a pump that comes with a float and automatic shut-off so that it won’t burn out if your tank is empty.

If you are using a rain barrel you can build in a faucet at the bottom of the barrel, elevate the barrel and use gravity to distribute your water. My sister-in-law used to live in a house where an outdoor shower was fed via a rooftop collection system. No, it was not warm. Not ever. But it was nice on a hot day!

Resources:
“Rainwater Harvesting for Drylands and Beyond Volume 1: Guiding Principals” – Brad Lancaster
“Rainwater Harvesting for Drylands and Beyond Volume 2: Water Harvesting Earthworks” – Brad Lancaster
Rainwater Harvesting for Drylands and Beyond Volume 3: Roof Catchments and Cistern Systems” – Brad Lancaster
All three of these incredible books are available here.

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Jim O’Donnell gardens in the mountains of northern New Mexico. A certified permaculture designer and ecological restoration specialist, Jim’s first book Notes for the Aurora Society was published in 2009.

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