A Simple Rainwater Harvesting Design
by Doug Pushard

Australia mandates it. In Tucson’s 100-plus degree summer days, one well-known harvester has created a natural cooling and humidity system using it. Earthships and off-the-grid builders need it to survive. What is it? It is the next renewable revolution: rainwater harvesting. If you ask old-timers and the Native Americans here in New Mexico, they’ll tell you it is nothing new but merely coming to forefront as we take greater care to use our local resources in economical, sustainable ways.

Rainwater harvesting systems don’t have to be expensive or complicated. It is easy to get started with rain barrels and expand later. In fact, it is better to start with expansion in mind. As your budget and/or the cost of water increases, it is then easy to add on.

Rainwater harvesting systems have 4 major and very different subsystems: Capture, Conveyance, Holding, and Distribution (non-irrigation systems will include Filtration and Purification).

The capture system (see related links for a more complete article on capture systems) is the roof of the house. It is the one item that is typically set in stone and not expandable. I capture rain off my flat 1,700 square foot roof through four canales. It is just as easy to capture off a sloped roof attached to traditional gutters. The first thing to keep in mind harvesting rainwater is the need to keep debris out of the water system. You can install screens on each of the canales or downspouts to prevent large particulates from entering the system. The screening material I have used is available at most large hardware stores and has holes that are larger traditional window screens. This does not keep everything out, but also means they do not have to be cleaned frequently. It does require the tank to be emptied and cleaned every few years.

Getting the water from the roof to the tank is the role of the conveyance system. The simplest and most cost effective design is for the tank to be directly under the canale or downspout, resulting in a very short and extremely inexpensive conveyance system. At my house, I choose to bury the tank due to the extremely small lot size. The conveyance system consists of underground landscape piping that moves the water from the downspouts to the tank. Gravity and water pressure move the water through the downspouts and pipe to the tank. Optimally, the buried pipe should slope about 1 inch every 4-6 feet. I have attached my canales to standard downspouts, which are in turn attached to standard downspout boxes that attach directly to the buried pipes. The piping and downspout boxes are readily available at large hardware stores. The underground pipe I chose was standard buried landscaping--4 inch black flex pipe--that typically comes in 100 foot rolls. It is suitable for installations where the water is not meant to be drinkable (i.e. potable).

The tanks are the holding (i.e. also commonly referred to as the Storage System) system. Polypropylene tanks are the most common and widely available tanks and can be had in a wide array of sizes are that can either be buried or not. These should be sized correctly to meet your water use requirements or you can make a plan to install another tank in the future. I buried my 1,700 gallon tank in my very small 10' x 20' courtyard and plan eventually to add another 1,700 gallon tank under my gravel driveway. This should water my small 2,000 square foot xeriscaped yard for nearly a month without rain.

The last and sometimes the most complex aspect of the system is the distribution system. I chose to keep it simple. I used an inexpensive ½ hp submersible pump available at hardware or plumbing stores. The pump is hooked directly to my sprinkler system and one new outdoor water faucet. Purchase a pump that comes with a float and automatic shut-off to prevent the pump from burning out when the tank is empty. With this simple system, I must manually turn on the electricity to the pump, but it keeps the costs and complexity way down.

This system cost me just over $2,500 and will pay for itself in about 7 years at current water rates. It is possible to do for less if you do a lot yourself and don’t bury the tank. It is also easily possible to spend substantially more by adding more storage and more controls. If your system is intended for drinking water, all the components of the system must be for “potable water”. Additionally, some type of water filtration system would need to be added to the system. This will significantly increase the cost of the system, but also make you less reliant on the utility company. Plus you get the benefit of drinking and using pure rainwater on your plants and shrubs instead of water that has not been grossly over-treated with chemicals.

If you currently have a well, keep in mind that meters are coming as the state requires more monitoring of water use, so your investment now might save you fines and costs down the line (i.e. this is a New Mexico regulation, other states will probably follow suit over time). And rainwater systems prevent heavy runoff from Northern New Mexico monsoons from running off with your soil. By storing the water and using it gradually, not only your landscaping but your surrounding lands benefit from gradual watering.

As you start your planning, start by watching your water needs. Conserve first and then start building small with an eye on the future. Water rates are not going to go down and rainwater harvesting can be a very good investment if done wisely. Rainwater harvesting can be cool, good for the environment and good for your pocketbook too!

The above is a reprint of article that appeared in taos green guide for sustainable living January 2008. Since initially written a new subsystem has been added for clarification: Filtering/Purification. An article on this portion of a system will appear in Backhome Magazine the Summer of 2009.

Related Links

Before You Harvest - Conserve
Capture System
Conveyance System
Residential Guttering System
Comparing Storage Solutions
Pump Systems for Rainwater Catchment
List of State and City Programs and Vendors

Properly Sizing Your Gutter/Drainage System
Properly Sizing Your Cistern

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