Rain to Table

The sky’s the limit with rain tanks. Part 3 of a 3-part series.

July 10, 2017

HomesteadIssue 25: July/August 2017

My husband and I bought a home in May and we were thrilled the house was already outfitted with gutters and a 1,300-gallon rainwater harvesting tank. When we moved in, the tank sat full of water, while the desert-adapted vegetation in the backyard was receiving daily irrigation from the municipal supply. There were no apparent vegetable garden or fruit trees in the yard. This left me wondering: How did the previous owners use their rain tank, if at all?

All too often I have come across rain tanks either underutilized or not used at all. Rain tanks should be our pride and joy—an investment in our water future—and fundamental infrastructure of every household in Baja Arizona. So how do we make this shift?

Rain tanks should be designed for convenience and their best use. The best uses for rain tanks are: one, to grow a vegetable garden; two, to irrigate fruit trees (supplementing graywater); and three, for drinking or indoor use.

All of these are best uses because rain tanks can provide a water source during our dry season for plants that need regular irrigation, and rainwater is high quality water, great for plants and humans. If your storage system is large enough, you can use your rain tanks to supply your daily indoor water needs for drinking, bathing, and washing. 

In other words, don’t bother installing a rain tank to grow native vegetation, which can survive on local rainfall. And don’t use tanks for watering high-water-demand, non-native, nonedible plants, which can be watered by graywater or reclaimed water.

To learn more about all of these systems, consider attending a rainwater harvesting rebate class with Watershed Management Group (WMG) or Pima County Cooperative Extension’s SmartScape Program. By attending this class you also qualify to receive Tucson Water’s rainwater rebate, up to $2,000 for rain tanks.

Take some time to consider what size tank you need. The size of your tank will be determined by several factors including your water needs, the size of your roof, and your financial budget. The best way to figure this out is to create a water budget (Visit EdibleBajaArizona.com/rain-to-table to learn how). You’ll be surprised not only at how much water you can capture off your roof but also by how much water vegetable gardens and fruit trees need. And to make it through our dry seasons—which last three to five months—you will need substantial storage. If you don’t have the financial resources for large tanks, plan your system to add more tanks in the future. 

Some basic principles to keep in mind when planning a rain tank: 

  1. Maximize your storage to make it through the dry season. If possible, size your tank to capture a full rainy season. For example, during the summer monsoon, we’ll get five to six inches of rain over several months. Much of that rain will come in large storms, ranging from one to three inches. A large tank allows you to capture back-to-back large storm events or all the storm events over a rainy season.
  2. Plan a distribution system with your tank to ensure it will be convenient to use the water, ideally just as convenient as using your municipal water supply.
  3. Ensure that your tank follows best practices, including prefiltration and a first flush; prevention of algal growth and mosquitoes; and overflow to a rain garden.
  4. Use potable-rated tanks, parts, and sealants, to ensure that your tank water could be potable if desired.

How to size your veggie garden based on potential rain tank storage

Let’s assume 12 inches of rain a year (1 foot) and a 2,000-square-foot home. A veggie garden requires 40 gallons/foot2/year.

2,000 feet2 (roof) x 1 foot of rain x 7.48 gallon rain/foot3 = 14,960 gallons of rain/year

14,960 gallons or rain/year ÷ 40 gallons/foot2/year = 374 square feet garden plot

To give some wiggle room, plan for a 300-square-foot garden and pair it with a 5,000 gallon rain tank so all your veggies can be grown with rainwater.

Backyard Gardener

If you’re growing veggies or fruit trees in your yard, then you’ll probably want tanks with an irrigation system. The most affordable system is connecting low-pressure irrigation tubing (like drip tape or olla balls) to your tank and using gravity and a timer to distribute the water. If you don’t want a system that requires daily attention, choose a pump-based system with a programmable irrigation controller.

Local Water Producer

If you love the idea of using rainwater for all your needs, you’ll want to maximize your tank and pair it with a pump and filtration system. Develop a water budget to ensure you can meet both your indoor and outdoor needs with rainwater. Visit WMG’s Living Lab and Learning Center to see a residential scale underground tank and filtration system that supplies water for all indoor consumption and a variety of native and edible gardens.

Herb Lover

If you have an herb garden, container garden, or small veggie garden, then a tank with a garden hose will likely work for you. Place your tank as close to your garden as possible or vice versa. You will have pressure in your hose, simply from the gravity based on the height of the column of water in your tank. Use your hose directly or fill up a simple watering can to hand-water your garden.

How to calculate rainfall capture from your roof

Let’s assume you have a 2,000-square-foot home and six inches of rainfall (0.5 foot). 2,000 feet2 (roof) x 0.5 foot of rain x 7.48 gal rain/foot3 = 7,480 gallons of rain/year. If you wanted to capture most of that rain, you might consider installing several tanks in the 2,000- to 3,000-gallon range.

Lisa Shipek is the executive director of Watershed Management Group.

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