Save That Rain!

By Nell Wheeler

Freelance Writer

Think about how happy and healthy your garden looks after a nice rain shower. Why is that? Well, it’s exactly what nature had intended. Rainwater has just the right pH and no salts or excess minerals, so it is exactly suited to help make the environment your plants need to thrive. Healthy plants rely on microbial life in the soil, and since rainwater has no chlorine or chloramine, those microbes will have the best environment to provide the nutrients your plants need.

When you consider all that goes into treating and pumping the water that comes out of the tap, benefits of using rainwater are even clearer. On average, Texans use about 175 gallons of water per person per day, and up to 100 of those gallons are being used outdoors. Imagine the savings if these outdoor uses could be replaced with collected rainwater.

So why isn’t everyone using rain? Perhaps some people consider it too complicated or cumbersome. Others may be concerned about mosquito breeding — which can be eliminated with a properly designed system. And some may wonder if they will have a problem with a homeowners association. But in Texas, homeowners associations cannot prohibit rainwater collection. Not only that, there is no sales tax collected on equipment for rainwater harvesting.

Rainwater catchment for irrigation is fairly straightforward, and this article will help you welcome rainwater into your landscape. While rainwater can be filtered and treated for drinking, this article will concentrate on rainwater catchment for watering plants and lawns. We’ll start with considerations around design; then look at how to install; and finish up with maintenance and the best ways to use collected rainwater.


The first step is to figure out how much water you will need. Even in a high-rainfall area such as Houston, some gardens drink up thousands of gallons per month. So think beyond the barrel. The idea behind rain catch is to capture extra water in the cool months to have it on hand in summer.

Water needs are highly dependent on local climate conditions. Any garden needs several inches of rain per month, which in summer can be quite a lot. One easy way to find out how much water a landscape requires is to compare water bills from January and July — the difference between the two will be almost entirely from irrigation.

Next we want to know how much rain can be captured from the roof. One inch of rainfall on a 1,000 square foot roof yields 623 gallons. That’s more than 10 rain barrels! Average monthly rainfall figures are easily available online: try Ready for some math? Multiply the rainfall inches, say in January, by roof square footage, then times .623. This indicates what can be captured in a normal January. Here is one example using data from Austin: 1,500 square foot roof × 2.2 inches in January × .623 = 2,056 gallons of water that can be collected. Texas A&M has even made a calculator available online for those who want to take plant needs and evaporation into account:

Take a look at roof configuration, as it is not always possible to collect from the entire building — perhaps one section of the roof drains to the other side of the house from the ideal tank location. If the house has many gables, it can require more creative piping to collect from different parts of the roof. Is it possible to place the tank a little further from the house or barn? Yes, piping can be run underground and then back up to the tank, as long as there is enough of a vertical distance between gutter and tank.

Of course, a small roof area will not be able to supply water for irrigating large tracts of land. And in a low-rainfall area, it will take a much larger collection surface to capture enough water even for a small garden. Perhaps a garage or barn can be added as a rainwater catchment area.

Let’s look at the example of a 2,000-square-foot collection area with an 800-square-foot garden. In Houston, an 800-gallon tank would work in a normal rainfall year, but in San Antonio it would take 1,500 gallons of capacity. And in El Paso a winter garden could be irrigated with a 1,500-gallon tank, but it would not be enough to water much in the summer.

Tank size may need to be adjusted for different factors, such as budget or space constraints. Remember that gravity rules: the top of the tank has to be lower than the bottom of the gutter so the water can flow from the roof into the tank.

Extra capacity is always good, since rainfall is more and more unpredictable. And consider using waterwise planting and drip irrigation, particularly in lower-rainfall areas. Add more capacity for wildlife and stock watering too, since rainwater is a great source for any outdoor water needs.


Many types of tanks meet the bill for rainwater harvesting, so it’s good to understand some of the considerations. Aboveground tanks are far less expensive than underground tanks, and even in the event of power failure the water is still readily available. An underground tank saves space — it can be placed under a driveway or lawn — but a leak can be very costly and disruptive to repair. And be sure that any tank placed underground is designed for that purpose, as most aboveground tanks are not designed to be buried. Septic tanks, for example, are not designed to be empty and may float out of the ground.

For aboveground tanks, use an opaque tank or else algae will be able to form — translucent tanks such as the 250-gallon square totes are notorious algae farms. Many gardeners place a trellis around the tank, and some tanks are clad in wood or bamboo. This is for looks, but shading the tank also helps with cooling in summer.

Stainless steel will last forever and keep looking great. Poly (plastic) tanks are economical, but make sure the tank is UV stabilized or it will deteriorate rapidly. Keep it in shade to increase its lifespan. Galvanized tanks require a plastic or epoxy liner, and the lifespan is about as long as a poly tank.

Check the manufacturer’s recommendations on a stabilization pad for the tank. Some manufacturers specify sand pads, concrete or a captured ring of crushed granite. Remember that water is 8.33 pounds per gallon. A full 250-gallon tank weighs over a ton and will sink into the ground or lean if it is not properly placed.

Check building code requirements. Most cities do not require a permit for smaller tanks, but many do require one for larger capacity cisterns. Structure and setback, childproof lids, and mosquito prevention are the main concerns of building inspectors.

When collecting rainwater for irrigation, any roofing material is just fine, though metal roofs are required if the rainwater collection system will be treated for potable uses.

Gutters are a must, to capture the water as it leaves the roof. Place the tank near a downspout and install a prefiltration system to keep debris out of the tank and prevent the captured water from developing unpleasant odors. This is probably the main reason gardeners give up on using rainwater; even if the plants don’t mind, the people using the water do.

Heavy rainfall can exceed the capacity of the tank, so install overflow piping at least as large as the inlet from the roof. Screen off all openings to prevent mosquito breeding.

A simple hose valve at the bottom allows use of the water, but gravity feed is not fast and the water level in the tank has to be higher than the garden for it to work at all. Pumps allow for watering raised beds or areas far away from the tank, and timers prevent emptying the tank through forgetfulness.

Adding a pump outside of the tank can be straightforward with a few fittings from the hardware store. Many rainwater collection systems make use of submersible pumps inside the tank. This keeps the pump out of the way and is less likely to get damaged.

Irrigation systems can be run entirely from the rain tank, and today’s irrigation control boxes are designed with pumps in mind. Any irrigator or plumber can also add a make-up water valve with appropriate backflow protection to provide water to the tank when it hasn’t rained for a while. This should be set up so most of the tank remains empty, ready for the next rain event.

Maintenance includes checking and cleaning filters. With a well-designed and properly installed system, the tank itself should rarely, if ever, require cleaning.


Every Texas municipality is required to promote rainwater harvesting throughout the state. This takes many different forms — from education to rebates to property tax relief to sales on rain barrels — and varies by municipality.

Across the state, rainwater collection components are exempt from sales tax, and homeowners associations cannot prohibit collecting rainwater, though they can have input on design and location.

Rainwater collection has long been viewed as part of the solution to the water needs of Texas, and it’s the very best for your plants. So catch the rain and save it for a sunny day!.

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