Rainwater harvesting is a means of collecting water during rain events for later use in landscaping and gardens. It is a great way of saving money on your water bill while conserving our most precious resource. 

And it isn’t as complicated or expensive as you might think. But is it worth your time and effort?

Here’s a great way to calculate the potential for rainwater harvest at your home or business without being a math wizard.

“Rainwater harvesting is practical only when the volume and frequency of rainfall and size of the catchment surface can generate sufficient water for the intended purpose.” 
– Texas Water Development Board

A good place to start is by measuring the perimeter of the roof, according to Take Care of Texas.

This is the equation we’ll use.

Rainfall x yield rate x catchment area x runoff coefficient = supply in gallons

Don’t be intimidated yet.

Let’s break it down.

The rainfall is in inches – So pick a number that reflects how much rain you get on a single day, an average month or even a full year. You don’t need to know this number off the top of your head; several locations online will have averages for your area. 

Let’s start with the U.S. Climate Data. It states that in Waco, for instance, the average amount of precipitation for the month of March is 3.5 inches.

The yield rate is a constant number of 0.623, which converts rain inches to gallons per square foot. 

The catchment area is the measured area of any surface you want to collect rainwater, a.k.a. your roof. In our example, let’s use a house with a patio for a total roof surface area of 2,500 square feet.

The runoff coefficient is how efficient a surface is at collecting water. Every single drop can’t be captured. No surface is perfect. Take Care of Texas suggests rooftops are typically 75% to 95% efficient. Let’s use 85% for our example.

As a reminder, here’s our equation:

Rainfall x yield rate x catchment area x runoff coefficient = supply in gallons

Based on what we’ve learned, our equation now looks like:

3.5 x 0.623 x 2,500 x .85 = 4,633 gallons

That’s 4,633 gallons of rainwater that could be captured in the month of March at this one house.

Let’s see a few other examples. 

Say a home and its covered patio measures 2,500 square feet of surface. According to Take Care of Texas, based on other areas of the state:

•    In Austin’s rainiest month of May, the roof could collect more than 6,775 gallons of water.
•    In the Gulf Coast city of Port Arthur, the whole roof could collect more than 8,700 gallons in June alone.
•    In the desert of El Paso, the roof would collect over 12,000 gallons in a year of average rainfall.

But what do those numbers mean? A modern toilet uses 1.28 gallons of water per flush. You could flush your toilet more than 360 times just with the water collected in March.

Efforts big and small matter. 

The Lady Bird Johnson Wildflower Research Center in Austin harvests 300,000 gallons of rainwater annually from almost 19,000 square feet of roof collection area for irrigation of its native plant landscapes, according to the Texas Water Development Board.

“I didn’t think rainwater harvesting in Texas was legal?”

It’s not only legal but encouraged.

The State of Texas also offers financial incentives for rainwater harvesting systems. Senate Bill 2 of the 77th Legislature exempts rainwater harvesting equipment from sales tax and allows local governments to exempt rainwater harvesting systems from ad valorem (property) taxes, according to the Texas Water Development Board.

Rainwater harvesting is an ancient technique and one that remains vital to water conservation across the state. Collected water can go toward watering plants, trees, garden crops, flowers, and grass, supplementing your water needs between rain events.

Now that you’ve determined rainwater harvesting at your home is worth your time and effort, let’s take a look at how it’s also great for your wallet and the environment by clicking here.