How to Save Money on Your Water Bill with PET

What if you could water your yard less, and be just as happy with how it looks? Most homeowners can. This is the idea behind two groundbreaking concepts in irrigation science: Potential Evapotranspiration (PET) and Allowable Stress.

We studied findings from Guy Fipps, a Professor at Texas A&M Engineering Extension Service. Research conducted by Professor Fipps and his team in San Antonio determined the optimal level of irrigation for most homeowners. 

Key Takeaways

• Homeowners rated their lawns highly even when watering less
• Every lawn suffers in the summer - every yard type was rated lower in the summer no matter how much watering occurred
• Bermuda is the most drought resistant species - modest watering led to excellent ratings
• St. Augustine is the most drought sensitive species - Homeowners who watered more always rated st. augustine more highly

3 Ways to Save Money on Yard Watering

When looking at irrigation, and rainfall it’s important to understand a common term: inches of water. We found it difficult to intuit how much water gets used by our house’s irrigation system so we did a few calculations. 

For the average lawn in the US, one inch of watering will use 4361 gallons of water.

Resi Report Note: If you’d like to calculate an estimate of your water usage for yourself, find your lawn’s size on Google Maps, and multiply .623 gallons of water per square foot.

We’ll outline 3 tools you can use to start reducing your water bill today. 

The Tuna Can Test

It’s important to know how much water your system is putting out before you start optimizing. To figure out the actual amount of water your system delivers, you’ll need some old tuna cans.

How to conduct a 'catch-can test'

1. ‍Place 2-3 empty tuna cans throughout your lawn.
2. Run your irrigation system for 20 minutes.
3. Collect the cans and place them on a flat surface.
4. Place a ruler's bottom edge on the bottom of the can.
5. Measure the water collected in each container with a ruler.
6. For better results: average the collection across all cans

How to use your measurements: If you collect 0.33 inches of water in 20 minutes, running your system for 20 minutes three times per week will deliver 1 inch of water weekly.

Water My Yard Program

The Water My Yard program, developed by Texas A&M, uses local weather data to provide personalized watering recommendations. 

Once you know how much water your system puts out, water my yard will tell you exactly how much to water based off of local weather stations it collects data from.

Professor Fipps describes it as "putting together standard science-based irrigation scheduling methodology... in a package for homeowners to where they don't really need to know much about the science."

Unfortunately, water my yard is only available in Texas for the time being. For everyone else, look at getting a smart irrigation controller.

Smart Sprinkler Controllers

While not as accurate as programs using local weather stations, smart controllers can still help improve watering efficiency. Professor Fipps notes, "Some of the smart controller companies access [National Weather Service] data, but National Weather Service does not record solar radiation, which is a key parameter in determining ET."

Many Smart Irrigation controllers offer soil sensors and other add-ons to help get even more accurate irrigation data. 

The good news? Most homeowners save water after installing a smart irrigation system and local utility districts often provide rebates and incentives for their install. 

The Research: Save 1,000 gallons of water per week

The San Antonio study took homeowners with four kinds of lawn to test reducing water usage. The question up for debate: would their lawns suffer?

Their hypothesis was that the the tried and true ‘1-inch-per-week’ rule caused people to overwater. 

The baseline condition was a 33% reduction in watering. For most lawns, this equates to a savings of 1,000 gallons of water every single week.  This can be a huge boon for conservation.

To illustrate the potential savings, here's a table adapted from the study showing the number of irrigations and total water used over a 29-week watering season (May to November):

Understanding PET is the key to saving water while keeping your lawn green and beautiful.

PET - the basis for Smart Irrigation Controllers

This groundbreaking study introduced many of the concepts used by smart irrigation controllers like Rachio, Orbit, and OtO.

What is PET?

"ET is how we scientists measure the amount of water that plants need." Professor Fipps explains. Potential Evapotranspiration (PET), as it’s more commonly known is a scientific measure of how much water a plant loses through evaporation from sunlight and transpiration from turning Carbon Dioxide into oxygen.

What is Allowable Stress?

Allowable Stress is an agricultural term. It refers to how much water stress a plant can handle before it affects its health or appearance. Professor Fipps notes, "The technical term for it is the allowable stress factor. How much stress can you allow your turf to have?"

Lawn Specific Recommendations from the San Antonio Study

One of the most interesting aspects of the San Antonio study was how homeowners perceived their lawn quality under different watering regimes.

"Before the project started, most homeowners participating in the study followed the standard recommendation of applying 1 inch per week. In comparison, the amount of water [for the baseline treatment] averaged about 2/3 inches [per week], a 33% savings”

They found that most homeowners were satisfied with their lawn's appearance, even when using less water than traditionally recommended.

St. Augustine Grass

St. Augustine is a thirsty grass. It was the only grass species in the study that watering more always led to higher ratings.  While it is more shade tolerant than Bermuda, it’s significantly less drought tolerant. 

Those homeowners who watered their St. Augustine lawns the most “showed a lower summer decline and quicker autumn recovery to higher ratings than at the beginning of the season."

Bermuda Grass

Bermuda grass, the undisputed king of golf courses. Bermuda performed well under reduced watering. 

The research found that “all Bermuda lawns recovered quickly in the fall.” But those who watered moderately in the summer “recovered more quickly than they other watering treatments.”

Buffalo Grass

Buffalo Grass, a lawn that’s more popular in the deep south is less common than Bermuda and St. Augustine.  

Buffalo grass showed impressive resilience in the study. Those who watered their lawn the most, and those watered at 50% both rated their lawns relatively similarly throughout the study. 

Note: adjust for Urban Environments

PET rates can vary significantly due to the built environment. Professor Phipps explains, "Urban areas are combinations of neighborhoods with various size lots... densely developed areas of cities are kind of like heat sinks."

If you live in a developed area with lots of concrete and small lot lines, your lawn may experience more evapotranspiration.

Addendum - Maintaining Your Irrigation System

Every year, homeowners lose hundreds of thousands of gallons to leaky, damaged irrigation systems. The scary part? Many don't realize it because their sprinklers run at night!

At least once per growing season it’s worthwhile to have a licensed irrigator inspect your system. This can save you thousands of gallons of water every year, and they’ll likely adjust your sprinkler system to ensure maximum efficiency.

To ensure your system is operating efficiently, Professor Phipps recommends 2 best practices:

1. Using proper filters: "These are screen filters to keep debris that might be in the water lines from clogging your emitters."
2. Installing pressure gauges: "You need pressure gauges so you can check those and see if the system is operating at the correct pressure."

Further Reading

The San Antonio study provides compelling evidence that using PET-based irrigation scheduling can lead to substantial water savings without significantly compromising lawn quality. 

If you’re interested in learning more about sprinkler system maintenance, read our sprinkler system maintenance checklist.

For those with Bermuda Lawns, check out our guide to watering Bermuda grass. 

Every lawn is unique, and factors like soil type, shade, and local climate can affect water needs. Start with these guidelines and adjust based on your lawn's response. With a little attention and the right approach, you can have a beautiful, water-efficient lawn that's kind to both your wallet and the environment.

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