Wednesday, May 10, 2017

Water Need and Water Use in Landscape Irrigation

By Michael Dukes

“Turf grasses need irrigation systems. Some grasses listed as Florida-Friendly need tremendous volumes of water. The homeowner also needs to fertilize and quickly address diseases and pests that will be present. Despite the homeowner's best efforts, all or part of the turf will need to be replaced every 4 to 5 years.”
- email from master gardener

A Need Benchmark
People often work under the assumption that turfgrass needs a lot of water. What is the definition of a lot? Normally, turfgrass is irrigated far more than it needs to be and this reinforces the perception that it needs a lot. There are lots of misconceptions about how much is used and how much is needed. If people knew how much is really needed, there would be a benchmark and you would have an idea of where you are relative to what is needed.

Michael Dukes and Bernard Cardenas in the research plots
An example is the comparison homes treatment (no technology, monitor-only) used in our Orange County smart irrigation study. We’re in our 6th year of data collection. Over that whole time period, the average application for those homes has been more than an inch per week – that’s over 52 inches a year, no matter how much rainfall. And we’re getting about 40+ inches of rainfall in a year. That’s 90 inches of water being applied to the landscape. But ET (evapotranspiration) of turfgrass is only maybe 30 inches in Orlando.

A properly installed weather-based irrigation controller
Timing is everything - for turfgrass in particular, with shallow root zones. In theory, if we get 40 inches of rainfall and ET is 30 inches, the amount of rainfall far exceeds the amount of water that plant needs. But it’s not timed perfectly. There are dry times and there are times when you get excess water. That’s where irrigation comes in. If you time that irrigation perfectly you’re in the mid-20 inches of water needed.

Ideally if you’re applying at the right time at the right amount, which a smart irrigation controller allows you to do automatically, you should be in that mid-20 inches of water applied. The comparison home treatments are applying over 50 inches. That’s why smart technology is a viable way of getting irrigation in the right ball park. They are not perfect, but when the problem is 2x and you have a solution that gets you to 1.3x or 1.5x you’ve made a huge dramatic improvement by doing one thing.

A Use Percentage
Image via Ewing
I often am asked how much water is used for irrigation, usually referring to single family homes. Often we see a mysteriously even number of 50% cited. The truth is that irrigation water use varies in time and space. In areas with more irrigation systems, there will necessarily be more irrigation water use. In drier years, irrigation water use will be higher. I believe the 50% number is a very convenient number to cite but here’s what we know based on data from Florida.

The reality is that in a given utility there will be a spectrum of users with most not over irrigating or perhaps irrigating at all. In Mackenzie Boyer’s recent work in Southwest FL we showed this for the Tampa Bay region, (Mining for water: Using billing data to characterize residential irrigation demand). That said, other utilities in the east and southeast part of the state have higher irrigation demand since their conservation programs typically aren’t as developed as in the SW region.

Additional research informing on water-use estimates:
Romero & Dukes (2016): A Method to Estimate Residential Irrigation from Potable Meter Data – Metered data from Orlando (1,781 homes) showed that irrigation accounted for 64% of total use.  The first paragraph also gives a good summary of several other studies that reported Florida residential irrigation use (ranging from 25-75% of total use).

Boyer et. al (2014): Irrigation Conservation of Florida-Friendly Landscaping Based on Water Billing Data – Again, the introduction summarizes key statistics from Florida studies.  For example, Haley et al. (2007) estimated 64% in central Florida, and Romero and Dukes (2014) estimated 32-63% in central Florida.

A good resource for how much to water your landscape depending on region in Florida, time of year and irrigation system can be found here.

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