COLLEGE STATION, Texas – Evapotranspiration, or ET, is the amount of water a plant actively pulls from the soil. Texas A&M AgriLife Research scientists are trying to utilize this basic measurement to help develop some simple formulas for watering lawns.
Operating on the principle that history is a good teacher, Charles Fontanier, AgriLife Research associate, and Dr. Richard White, AgriLife Research turfgrass physiologist, are conducting studies at the Texas A&M AgriLife Turfgrass Field Lab to determine if historical evapotranspiration data can be used to predict water needs when irrigating St. Augustine lawns.

The goal is to try to create a message that is easy to understand for both homeowners and landscape contractors that will also promote water conservation and healthy turfgrasses,” Fontanier said.

In general, daily reference evapotranspiration is estimated using various weather data, and from there, scientists can relate reference evapotranspiration to the grass or the plant being grown.

“From the reference ET, we can adjust it to the different types of grasses we are growing, as well as perhaps a microclimate: if it is a sunny spot or a shady spot,” Fontanier said. “So we can adjust the number accordingly. If the number for a warm-season turfgrass in Texas is 0.6, then 60 percent of the weather station output is what we should be applying to our grasses.”

“And ideally, we should be adjusting irrigation amounts based on what the weather station is telling us on a daily or weekly basis,” he said. “But keeping up with real-time ET data can be too time consuming. So our goal for this study was to demonstrate and quantify the effects of using historical average water needs as a predictor of actual plant water needs.”
Fontanier and White have used historical averages to set the sprinkler system run times each of the last two growing seasons. For example, on average over the past 47 years, St. Augustine grass has needed about 4.25 inches of water in July. They have used that number to irrigate the grass each July.

Their four treatments in the study were: reference evapotranspiration from the weather station or 100 percent, which would be an over-watering scenario; turf coefficient, which is what is theorized that the turfgrass actually needs; and two deficits, 40 percent and 60 percent of the turf coefficient.
“Looking at the two years we’ve had recently and comparing the differences, in 2011 when we really had severe conditions, our main goal with any irrigation was really just bud and crown survival,” Fontanier said.

“If we kept enough plants alive, we could get regrowth once rains did come. In fact that is what we found. Even though we lost a tremendous amount of density in our deficit irrigation treatments, the grass survived and by March, we had close to 100 percent stands.”
Superficially, all the plots looked the same, he said. That’s the important message for areas that can tolerate that seasonal reduction in quality – that St. Augustine grass will survive “if you just moderate your irrigation and keep it moist enough that we have enough buds to stimulate regrowth.”

A second aspect determined from the study is that nitrogen fertilizer is key to the speed of that recovery, Fontanier said. The high fertilizer experiments show greater recovery.
“If you look at 2012, where we did get occasional rains, our water conservation treatments or deficit irrigation treatments not only survived, they actually look pretty good,” he said. “So what we did using historical ET, we were probably under-irrigating in 2011 by maybe 20 percent and over-irrigating in 2012 by about 10 percent.

“But if you look at the long haul, we think we will be right on the money or at least promoting some level of conservation utilizing historical ET as a base for irrigation,” Fontanier said.

“Historical and real-time ET data for locations across Texas are readily available online at the Texas ET Network. If these data are not available to your region, a general rule of thumb is that Texas lawns need about 1 inch of water a week from June through August. Sticking with these easy-to-remember strategies should, in the long run, help homeowners conserve water.

“And if you bring it down to a deficit from that, we can still have water conservation and the grass will stay alive,” Fontanier said. “The key is to make sure you have some moisture in the ground to maintain bud survival.”