Irrigating turf with recycled water is a key step on the path toward sustainability. But a giant leap is what New Mexico State University turfgrass expert, Bernd Leinauer, now has in mind. In a paper published July 9 in Crop Science, he and co-author Elena Sevostianova detail their recipe for a “greener” lawn: fertigation, drip irrigation, and decentralized water treatment.
Fertigation supplies fertilizers to plants in irrigation water. Drip irrigation delivers water directly to plant roots, instead of sprinkling plants from above. Scientists have studied the effectiveness of both techniques extensively, Leinauer says, especially in agricultural systems. What’s making them relevant now to urban areas are new decentralized, water treatment technologies. Once installed in a housing development or other location, these small-scale systems can “tailor” the nutrient levels in recycled water for different purposes or times of year.
In Leinauer’s and Sevostianova’s vision, a decentralized treatment system at a subdivision would be set to generate recycled water during summer containing 15 parts per million (ppm) of the nutrient, nitrate. Residents would then use this tailored water to fertigate their lawns—or, irrigate and fertilize them at the same time. Then, when the summer growing season was over, nitrate levels in the effluent water could then be dialed back down.
Drip irrigation, meanwhile, would be used to overcome people’s squeamishness about sprinkling their yards with treated wastewater from their own homes. Because drip systems put water directly into the soil, Leinauer says, homeowners wouldn’t come in contact with it.
Combining the three approaches could solve a lot of issues, he adds. Right now, many New Mexico cities remove nearly all the nitrate from wastewater—an expensive and energy-intensive step designed to prevent pollution of surface- and ground- waters when the effluent is used for irrigation or discharged downstream.
“But from a turf perspective that doesn’t make a whole lot of sense,” Leinauer says, because golf course managers then have to apply nitrate fertilizers anyway to keep turf thriving. “So, why not leave the nitrate in the water?” he asks, “Then, the water already contains a fertilizer that the golf course operator doesn’t have to buy or manage."
One critical question, however, is whether drip-irrigating with water containing relatively high levels of nitrate will cause significant losses of the nutrient to the subsoil and eventually to groundwater. As part of a larger National Science Foundation-funded project called “Re-Inventing the Nation's Urban Water Infrastructure” (ReNUWIt) Leinauer is now studying this question at a New Mexico State test facility.
Three times each week for the past two years, a small-scale treatment system at the site has produced recycled water at 15 ppm nitrate. The water is then pumped through a drip system to plots of both cool season (tall fescue) and warm season (Bermuda grass and buffalo grass) turf grasses.
So far, the turf plots being drip-irrigated with tailored water are just as green and healthy—or even more so—than those receiving potable water and mineral fertilizers, Leinauer says. He and his collaborators also see little evidence of greater nitrate loss from the fertigated, drip-irrigated plots.
Still, he cautions, the results are preliminary and there are other challenges to address, as well. Besides nitrate, for example, wastewater tends to be high in salts, meaning that soil salinity needs to be carefully managed when irrigating with it. The turfgrass industry is also pretty resistant to drip irrigation, Leinauer says, despite many studies showing its superior efficiency compared to watering with sprinklers.
These problems must be solved, though, as water supplies continue to decline. In New Mexico, in fact, demands on potable water from agriculture and a growing populace are so great that “basically the only water left for the landscape is treated effluent,” Leinauer says. But the issue is hardly unique to his region, which is why he’s hoping that researchers around the country will embark on similar studies.
“We’re doing our part here in the Southwest, but our part is completely different from, let’s say, Boston, Massachusetts, or the Midwest,” he says. “So, these questions need to be investigated more thoroughly on a regional basis.”
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