Saving Water on Lawns with Shade

A University of Utah study re-evaluated lawn watering recommendations by measuring water use by lawns in Los Angeles

The standard model of turfgrass water needs, they found, lacked precision in some common urban southern California conditions, like the Santa Ana winds, or in the shade. “The current method of estimating water use is very arbitrary,” says postdoctoral scholar Elizaveta Litvak, first author on the new study.“ 

Field of steam

Scientists study how much water plants lose so that landscape managers can know how much water they need to put back in. Water evaporates in a straightforward physical process that depends primarily on the temperature and humidity of the air. But plants also lose water through transpiration, breathing out water vapor as part of their metabolism. Transpiration is less predictable because it’s controlled by the plant’s biological processes. Plants may restrict transpiration in drought conditions, for example. Transpiration rates can also vary from species to species.  The combined measure of evaporation and transpiration is called “evapotranspiration” (ET) and it’s a difficult quantity to measure. “There is no direct way to measure it,” Litvak says. “It’s like trying to capture what’s not capturable.”

The lawns of LA

The present-day turfgrass ET model serves for estimating water use by forests and agricultural crops in various weather conditions. Urban lawns, however, are much smaller than crop fields. Shade from landscape trees and buildings may create the opposite effect and mitigate ET rates, perhaps on an hourly basis.

Because of the impracticality of building water vapor flux towers in residential lawns, Pataki and Litvak turned to the simple chamber method for measuring ET. They built a small clear box a bit bigger than a shoebox equipped with sensors that would instantaneously measure temperature and humidity within the box and, when set on a lawn, would provide a rapid estimate of ET. They carefully calibrated the method by weighing lawn pieces on a precise scale to capture weight losses caused by ET.

Throw some shade

Litvak and Pataki found that their measurements of turfgrass ET didn’t always match modeled predictions. The model’s recommendations for watering shaded lawns are very vague, Pataki says, and most lawns are shaded, to some degree at some parts of the day by trees or buildings. Shade dramatically reduces plants’ water needs, so a partly-shaded lawn will need less water than unshaded.

Litvak says that the model performs best on dense unshaded grass, possibly intentionally overwatered to keep it green. On such lush lawns, actual ET is very close to predicted. Litvak and Pataki developed an approach to adjust ET rates for different urban conditions, including dry air or partial shade, that could lead to adjusted watering recommendations. “At some level, modeled methods work,” Litvak says. “But when we transition to really low humidity, they just don’t match our findings.”

“The water use we measured in the field was higher than the recommendations that California was already making,” Pataki says. “So if all people did was follow the recommendations, that’s significant water saving right there. Plus the recommendations could be improved as well.”

Pataki and Litvak have one resounding suggestion for managing your grass in the summer: “Shade your lawn!” Then dial back irrigation water use accordingly.

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Taxonomy

• Conservation
• Landscape Horticulture
• Irrigation Scheduling