Thirsty grass.

• Author: Litvak, Elizaveta
• Position: Eye on Ecology

HOW MUCH WATER does your lawn really need? We conducted a study that reevaluated lawn watering recommendations by measuring water use by lawns in Los Angeles, Calif. The standard model of turfgrass water needs, we found, lacked precision in some common urban Southern California conditions, like the Santa Ana winds, or in the shade. Actually, the current method of estimating water use is very arbitrary, and there has been no scientific ground for more precise recommendations.

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 is controlled by the plant's biological processes. Plants, for instance, may restrict transpiration in drought conditions. Transpiration rates also can vary from species to species. The combined measure of evaporation and transpiration is called "evapotranspiration" (ET) and it is a difficult quantity to measure, as there is no direct way to measure it--it is like trying to capture what is not capturable.

Researchers attempt to determine a crop's ET rates by placing a large greenhouse-like chamber over an area of plants and measure how temperature and humidity change within the chamber, but the presence of such a large chamber alters the plants' environment so much that the method is far from ideal.

The current state-of-the-art method involves placing tall towers, festooned with scientific instruments, in large crop fields. The towers measure slight atmospheric changes multiple times per second, and researchers feed the data into a mathematical model to compute ET rates for that field.

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. Although turfgrass is the largest irrigated crop in the U.S., researchers are not certain how the ET model holds up in the fragmented urban landscape. Tall water vapor flux towers are not the answer: you cannot use that method in urban lawns because the grass area is too small. It just cannot be done where lawns form a patchwork with streets and buildings. People had not gone into cities and measured what turfgrass...