University of Utah scientists have conducted the first study to document landscape water use on a city-wide scale. Among other findings, trees are shown to be an excellent water-saving tool in grassy landscapes.
In 2010, Los Angeles used enough water irrigating lawns to meet the needs of nearly a half-million average households for a year.
That’s according to a new study by scientists at the University of Utah, who conducted what they say is the first city-scale assessment of water consumed by landscaping. Their findings show that Los Angeles’ landscaping consumed the equivalent of 100 gallons per person each day, with lawns accounting for 70 percent of that.
Urban trees, it turns out, consume relatively little water. And by providing crucial shade, trees can actually make lawns less thirsty.
The measurements were done before the worst drought in history gripped California, resulting in strict water conservation measures in Los Angeles and elsewhere. So it’s likely lawns got a lot less thirsty during the drought.
Nevertheless, the study provides an important look at the water-supply implications of our obsessions with verdant lawns, and may help guide future urban planning and conservation actions.
To learn more, Water Deeply recently spoke with Diane Pataki, a coauthor of the study and a professor of biology and urban planning at University of Utah who previously worked at University of California, Irvine.
Water Deeply: What was your goal in this study?
Diane Pataki: The goal was to estimate the flux of water from Los Angeles to the atmosphere, which is called evapotranspiration. We had spent many years measuring the transpiration of trees in L.A., and the evapotranspiration of lawns.
It hadn’t actually been measured in real urban environments. There were a bunch of studies of landscaping plants under controlled conditions in experimental facilities. But there weren’t very many measurements out in the urban area itself.
We spent a number of years making measurements of what plants were doing in L.A. in terms of water use, because the species composition of plants in L.A. is strange for an ecologist. It’s really diverse. There are species from all over the world. There are trees literally from every continent except Antarctica in Los Angeles. So there really was no way of knowing ahead of time how much water trees use in L.A.
Water Deeply: What did you learn about water use in this urban setting?
Pataki: We very rarely found trees or lawns that were water-stressed. Most landscapes in L.A. are over-watered. Plants were transpiring kind of the maximum amount that they physically could.
So we came up with an estimate for the whole city, which is 100 billion gallons of water in a year [or about 306,000 acre-feet], and 70 percent of that is contributed by lawns. So lawns are the majority of the landscape water use. It is a lot of water, and something like half the water used in L.A. is used in outdoor irrigation. So, yes, there’s a lot of room there to conserve.
Lawn evapotranspiration is very, very high in L.A. Trees actually don’t use that much water when compared to lawns. There are a lot of trees in L.A., but their transpiration rates are much lower than lawns. Part of that is because their leaf area is lower than lawns.
Water Deeply: How rare is this type of measurement?
Pataki: To the best of our knowledge, these are the first estimates of municipal-scale evapotranspiration that discern relative contributions from different growth forms and tree species.
Water Deeply: Why has there been so little measurement of landscaping water use at this scale?
Pataki: The methods we used to make those measurements are not that complicated. They’re commonly used in natural areas and agricultural areas. But people hadn’t really been setting out the equipment in L.A.
There were problems like, what if your equipment gets stolen, which did happen to us. It was hard for us to find places to set up our equipment. In one case, we wanted to measure street trees. That was the hardest thing – trees that are right there on the sidewalk. How are you going to put your sensors there? So we ended up putting our data logger in someone’s apartment and stringing cables from street trees into her apartment, and left these probes in the trees for months. That was one of the few places where we could measure street trees.
Lawns are tricky, because with trees we can put probes inside the wood. You can’t do that in grass. The method people usually use is to put sensors in the atmosphere. But to make it work, you need a huge lawn area, much bigger than most urban lawns. So we had to revive a really old method, which is to put a clear chamber over the lawn and measure the change over time. And you have to do it really fast, like in 30 seconds.
Water Deeply: What changes do you think occurred due to conservation measures imposed during the drought?
Pataki: We’re actually studying that now. What we’re specifically studying is the result of turfgrass removal programs. We’re trying to document what happened. Both my group and some folks at UCLA have been using Google Street View to actually measure what kind of landscapes people replaced their lawns with. We’ve got to do that first, then we can get a calculation of water savings.
At some point, we need to go and physically show up and knock on peoples’ doors and ask if we can sample their yard. Then we really get into species and exact plant cover.
Water Deeply: You estimate landscaping water use in 2010 at 100 gallons per capita per day. That sounds like a lot.
Elizaveta Litvak, coauthor of a new study on water consumption by Los Angeles lawns, measures solar irradiance at the L.A. Arboretum and Botanic Garden. (Photo Courtesy University of Utah)
Pataki: That’s definitely a lot of water, and that’s just the amount of water that actually is going to the atmosphere. It’s not the amount of water people are irrigating. People are irrigating more than that. Some of it runs off into the street, and some of it just drains below the plant rooting zone.
Now, in theory, that’s the amount of water you’d have to allocate to keep the vegetation exactly the same as it is. So the first layer, in thinking about conservation, is to get rid of the over-irrigation. The second layer is to think about how many lawns could be removed.
The other thing our study showed is that if you add trees to lawns you actually save water. Which is a little counterintuitive. But because trees reduce solar radiation to lawns so much, the lawn can receive 50 percent less water.
Water Deeply: You also linked your results to median household income across different sectors of L.A. What did those results show?
Pataki: We looked at evapotranspiration as a function of median income in the city council districts in L.A. There’s already been a lot of work on the inequities in the tree canopy in L.A. So we already know the more affluent council districts have more trees.
We found the wealthiest areas have twice the evapotranspiration of the poorest neighborhoods. Primarily that’s a function of tree canopy and bigger lot sizes. You end up with more vegetated area in wealthier areas, and larger trees, too.
So you get much bigger fluctuation of water to the atmosphere in wealthier neighborhoods. Because that does have a cooling effect, that’s going to mean more cooling in wealthier neighborhoods.
Water Deeply: What lessons from your work can be applied to water conservation going forward?
Pataki: There certainly seems to be a lot of leeway to save water. This isn’t going to be the only drought, and snowpacks are expected to continue dwindling, which is the water supply for most of these urban areas.
So, going forward, there are lots of reasons to think more conservation will be needed. And there are a lot of opportunities for conservation. One thing I think we saw in the drought is if we want to preserve the tree canopy of L.A., we might need to really think about managing tree water use.
Sometimes, when people remove their lawn or shut off irrigation, we lost some trees too. Trees are relying on over-irrigation. Trees will need to be irrigated infrequently but deeply, and with something that puts more water in a single watering than just lawn sprinklers.
One more thing we found is the best system for watering your lawn – if you need a lawn – is the system that has a soil moisture sensor in the ground.
Water Deeply: Now that the drought is over, do you think people will return to the kind of over-irrigation and thirsty lawns you measured in 2010?
Pataki: That’s definitely a question we’re trying to answer. We’re interested in people’s behavior around landscaping. We want to know why people prefer certain types of plants, and we want to know if the drought has changed peoples’ esthetics. It’s hard to offer people recommendations for alternatives when you don’t really know why they were attracted to lawns in the first place.
L.A. was not a gravelscape city, unlike some other Southwest cities like Phoenix. And now that these other types of landscapes are more common, is that changing people’s visual preferences? I think that’s going to be a big question going forward. And it turns out that doesn’t cool things down very much. It really is shading that cools things down.