Dr. Elizaveta Litvak and Prof. Diane E. Pataki, University of Utah, U.S.A. Sprinklers at the Los Angeles County Arboretum and Botanic Garden. Photo: Elizaveta Litvak Well-groomed lawns have become ubiquitous in cities worldwide in multiple geographic locations and climate zones. In the United States (U.S.), turfgrass lawns occupy an area larger than any irrigated crop. 1 Yet in the naturally dry southwestern U.S., lawns cannot be sustained without irrigation. In Los Angeles, California, the municipal water supply depends on the most extensive water delivery system in the United States, which imports water from northern California and from the beleaguered Colorado River. 2 Yet, more than half of household water in Los Angeles is used for landscape irrigation. Currently, California is experiencing the most severe drought on record, leading to mandatory restrictions in outdoor watering. 3 In an effort to save water, the Los Angeles Department of Water and Power offered a monetary incentive for replacing turfgrass lawns with California Friendly plants, mulch, permeable pathways, and drip irrigation since turf grass is very expensive to maintain, requiring heavy water use and lots of attention. 4 However, while the water requirements of turfgrass have been thoroughly studied in agricultural and experimental settings, lawn evapotranspiration rates in cities under real climatic, management, and environmental conditions are often unknown. So how much water do lawns actually require in cities located in dry regions, such as Los Angeles, and how do these requirements compare to typical irrigation rates? Watering recommendations developed for California are used in many regions in the U.S. as well as worldwide, so the actual water use of urban lawns in Los Angeles has broad applications. Global Water Forum www.globalwaterforum.org 1
Monitoring lawn water use Our measurements in Los Angeles have shown that lawns can indeed transpire large amounts of water at times, but can also have surprisingly low water requirements, depending on the site. In cities, lawns are often shaded. Some shade comes from landscape trees, and some from buildings and other built objects. Shaded lawns require dramatically less water than unshaded lawns. In fact, most shaded lawns likely use less water than currently recommended by local water agencies. 5 However, Unshaded turfgrass lawn. Fullerton Arboretum, whether shaded lawns have a comparable aesthetic Fullerton, CA, USA. Photo: Elizaveta Litvak appeal to large expanses of unshaded turfgrass likely varies by location and demographic. Conversely, lawns in dry regions across the U.S. and worldwide have unexpectedly high water use under extremely dry atmospheric conditions. 5 In Los Angeles, such conditions usually appear in the late summer and through the fall, caused by local off-shore winds (i.e. Santa Ana winds ). Because most models used to predict lawn water use were developed in relatively humid conditions, they tend to underestimate lawn water requirements under the localized weather conditions that are unique to dry climates. Better watering practices Our empirical tests of water use patterns of urban lawns under real urban conditions provide clear hints for water. First, current methods usually overestimate turfgrass water use and result in excessive municipal irrigation. 5 In turn, over-irrigation is responsible for significant water consumption by unshaded lawns. 5 Before mandatory restrictions, Los Global Water Forum www.globalwaterforum.org 2
Angeles landscapes received about 440 mm of irrigation water per year. 6 According to our estimates, even the most conservative requirements of Los Angeles landscapes do not exceed 370 mm per year, with turfgrass responsible for about 70% of the total (article in prep.). Therefore, adjusting irrigation to match actual landscape needs would lead to savings of at least 70 mm per year. This amounts to an average of 52 million gallons of water per day. In addition, shaded lawns require even less water, and their irrigation should be adjusted according to the amount of shade. 5 Merely taking the current extent of shaded lawns into account may save another 8 million additional gallons per day for the city of Los Angeles. 5 Although it may seem counterintuitive, our results indicate that landscape trees use much less water than irrigated turfgrass under dry atmospheric conditions. 7 The net effect of adding trees to turfgrass lawns can be a reduction in landscape water use of up to 50%. 7 Therefore, intentional shading of turfgrass can be adopted as a powerful water measure. Because shade from trees also mitigates hot urban conditions, Shaded turfgrass lawn near a building at the strategically planting trees over turfgrass may be University of California, Irvine. Photo: Elizaveta particularly beneficial for cities in dry regions. Trees Litvak are highly valued by shaping psychologically comfortable and healthy environments, in addition to aesthetics. While urban communities may be resistant to some landscape modifications, the act of planting a tree is appealing to many people, and planting trees on existing turfgrass may become a part of municipal tree-planting programs. This measure would not just remove high water use lawns, but also relatively easily replace them with efficient landscapes able to alleviate urban Global Water Forum www.globalwaterforum.org 3
microclimates and improve human thermal comfort. What about the water savings of complete lawn removal and replacement by other types of vegetation? This could achieve up to a 62% additional reduction in water consumption. 8 Ironically, similar to turfgrass, municipal recommendations currently exceed the actual water needs of xeric landscapes. 8 However, it is important to avoid over-irrigating these new landscapes as plants native to dry environments are able to take advantage of additional irrigation water when it is accessible. In other words, many xeric landscape species have unusually high transpiration rates when over-watered. For example, in a common garden study of horticultural shrubs, desert shrub transpiration was almost twice as large as transpiration of species from wetter regions when both were well-watered. 9 Therefore, lawn removal alone may not achieve desired water savings without a concerted effort to disseminate information about designing and managing lawn alternatives. Conclusions With rapid growth of urban populations in dry regions coupled with shrinking water availability, low-cost and easy-to-implement water- measures are critically needed. In Los Angeles, significant water savings can be achieved by changing irrigation practices, and without removal of lawns and other vegetation. In fact, adding vegetation that provide shade or placing lawns near buildings can lower water use if irrigation practices are adjusted accordingly. Lawn removal canurban landscapes in Los Angeles are sustained certainly facilitate water, but property by irrigation. Photo: Joseph Plots, panoramio.com, selected for Google Maps and owners and managers will need sources of Google Earth information that include sensible alternatives and best management practices. Avoiding over- Global Water Forum www.globalwaterforum.org 4
irrigation, improving methods used to estimate landscape water needs, and strategically shading turfgrass all have a strong potential to lower irrigation water use in Los Angeles and other dry cities. References: 1. Milesi C., Running S.W., Elvidge C.D., Dietz J.B., Tuttle B.T. and Nemani R.R. 2005. Mapping and Modeling the Biogeochemical Cycling of Turf Grasses in the United States. Environmental Management 36(3): 426-438. 2. California Department of Water Resources. California State Water Project Today. 3. California Government Operations Agency. July 2016. Statewide Water Conservation Grows to 28 Percent in May; Urban Water Suppliers Stress Test Data Under Review. Available at http://drought.ca.gov/topstory/top-story-62.html 4. Los Angeles Department of Water and Power. Turf Replacement. Available at www.ladwp.com/ladwp/faces/ladwp/aboutus/a-water/a-w-/a-w-c-landscap?_ adf.ctrl-state=9uh3id3n0_4&_afrloop=306902721538934 5. Litvak E. and Pataki D.E. 2016. Evapotranspiration of urban lawns in a semi-arid environment: An in situ evaluation of microclimatic conditions and watering recommendations. Journal of Arid Environments 134: 87-96. 6. Mini C., Hogue T.S. and Pincetl S. 2014. Estimation of residential outdoor water use in Los Angeles, California. Landscape and Urban Planning 127: 124-135. 7. Litvak E., Bijoor N.S. and Pataki D.E. 2013. Adding trees to irrigated turfgrass lawns may be a water-saving measure in semi-arid environments. Ecohydrology 7(5): 1314-1330. 8. Volo T.V., Vivoni E.R. and Ruddell B.L. 2015. An ecohydrological approach to conserving urban water through optimized landscape irrigation schedules. Landscapes and Urban Planning 133: 127-132. 9. Goedhart C.M and Pataki D.E. 2012. Do arid species use less water than mesic species in an irrigated common garden? Urban Ecosystems 15(1): 215-232. Global Water Forum www.globalwaterforum.org 5
Elizaveta Litvak is a postdoctoral fellow at the University of Utah. Her research is focused on urban transpiration and its physiological and environmental controls. Diane E. Pataki is a Professor at the University of Utah. Her research focuses on the role of vegetation in the functioning of cities in semi-arid regions. The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here. Global Water Forum www.globalwaterforum.org 6