Between 40% and 70% of water used in urban settings in the United States is applied to landscape plantings (Cabrera et al., 2013; Haley et al., 2007; Kjelgren et al., 2000; St. Hilaire et al., 2008). Water conservation in urban landscapes in California is especially important because of a limited water supply, cyclical droughts, population increases, and a water distribution problem requiring transporting large volumes of water from Northern to Southern California. The population of California is expected to increase from 39 to 60 million by 2050 (Dieter and Maupin, 2017). Since 2005, nearly half of the population growth in the state has occurred in inland Southern California and the Central Valley because of less expensive and more plentiful land than along the coast (Hanak and Davis, 2006). In addition, because inland landscapes tend to be larger and ET rates higher than those in coastal areas, more water is required for their irrigation.
Climate change poses additional challenges to urban landscapes as rising temperatures coupled with limited water exacerbates the need to increase and diversify the palette of trees and other ornamentals adaptable to harsh urban conditions (Bohn et al., 2018; Hanak and Lund, 2008). Furthermore, Fall 2011 through Fall 2015 was the driest 4-year period in recorded history in California since the beginning of weather tracking in 1895, exacerbated with record high temperatures in 2014 and 2015 (Hanak et al., 2015). Although precipitation in 2016 and 2017 rose to near-average levels in much of northern California, all of central and southern California continue to experience moderate or severe drought as of 10 Mar. 2018 (Fenimore, 2018).
An increase in California’s population coupled with a multiyear drought in the 1980s requiring greater landscape water conservation led to the enactment of the California Assembly Bill 325 (Water Conservation in Landscaping Act), which became effective in 1993. The act required the California Department of Water Resources (CDWR) to develop a Model Water Efficient Landscape Ordinance (MWELO), intended to increase water conservation in urban landscapes. This included reducing water waste in landscape plantings and listing landscape plants within WUCOLS water-use categories to supplement the small number of actual plants whose water use had been measured in field studies, a lengthy and resource-intensive process.
The assumed a leadership role in WUCOLS, bringing together 36 experts from the landscape industry who categorized thousands of plants in six climate zones (north central valley, central valley, south coastal, south inland valley, high and intermediate desert, and low desert) as very low, low, moderate, or high water users. Since the inception of WUCOLS, additional species were added and, when deemed necessary, plants were recategorized (Costello and Jones, 2014; Costello et al., 1991, 2000). Presently, WUCOLS includes more than 3500 plants categorized in its searchable database (Costello and Jones, 2014).
Irrigating landscape plants less than their ET rate is an effective water-saving strategy that has become common in arid states under conditions of drought and water restrictions. Because landscape plants are planted for their ornamental value rather than for producing crops that require large amounts of water at certain life cycle stages, significant water savings can be realized through this practice (Costello et al., 2000; Hartin et al., 1993, 2015). Fortunately, many landscape plants retain acceptable health and appearance when subjected to deficit irrigation (Hartin et al., 1993, 2015; Kjelgren et al., 2000; Montague et al., 2004, 2007; Pittenger et al., 2001, 2002, 2009; Sun et al., 2012).
As previously mentioned, determining the minimum irrigation requirements for the thousands of native and introduced landscape plant species growing in arid climates is a formidable task. Adding to the burden are landscape areas (LAs) that contain plantings of multiple species, planting densities, and microclimates that comprise the vast majority of these landscapes (Nouri et al., 2012, 2016; St. Hilaire et al., 2008). The common agronomic practice of determining crop coefficient (Kc) values that relate the water requirement of a specific crop to reference ET illustrated by Eq.  is inadequate for correctly estimating irrigation requirements of these complex mixed landscapes (Allen et al., 1998; Nouri et al., 2012; Snyder and Eching, 2006).
Turfgrass researchers (Gibeault et al., 1985; Harivandi et al., 2009) determined that monocultures of warm and cool season turfgrass plantings that conform to Kc standards under California conditions have Kc values of 0.6 and 0.8, respectively, and often perform adequately at somewhat lower values situationally. A lawn watering guide for the general public based on these Kc values, a distribution uniformity (DU) of 80%, and historical ETo data were also developed (Hartin et al., 2001). Other work determined that bermudagrasses (Cynodon sp.) and seashore paspalum (Paspalum vaginatum) outperformed several other turfgrasses and alternative groundcovers under deficit irrigation (Gibeault et al., 1989) and quantified the performance of several turfgrass cultivars under varying irrigation regimes and mowing heights (Richie et al., 2002).
Many UC researchers have conducted studies on the health, performance, and aesthetics of non-turfgrass landscape plantings under minimal irrigation conditions. Methods include measuring the performance of monocultures of landscape plants through species evaluations under varying levels of ETo and developing WUCOLS for landscapes with mixed species, densities, and microclimates (Costello and Jones, 2014; Costello et al., 1991, 2000). Additional studies have increased the adoption of technologies and practices that reduce water waste in turfgrass and non-turfgrass landscapes, and improve DUs and IE of sprinkler-irrigated landscapes through intense on-site training (Hartin and McArthur, 2007; Hartin et al., 2017; Reid et al., 2017).
A summary and brief review of these UC-led studies follows.
AllenR.G.PereiraL.S.RaesD.SmithM.1998Crop evapotranspiration: Guidelines for computing crop water requirements. FAO Irr. Drainage Paper 56
BaumM.C.DukesM.D.MillerG.L.2003Residential irrigation uniformity and efficiency in Florida. Amer. Soc. Agr. Eng. Florida Section Mtg. Paper FL03-100
BohnS.BonnerD.ChappelleC.MejiaM.DanielsonC.Escriva-BouA.NaoG.GrayB.HanakE.HillL.2018California faces growing water management challenges. Public Policy Inst. California San Francisco CA
California Department of Water Resources2009Model water efficient landscape ordinance. 20 May 2018. <https://www.water.ca.gov/LegacyFiles/wateruseefficiency/docs/MWELO09-10-09.pdf>
CostelloL.R.JonesK.S.2014WUCOLS IV: Water use classification of landscape species. 20 May 2018. <http://ucanr.edu/sites/WUCOLS/>
CostelloL.R.MathenyN.P.ClarkJ.R.1991Estimating water requirements of landscape planting; the landscape coefficient method. Univ. California Div. Agr. Natural Resources Publ. 21492
CostelloL.R.MathenyN.P.ClarkJ.R.2000The landscape coefficient method. In: A guide to estimating irrigation water needs of landscape planting in California. 20 May 2018. <http://www.water.ca.gov/wateruseeffciency/docs/wucols00.pdf>
DieterC.A.MaupinM.A.2017Public supply and domestic water use in the United States 2015. U.S. Geological Survey Open-File Rpt. 2017-1131. 20 May 2018. <https://pubs.er.usgs.gov/publication/ofr20171131>
FenimoreC.2018Drought monitor. 20 May 2018. <http://droughtmonitor.unl.edu/CurrentMap/StateDroughtMonitor.aspx?CA>
GibeaultV.A.MeyerJ.L.YoungnerV.B.CockerhamS.T.1985Irrigation of turfgrass below replacement of evapotranspiration as a means of water conservation; performance of commonly used turfgrasses. Proc. 5th Intl. Turfgrass Res. Conf. 347–356
HanakE.DavisM.2006Lawns and water demand in California. California economic policy. Vol. 2. Public Policy Inst. California San Francisco CA
HanakE.LundJ.R.2008Adapting California’s water management to climate change. Public Policy Inst. California San Francisco CA
HanakE.MountJ.ChappelleC.2015California’s latest drought. Public Policy Inst. California San Francisco CA
HarivandiM.A.BairdJ.HartinJ.HenryJ.M.ShawD.2009Managing turfgrass during drought. Univ. California Div. Agr. Natural Resources Publ. 8395
HartinJ.S.McArthurK.A.2007Conserving water and improving plant health in large southern California landscapes. Final project report. California Dept. Water Resources Office Water Use Efficiency Sacramento CA
HartinJ.S.MeyerJ.L.GibeaultV.A.1993Minimum irrigation requirements of four species of landscape trees. Research report. Univ. California Agr. Natural Resources South Coast Field Station Irvine CA
HartinJ.S.MeyerJ.L.GibeaultV.A.2001Lawn watering guide. Univ. California Div. Agr. Natural Resources Publ. 8044
HartinJ.OkiL.FujinoD.FaberB.2015Drought tip: Keeping plants alive during drought and/or water restrictions. Univ. California Div. Agr. Natural Resources Publ. 8553
HartinJ.OkiL.FujinoD.ReidK.IngelsC.HaverD.2017Evapotranspiration adjustment factor study: Final project report. California Dept. Water Resources Office Water Use Efficiency Sacramento CA
MontagueT.KjelgrenR.AllenR.WesterD.2004Water loss estimates for five recently transplanted tree species in a semi-arid climateJ. Environ. Hort.22189196
MontagueT.McKenneyC.MaurerM.WinnB.2007Influence of irrigation volume and mulch on establishment of selected shrub speciesArboricult. Urban For.33202209
NouriH.BeechamS.KazemiF.HassanliA.2012A review of ET measurement techniques for estimating the water requirements of urban landscape vegetationUrban Water J.10247259
NouriH.GlennE.P.BeechamS.ChavoshiS.BoroujeniS.SuttonP.AlaghmandS.NooriB.NaglerP.2016Comparing three approaches of evapotranspiration estimation in mixed urban vegetation: Field-based, remote sensing-based and observational-based methodsRemote Sens.8492
PetrassL.A.TwomeyD.M.HarveyJ.2014Understanding how the components of a synthetic turf system contribute to increased surface temperatureProcedia Eng.72943948
PittengerD.R.RichieW.E.HodelD.R.2002Executive summary: Performance and quality of landscape tree species under two irrigation regimes p. I6–I7. In: R.L. Green D.R. Pittenger and W.E. Richie (eds.). Turfgrass and landscape irrigation studies progress report. Univ. California Coop. Ext. Riverside
ReidK.FujinoD.OkiL.HartinJ.BakerB.DuenowB.2017Maintaining urban landscape health and services on reduced irrigation: A multi-site study in best management practices. ISHS 1st Intl. Symp. Greener Cities for More Efficient Ecosystem Services in a Climate Changing World abstr. S2C5 p. 103. 31 May 2018. <https://docs.wixstatic.com/ugd/aa54ce_e065d07c21f84e2aa481c80e9baefb4f.pdf>
RichieW.E.GreenR.L.KleinG.J.HartinJ.S.2002Tall fescue performance is influenced by irrigation scheduling, cultivar, and mowing heightCrop Sci.4220112017
SisnerozJ.OkiL.FujinoD.ReidK.2018UC landscape plant irrigation trials. California Center for Urban Horticulture. 22 May 2018. <https://ccuh.ucdavis.edu/sites/g/files/dgvnsk1376/files/inline-files/Trials%20Overview%20Printer%20Friendly.pdf>
SnyderR.L.EchingS.2006Urban landscape evapotranspiration. Vol. 4: Reference guide p. 691–693. In: California water plan update 2005. California Dept. Water Resources Bul. 160-05
St. HilaireR.ArnoldM.WilkersonD.C.DevittD.A.HurdB.H.LesikarB.J.LohrV.I.MartinC.A.McDonaldG.V.MorrisR.L.PittengerD.R.ShawD.A.ZoldoskeD.F.2008Efficient water use in residential urban landscapesHortScience4320812092
SunH.KoppK.KjelgrenR.2012Water-efficient urban landscapes: Integrating different water use categorizations and plant typesHortScience47254263
WilliamsC.F.PulleyG.E.2003Synthetic surface heat studies. 22 May 2018. <http://cahe.nmsu.edu/programs/turf/documents/brigham-young-study.pdf>