Irrigation 78815302292 Main

Energy and Buildings 107 (2015) 335344

Contents lists available at ScienceDirect

Energy and Buildings

j ourna l ho me page: www.elsev ier .com/ locate /enbui ld

Optimi adand wa

JiachuanSchool of Susta , Tem

a r t i c l

Article history:Received 13 AReceived in reAccepted 22 AAvailable onlin

Keywords:Building energEnergy-water EnvironmentaUrban canopyUrban irrigatio

lps toe bal

focue urbaenix. on ha

air tmon

ucinge conmparerma

1. Introduction

From 19average annencing highfor 6776% to increase ings are thenal energyUnion is in a growing ccontributorclimate chadown the dber of studidecades to cof reectiveing new buiefciency o

While nution have beschemes on

CorresponE-mail add

Building energy consumption in cities is closely related to environ-

http://dx.doi.o0378-7788/ 84 to 2004, global energy use has been growing at anual rate of 2%, with the developing countries experi-er increasing rates [1]. At present, urban areas accountof global energy use [2], with the percentages expectedunder future urban expansion. Within the cities, build-

dominant energy consumers. Around 40% of the total consumption in the United States and the Europeanthe building sector [3,4]. In recent years there has beenoncern about the energy consumption as it is the largest

to global CO2 emissions, which is the leading cause ofnge [2]. To reduce greenhouse gas emissions and to slowepletion of non-renewable energy resources, a num-es and initiatives have been carried out during the pastut back building energy consumption, including usage

materials [5], deployment of green roofs [6], introduc-lding design requirements [7], and improving operationf building services [8].merous means for reducing building energy consump-en investigated, the impact of various urban irrigation

building energy efciency has been less explored.

ding author.ress: [email protected] (Z.-H. Wang).

mental temperatures [9], on which irrigation has cooling effectsby increasing the supply of surface moisture for evapotranspira-tion. Irrigation-induced cooling on near-surface temperature overagricultural land has been extensively documented in both obser-vational [10] and modeling [11,12] studies. In summers, dailymaximum air temperature over 100% irrigated area can be 5 Ccooler than that over non-irrigated area in California [12]. On theother hand, though the importance of irrigation in modeling urbanenergy and water budget has been increasingly recognized [13,14],the explicit impact of irrigation on urban environmental temper-ature and building energy consumption has rarely been studied.Irrigation of private gardens consumes 1634% of the total watersupplied to an urban area, let alone the water used for irrigatinglarge open space such as public parks and golf courses [15]. For res-idential areas within the city of Los Angeles, nearly 225 106 m3of water was used for irrigation per year [16]. Such amount ofirrigation can increase evapotranspiration and cool the urban envi-ronment considerably, leading to signicantly lower cooling load,especially in densely built areas.

Under the challenge of future climate change, water becomesa more precious resource in cities [17]. Current irrigation prac-tices in most cities are scheduled between sunset and sunrise inorder to avoid rapid moisture loss. However, from an energy sav-ing perspective, irrigation should be conducted during daytime asevaporative cooling is driven by available solar radiation at the

rg/10.1016/j.enbuild.2015.08.0452015 Elsevier B.V. All rights reserved.zing urban irrigation schemes for the trter consumption

Yang, Zhi-Hua Wang

inable Engineering and the Built Environment, Arizona State University, PO Box 873005

e i n f o

pril 2015vised form 29 July 2015ugust 2015e 24 August 2015

y efciencytrade-offl sustainability

modeln

a b s t r a c t

Irrigation of green spaces in cities hehot seasons, but requires an intricatobjective for agricultural irrigation isparadigm. In this study, a cutting-edgcontrolled irrigation schemes for Phofor evapotranspiration, urban irrigatiyear. Maximum reduction in canyonto the condition without irrigation. Airrigation is the most efcient in redThe total annual saving depends on thup to about $1.19 m2 wall area as cocan substantially enhance outdoor the-off between energy

pe, AZ 85287-3005, USA

reduce thermal stress and building energy consumption inance between energy and water resource usage. While thesed on the yield of produces, urban irrigation needs a newn canopy model is applied to assess the impact of a variety ofResults show that by increasing surface moisture availabilitys a cooling effect on the built environment throughout the

emperature can be more than 3 C in summer as comparedg all investigated schemes, the soil-temperature-controlled

the annual building energy consumption and the total cost.trolling soil temperature for irrigation activation, and can beed to the current irrigation practice. In addition, the schemel comfort of pedestrians in summers.

2015 Elsevier B.V. All rights reserved.

336 J. Yang, Z.-H. Wang / Energy and Buildings 107 (2015) 335344

surface. In this case, irrigating urban vegetation leads to improvedbuilding energy efciency, albeit the trade-off and balance betweenwater and energy resources need to be carefully measured. Differ-ent from agricultural irrigation whose objective is mainly on theyield of produces [18], urban irrigation apparently needs a newparadigm by considering the environmental sustainability of cities(e.g. mitigate urban heat islands and save building energy con-sumption).

It is therefore imperative to understand the relationshipbetween water and energy consumption in the urban envi-ronment to develop an optimal urban irrigation scheme. Inthis study a state-of-the-art urban canopy model is employed[1921], with realistic representation of urban hydrological pro-cesses, to identify the environmental impact of urban irrigationin the Phoenix metropolitan area. A variety of uncontrolledand controlled irrigation schemes is investigated, including (1)daily constant scheme, (2) soil-moisture-controlled scheme, and(3) soil-temperature-controlled scheme. Considering the seasonalvariation of meteorological conditions and irrigation demands, thenet saving of individual scheme is quantied at an annual scale.The trade-off between water and energy consumption is quantiedby adoptingronmental thermal com

2. Numeri

2.1. Irrigati

Here theThe simulathas a populmost popularid envirocompared tbuilding enIn Phoenix,landscapes.use native aconsist of tuhelps to conable enviroimproving s[26].

A schemin Fig. 1. Foferent urbais the baselperiod. Sche

irrigation practice over mesic residential landscapes in Phoenix.Daily irrigation amount is estimated by dividing monthly irrigationdata from an in situ measurement by the number of days in eachmonth [27]. Following a previous study, irrigation is scheduled at8 pm local time every day in this scheme [28]. Sensitivity analysisnds that the irrigation time at night has limited impacts on modelresults. Scheme 3 is a soil-moisture-controlled scheme proposed asa potential urban irrigation paradigm. The idea is to maintain soilmoisture at a certain level to keep evaporative cooling effective allthe time. Whenever the moisture content of top soil layer (top)drops below a critical value, irrigation is carried out to increasethe moisture. The amount of irrigation each time is set to be thesame as that in the daily constant scheme. Scheme 4 is similar tothe soil-moisture-controlled scheme but uses the soil temperatureas the controlling variable. Targeted on reducing urban environ-mental temperature during hot periods, the scheme activates urbanirrigation once the temperature of top soil layer exceeds a thresh-old value. Each time the irrigation amount also equals to the dailyirrigation amount of Scheme 2. During prolonged daytime periodof hot summers, this scheme may easily lead to over irrigation. Toavoid waste of water resource, the irrigation amount is then reg-

by ence br. Foly locted t bioo et

of iix. Ty

0.24 poinoistual ant to tl-temrst

odel

uanmewurba

mod featul andsoluten vaions

Fig. 1. A sche presendimension (ca dth). the combined monetary saving as a measure of envi-co-benet. The indirect benet of irrigation on outdoor

fort of pedestrians is also discussed.

cal simulations

on schemes

Phoenix metropolitan area is selected as the study area.ion period was one entire calendar year, 2012. Phoenixation of more than 1.5 million in 2013, and is the sixthous city in the United States [22]. Located in a semi-nment, Phoenix has a tremendous demand for coolingo other cities [23], thus providing a large potential forergy saving through optimizing irrigation schemes [24].

xeric and mesic are two typical vegetated residential Xeric sites usually comprise drip-irrigated, low water-nd/or desert-adapted plants, while mesic sites mainlyrf grass and shade trees [25]. Though xeric landscapingserve water resource, mesic landscaping provides valu-nmental services by, e.g. reducing urban warming andtormwater management, and is esthetically appealing

atic of irrigation in the urban canopy layer is showncusing on irrigation of mesic neighborhoods, four dif-n irrigation schemes are tested for Phoenix. Scheme 1ine case with no irrigation during the entire simulationme 2 is a daily constant scheme that represents current

ulateddifferesmallesistentcondusuppor

VolimpactPhoen0.15 towiltingling mResidurespecthe soias the

2.2. M

To qing frain the canopymodelnaturalytical has becondit

matic of lawn irrigation in residential areas. The two-dimensional big canyon renyon length) much larger than the planar dimensions (building height and road wiither the daily irrigation amount of Scheme 2 or theetween top and saturated soil moisture, whichever isr cool to cold months where soil temperature is con-wer than the threshold value, essential irrigation isto maintain soil moisture above the wilting point tological functions of mesic vegetation.al. [29] have conducted a comparative analysis of therrigation scheduling at both mesic and xeric sites inpical wilting point for mesic site is found to be from. In this study, the lower bound value 0.15 is used as thet and the upper bound value 0.24 is used as the control-re for the soil-moisture-controlled irrigation scheme.d saturated soil moisture is set to be 0.10 and 0.50. Withhe threshold soil temperature for irrigation activation inperature-controlled scheme, a value of 22 C is adopted

step to illustrate performance of the scheme.

evaluation

tify the impact of urban irrigation, an integrated model-ork that physically resolves energy and water transportn environment is needed. Here a state-of-the-art urbanel (UCM) developed by Wang et al. [1921] is used. Theres detailed description of hydrological processes over

engineered surfaces, sub-facet heterogeneity, and ana-ions to heat diffusion equations. Capability of the modellidated by eld measurements under different climate[28]. Detailed computational processes of the model can

tation is adopted to represent the urban area with the longitudinal

J. Yang, Z.-H. Wang / Energy and Buildings 107 (2015) 335344 337

(oC

)

45

55

Observ ation

Prediction

(a)

be found inAccuracy oets of Phoeirrigation osumption. Pnd

tem

per

ature

25

35Gro

u

5

15

Jan Feb AprMar AJulJunMay

Can

yo

nai

rte

mp

erat

ure

(oC

)

5

15

25

35

45


Sen

sib

leh

eat

flu

x(W

m-2)

0

30

60

90

120

150

Jan Feb Ap rMar JulJunMay

Lat

ent

hea

tfl

ux

(Wm

-2)

0

20

40

60

80


(c)

(b)

(d)

Fig. 2. Comparison of predicted and observed average (a) Tg , (b) Tcan , (c) H, and (d) L

original paper [1921] thus are not duplicated here.f the UCM in capturing the energy and water budg-nix is crucial to accurately assess the impact of urbann environmental temperature and building energy con-revious studies have evaluated the performance of the

UCM for Phthe monthldemands, tvious studieis obtained OctSepug DecNov

OctSepug DecNov

OctSepAug DecNov

OctSepug DecNov

E in Phoenix during the entire simulation period.

oenix during a short summer period [6]. Consideringy variation of meteorological conditions and irrigationhe UCM is tested with calibrated parameters from pre-s at an annual scale. Half-hourly meteorological forcingfrom the eddy-covariance tower deployed at Maryvale,


x

x

x

0.4

0.5

Dail y con stant

Soil -moisture-con trolle d

Soil-temperature-controlled

x

A

xxx

A

(a)

amon

West Phoen1 km 1 kmbare soil, anbalance of year 2012 (Scheme 2) for soil moground temheat ux (Hpoints are dis clear fromtions reasonbetween obNovember. and uncertasimulation 12.51 W m

The calibraanalysis.

3. Results

3.1. Compa

On the bcing energyis conducteon environmoutdoor thecover of the12% [31]. Aous desert cMaster Planof 25% by A

reastativinat

to re 3 sptio

for dx xxx

x xx x

x

x

xx x

x x xx x

xx

x

xxx

xxx x

xxx

x

xx

top

(m3

m

3)

0.1

0.2

0.3

No irr igation

Jan Feb AprMar JulJunMay

x x x x xx x x x

x

x x x x x

x x x x x

x x x x x xx x x x

x x x x

Wat

erco

nsu

mpti

on

(m3

m

2)

10-4

10-3

10-2

10-1

Jan Feb AprMar JulJunMay

(b)

Fig. 3. Simulated temporal distribution of (a) top , and (b) water consumption

ix. The experiment site has a footprint area of about, of which about 48.4% is impervious surface, 36.8% isd 14.6% is vegetation [28]. Local-scale surface energythe area has been measured for the entire calendarsee [30] for more details). Daily constant irrigation (i.e.is added into the model to represent practical supplyisture. Comparison of predicted and observed average

the inca vegea combis used

Fig.consumferent perature (Tg), canyon air temperature (Tcan), sensible), and latent heat ux (LE) is shown in Fig. 2. Gaps in dataue to failure and maintenance of individual sensors. It

the graphs that model predictions agree with observa-ably well except for LE in certain months. Discrepancyserved and predicted LE is about 30% in October andThis deviation is largely caused by the spatial variabilityinty in precipitation and irrigation data. For the entireperiod, root mean square errors are 1.39 C, 1.02 C,2, and 7.36 W m2 for Tg, Tcan, H, and LE, respectively.ted input parameters are then adopted for subsequent

and discussion

rison between different irrigation schemes

asis of the demonstrated skill of the UCM in reprodu- and water budgets for Phoenix, a series of simulationsd to investigate the effect of various irrigation schemesental temperature, building energy consumption, andrmal comfort at an annual scale. In 2010, vegetative

Phoenix metropolitan area was estimated to be aboutiming to create a healthier, more livable and prosper-ity, the City of Phoenix has initiated a Tree and Shade

to achieve the recommended average tree coveragemerican Forest for southwestern cities [31]. Projecting

water use consumptiocontrolled water consis similar ttemperaturvariation, wowing to eland Augustmore thanprole of temperaturhave signiurban canyheating andchanging thSecond, diffof longwavresults andin subseque

3.1.1. EffectBy reple

irrigation hNote that thwilt of vegas long as xx

xx

x

x

xx x

x

x xxx

x

x xxx

x

x

xx

OctSepug DecNov

x x x xx x x x

xx x x x

x

x x x xx

x x x x

OctSepug DecNov

g different irrigation schemes in Phoenix in 2012.

e onto residential area, mesic neighborhoods may havee cover of more than 30%. For subsequent simulations,ion of 35% vegetative cover and 65% impervious surfacepresent mesic residential landscape in the near future.hows the temporal distribution of top and watern of all schemes. The annual variability is markedly dif-ifferent schemes: for daily constant irrigation scheme,

pattern roughly follows a bell curve, with the peakn in the pre-monsoon summer, June; the soil-moisture-scheme maintains top at a relatively constant level,umption increases with soil temperature and the trendo that of daily constant scheme. Irrigation of the soil-e-controlled scheme has the most drastic seasonalith water use mainly concentrated in the summer

evated temperatures. Peak water consumption in July for the soil-temperature-controlled scheme is 4 times

that of other two schemes. With vastly differenttop and water consumption, it is expected that soil-e-controlled and soil-moisture-controlled irrigationcantly different impacts on thermal condition in theon. First, various irrigation schedules modify surface

turbulent mixing in the urban canyon, subsequentlye heat exchange between wall surface and canyon air.erent cooling of the ground surface impacts the amounte radiation emitted toward building surface. Detailed

discussion on the difference in heat transfer are shownnt sections.

of irrigation schemes on environmental temperaturesnishing soil moisture for evapotranspiration, urbanas direct cooling impacts on the ground temperature.e UCM does not dynamically simulate the growth andetation. Vegetation is assumed to be fully functionaltop is maintained above the wilting point of mesic


Fig. 4. Monthlsubplots.

landscape. temperaturthe no-irrigcomplicatedgeometry acomputatioevaporativetent of top Tg among dmagnitude has a largeter, whereagreatest coois about 2.1moisture ccontrolled sulated by acooling in suinteraction cooling impreduces theperatures suy reduction in (a) Tg , (b) Tw , and (c) Tcan by various irrigation schemes as compared to the

Fig. 4 demonstrates the reduction of Tg, Tcan, and walle (Tw) by various irrigation schemes as compared toation case. Calculation of these temperatures involves

energy and moisture transport in cities due to urbannd thermal interaction. Please refer to [20] for detailednal process. Under the same meteorological condition,

cooling is determined by the volumetric moisture con-soil layer. Consequently the magnitude of reduction inifferent schemes (Fig. 4a) follows closely the relativeof top in Fig. 3a. The soil-moisture-controlled schemer reduction of Tg than other schemes during the win-s the soil-temperature-controlled irrigation induces theling in the summer. Maximum monthly reduction in TgC in the winter and about 6.3 C in the summer. Whenontent is relatively constant (e.g. the soil-moisture-cheme), evapotranspiration of urban vegetation is reg-vailable radiation at the surface, resulting in the largermmer compared to other seasons. Through the thermalinside the street canyon, urban irrigation has indirectacts on building surface as lower ground temperaturermal radiation emitted toward the wall. Reduced tem-bsequently weaken the sensible heat ux arising from

ground andEffect of diFig. 4b andTcan and Twmonthly cowhich is signoteworthythe evapotrhere represwith intera

3.1.2. Effectconsumptio

Fig. 4 clenvironmentemperaturseasons, it cool to coldon buildingequation th

T(x, t)t

= baseline (no-irrigation) case. Scale of the vertical axis is different for

wall surfaces, leading to the cooling of canyon air.fferent irrigation schemes on Tcan and Tw is plotted in

c, respectively. Monthly variation of the reductions inis nearly identical to that of reduction in Tg. Maximumoling in June is less than 4.0 C for Tcan and 3.0 C for Tw,nicantly lower than the direct cooling effect on Tg. It is

that the cooler canyon air and wall surface in turn affectanspiration process of ground vegetation, thus resultsent the effect of urban irrigation in a built environmentctive exchange of thermal energy.

of irrigation schemes on building energynearly illustrates that urban irrigation cools the builtt throughout the annual cycle. Reduced environmentale can save cooling load of buildings during warm to hotnevertheless increases heating demand of buildings in

seasons. To quantify the net impact of urban irrigation energy efciency, the one-dimensional heat conductionrough walls is solved as

w2

x2T(x, t) (1)


where T is temperature inside the building envelop as a function ofposition x and time t, and = k/c is the thermal diffusivity, with the density, c the specic heat, k the thermal conductivity, andthe subscript w denoting walls. The heat equation can be solvedanalyticallyintegral. Gifact that cothe thermacomputatioto solve thnumber of i-th layer at

Tji= Tj1

i+

where x aequation isboundary c

Buildingequipment,mal load aswhereas thlogical condare expecteing and cofrom the buture for occcombinatioExternal loalope (roof, loss/gain thloads includNote that twater budging interiorfactors are tion. Firstlythe studiedmajor buildenergy-inteinated by exis ignored. Tconstitutesand dry climmitted throanalysis indeffects on rground oosimulated swindows. Lvariation ofBecause theunity, actuathan the heon these asas the heatby

Qjin

=kw

(T

d

where Qjin

is

j, din is the perature ofEq. (3), and

indicates a cooling demand of the building, while a negative valuemeans a heating demand. In the UCM, heat transfer within build-ing wall is computed using a multi-layer algorithm, which enablescapturing evolution of temperature and heat transfer within the

s comion wndatall s

oor hor thconsd), aeren

coolled

met.22 mil-temvel antlum e aner h

ratur the e in sconsto th

is m, totah m

Effect savint wl an aion ning oresouenix watses rter te, sental ency

al = P

Pwatusagfveg i

is th to ag dem

etetiplieea (war pehe brpneirederviccity aal va using Greens function approach [19] using convolutionven the temporal scale of this study (annual), and thenvolution requires the saving of all temporal history ofl eld evolution inside the wall (for 1 year) and is notnally effective, the nite difference method is adoptede heat equation, by discretizing the wall into a nitelayers. The temperature prole inside the wall in the

time instant j is solved as

wTj1i+1 2T

j1i

+ Tj1i1

(x)2t (2)

nd t are the space and time intervals respectively. This solved with a constant inner wall surface temperatureondition.

energy consumption mainly consists of thermal, plug, and lighting loads. This study focuses on the ther-

a strong function of environmental (outdoor) factors,e rest are relatively independent of outdoor meteoro-ition thus their differences due to irrigation schemesd to be small. Thermal load is the amount of heat-oling energy that needs to be added to or removedilding to maintain thermal comfort and control mois-upants. Thermal load of buildings is determined by then of external thermal load and internal thermal load.ds are made up of heat transmitted through the enve-wall, and ground), solar gain through windows, heatrough leaks, inltration and ventilation, while internale heat generated by people, lighting, and equipment.hough the UCM is capable of predicting energy andets for outdoor environment, its capability in simulat-

building physics is rather limited. Therefore, severalneglected when estimating building energy consump-, internal thermal load of buildings is neglected. For

residential area in Phoenix, single family house is theing type [30]. Sparsely populated with little activity andnsive equipment, single family house is generally dom-ternal thermal loads. Secondly, latent load of buildingshis assumption is acceptable for Phoenix as latent load

only about 21% of the annual ventilation load under hotate [32]. Thirdly, the contribution of heat ux trans-

ugh roof and ground oor is not accounted for, as piloticates that irrigation in the street canyon has limitedoof temperature and soil temperature under buildingr. Fourthly, due to model limitation, windows are noto that subsequent results represent buildings withoutastly, the efciency of air conditioning system and the

the building interior temperature are not considered. efciency of air conditioning system is always less thanl energy used for heating and cooling would be largerat ux transmitted through building envelope. Basedsumptions, building energy consumption is estimated

ux transmitted through the wall in this study, given

jin

Tb)

in/2(3)

the heat ux entering the building via walls at time step

thickness of the innermost (discrete) layer, Tjin

is tem- the innermost layer at the same time calculated using

Tb is the inner wall surface temperature. A positive Qin

wall ainsulatommeInner wby indtems fwater demanby diffcase. Incontrosquarethan 0the soture lesignicmaximand ththe othtempewell asschemto the pared savingperiod116 kW

3.1.3. The

comitato coosumptthe savwater in Phogroundsurpasing waestimaronmeconsist

costtot

wheretricity areas, and Wis usedheatincubic mis mulface arin dollfrom twww.sis acquwaterselectriseasonpared to a single-layer algorithm. In this study, a R5all sheathing is considered based on Energy Star rec-

ion and a typical value of 0.9 W m1 K1 is used for kw.urface temperature is assumed to be maintained at 24 Ceating, ventilation, and air-conditioning (HVAC) sys-

e entire simulation period. Fig. 5 presents the monthlyumption, heating penalty (additional building heatingnd cooling saving (reduced building cooling demand)t irrigation schemes as compared to the no-irrigationl to cold season (NovemberMarch), the soil-moisture-scheme consumes about 0.29 cubic meter water perer vegetated ground area for irrigation, notably larger3 m2 in daily constant scheme and 0.16 m3 m2 inperature-controlled irrigation. Relatively high mois-

maintained by the soil-moisture-controlled schemey increases the heating demand of buildings. Monthlypenalty can be up to about 6.3 kWh m2 in early springnual heating penalty is more than 45 kWh m2. Onand, with irrigation concentrated in summer, the soil-e-controlled scheme has the least heating penalty aslargest cooling saving. Total water consumption of theummer is 1.23 m3 m2, which is about tripled comparedumption of 0.38 m3 m2 in other two schemes. Com-e control case (no-irrigation), the maximum monthlyore than 20 kWh m2 in June. For the entire simulationl heating penalty and cooling saving is about 32 and

2, respectively.

of irrigation schemes on net costng of summer cooling load by lawn irrigation is con-ith the cost of increased water usage: it takes waterrid city. The trade-off between water and energy con-aturally leads to the classic question of cost-benet: Isf cooling energy from urban irrigation worth the cost ofrces? Water conservation has been a primary concernas the city receives water from upstream basins ander pumping. Outdoor water use per capita in Phoenixates in other cities. A cost-benet analysis by comb-and energy consumptions can provide a quantitativerving as a reasonable economic measure of the envi-sustainability of various irrigation schemes. For unit, total cost is given by:

water

(w

h

)fveg

t

W + Pelectricity

t

Qin (4)

er and Pelectricity are the unit prices of water and elec-e respectively, w/h is ration between ground and walls the areal fraction of vegetation over ground surface,e water consumption rate. The absolute value functionccount for electricity consumption by both cooling andand of the building. Note that water cost has a unit of

r water per square meter vegetated ground area, andd by the fraction of vegetated ground area to wall sur-/h)fveg for unit conversion. The resulted total cost isr square meter wall area. Electricity price is obtainedasic plan of local company Salt River Project (http://t.com/prices/home/basicfaq.aspx#2) and water price

from the city of Phoenix (https://www.phoenix.gov/es/customerservices/rateinfo). Table 1 summarizes thend water prices for Phoenix, both prices have a strongriation with high charges in summer. Note that the Salt


Fig.

Table 1Summary of e

Month

January February March April May June July August September October November December

River Projewhich electof electricithowever, coof this stud 5. Monthly (a) water consumption, (b) heating penalty, and (c) cooling saving by variou

lectricity and water prices in Phoenix.

Electricity price ( kWh1) Water price ($ m3)

8.03 1.008.03 1.008.03 1.008.03 1.19

12.31 1.1912.31 1.3212.83 1.3212.83 1.3212.31 1.3212.31 1.198.03 1.198.03 1.00

ct also offers a Time-of-Use plan and an EZ-3 plan inricity price is higher during on-peak hours. The choicey plans may affect the results of total monetary saving,mparison between different plans is beyond the scope

y.

Monthlyas compareshow that more total $2.5 m2 inAugust. In nates, additotal cost (moisture-coof the soil-tTable 2 sumand total comoisture-codaily consttotal cost, pcool seasonicantly largother two ssaving in coirrigation scost of mesoil-tempers irrigation schemes as compared to the no-irrigation case.

saving in total cost of different irrigation schemesd to the no-irrigation case is shown in Fig. 6. Resultsduring hot seasons, irrigating more water leads tosaving. Maximum monthly saving can be up to about

the soil-temperature-controlled scheme for June andcool to cold months when heating demand domi-tional moisture from irrigation results in increasednegative values in Fig. 6). Monthly cost of the soil-ntrolled scheme is about $0.13 m2 higher than thatemperature-controlled scheme throughout the winter.marizes the annual water use, electricity consumption,st of all schemes. Among investigated schemes, the soil-ntrolled scheme has the largest total cost. Compared to

ant irrigation, it consumes more water and has higherrimarily due to the increased heating penalty durings. The soil-temperature-controlled scheme has a signif-er annual water usage, which is 60% more than that ofchemes. However, the cost of water can be offset by theoling energy. Overall, the soil-temperature-controlledcheme is the most efcient in reducing annual totalsic neighborhoods. Table 2 shows that saving by theature-controlled irrigation is relatively insignicant as


s com

Table 2Summary of airrigation sche

Water usage(m3 m2)Energyconsumptio(kWh m2)Annual totalcost ($ m2)

compared tble reason iplanned asimportant irrigation ising the sumbenets of acost-effecticost are achin section 3ciency of aitrade-off angated irriga

3.1.4. EffectIn additi

ing energy for thermalWith a largeof clear daythe United comfort duenvironmenin urban areincluding bradiative ex[34]. Evaluausing varioudeveloped birrigation oof the rate aenvironmen

ITS = E ex

y of m

y ry

t ber

er ber ber

E is the cooling rate produced by sweat which is requiredilibrium, and Emax is the evaporative capacity of the air. E isy

W + Rn + C (6)Fig. 6. Monthly total saving by various irrigation schemes a

nnual water usage, energy consumption, and total cost of all studymes.

No-irrigation

Dailyconstant

Soil-moisture-controlled


0 1.04 1.09 1.79

n1405.8 1335.7 1336.3 1321.6

151.29 143.28 143.32 142.44

o the total cost of daily constant irrigation. One plausi-s that the current irrigation practice in Phoenix is well

water is a precious resource for the desert city. It isto keep in mind that the soil-temperature-controlled

more effective in reducing urban temperatures dur-mer than the current irrigation scheme, thus providing

better living environment to residents, and being moreve. At last, it is worth to point out that results on netieved based on the assumption and simplication listed

Table 3Summar

Month

JanuarFebruaMarchApril May June July AugusSeptemOctobNovemDecem

wherefor equgiven b

E = M

.1.2. Modication of any condition, especially the ef-r conditioning system, may revamp the energy-waterd lead to signicant variation in efciencies of investi-tion schemes.

of irrigation schemes on outdoor thermal comforton to alleviating environmental temperature and build-demand, urban irrigation has important implications

comfort of pedestrians in outdoor urban environment. city size, warm and dry climate, and signicant amounts, Phoenix is among the hubs of urban heat islands inStates where people experience intense thermal dis-ring hot days in outdoor or non-air-conditioned indoorts [33]. The quantication of outdoor thermal comfortas is complicated, due to many environmental factors,ut not limited to temperature, humidity, wind speed,posure, and ambient evaporative and sensible uxestion of the outdoor thermal comfort may be performeds indices. In this study, the Index of Thermal Stress (ITS)y Givoni [35] is selected to identify the impact of urban

n outdoor thermal comfort for Phoenix. ITS is a measuret which the human body must give up moisture to thet in order to maintain thermal equilibrium, dened by

p[0.6

(E

Emax 0.12

)](5)

where M iformed intoexchanges calculationfound in [36a constant 7employed tin urban ca

The ITS fing) in thedifferent irrconducted subjective tindicates a hot. Followwill be veryReduction the no-irrigtal temperareduction ito four majrelative humirrigation uoutdoor thesoil-temperof other twpared to the no-irrigation case.

onthly averaged ITS of all study irrigation schemes.

ITS (W m2)

No-irrigation

Dailyconstant

Soil-moisture-controlled


172.2 169.6 168.9 170.7237.7 233.8 233.0 235.2338.4 333.5 332.3 335.2452.8 438.7 436.9 439.6578.0 554.4 553.9 555.1658.9 627.1 626.4 625.3628.8 605.2 603.2 602.9614.0 595.2 594.0 590.7531.5 533.4 534.1 529.1358.6 345.7 346.4 348.7235.5 229.5 229.0 231.8151.7 149.8 149.0 150.3s the bodys metabolic rate, W is the energy trans- mechanical work, Rn and C are the environmental heatdue to radiation and convection, respectively. Detailed

of Emax, Rn and C using meteorological variables can be], where the net metabolic heat gain (M W) is taken as0 W m2 for the pedestrian. Pearlmutter et al. [37] havehe index to assess outdoor thermal comfort conditionnyon with different geometries.or pedestrians doing gentle outdoor activities (e.g. walk-

street canyon is calculated. Monthly averaged ITS inigation schemes are summarized in Table 3. Givoni [35]a series of empirical experiments to correlate ITS withhermal sensation. An ITS between 280 and 400 W m2

hot thermal condition and above 400 W m2 is verying their denition, outdoor thermal comfort condition

hot for pedestrians from April to September in Phoenix.of ITS by different irrigation schemes as compared toation case is shown in Fig. 7. By reducing environmen-ture and increasing humidity, urban irrigation leads ton ITS throughout the year except for September. Dueor rainfall events, September has a signicantly higheridity than other months. Further moisture brought by

nder the humid condition thus results in degradation ofrmal comfort. In hot summer, reduction of ITS by theature-controlled scheme is more signicant than thato schemes. Maximum reduction is about 35 W m2 in


s as co

Sav

ing

($m

-2)

19 -8

-4

0

4

8

12

Fig. 8. Annualtemperature-cas compared t

June. It is nrather subjof pedestriathis study iirrigation o

3.2. Optimairrigation

The comthe soil-temin terms ofscheme is tas to minimcool a city iof water-enthe soil-temcombined squestion, a atures (in ais carried ousoil-temper

Fig. 8 decombined cschemes asthe graph dthe soil-tem

hotre, craturs a fuvatining tergy2 wea is

withe, theined

ratur. It iFig. 7. Monthly reduction of ITS by various irrigation scheme

energy us e

water use

total

duringperatutempetrend aan actiactivatless en$9.20 mwall arparingschemdetermtempeenergyActivat ing top-soil tem peratu re (oC)

20 21 22 23 24 25 26 27

saving in cost of water consumption, energy cost and total cost by soil-ontrolled irrigation scheme with various activating soil temperatureso the no-irrigation case.

oteworthy that outdoor human thermal comfort is aective measure that is related to physiological aspectsns, which can vary from region to region. The results ofndicate a rather qualitatively positive impact of urbann outdoor thermal comfort in Phoenix.

l soil temperature for the soil-temperature-controlled

parative analysis in previous section indicates thatperature-controlled irrigation is an optimal scheme

annual total saving. The governing mechanism of thiso generate saving in cooling energy in summers as wellize heating penalty during winters. The use of water ton summers necessarily points to the intricate balanceergy nexus. Is there an optimal temperature regulatingperature-controlled irrigation that can maximize theaving of energy and water resources? To address thisset of simulations with six controlling top-soil temper-ddition to the initial controlling temperature of 22 C)t, namely 20, 21, 23, 24, 25, and 26 C, above which theature-controlled irrigation will be activated.monstrates the annual saving in energy, water and theost by different soil-temperature-controlled irrigation

compared to the no-irrigation case. Positive values inenote net saving. At a lower activating temperature,perature-controlled scheme consumes more water

perature devastly for dhere using rst step toefciency. Ftemperatur

4. Conclud

In this to identify Phoenix mtrolled andlandscapes (2) soil-mocontrolled urban areatranspiratiocan be motemperaturenergy conno irrigatioinvestigatescheme actserve watebetween eabout $1.1practice (dsoil-temperenergy-watdetermine t

It is nota qualitativtrade-off inestimating mpared to the no-irrigation case.

periods. Due to the nonlinear distribution of tem-ost of water decreases more rapidly at a lower soile. The combined annual saving exhibits a nonlinearnction of activating soil temperature. Water usage withg temperature of 26 C is only about 18% of that with anemperature of 20 C. The latter consumes 17.1 kWh m2

than the former. Maximum annual saving is aboutall area at 23 C, while minimum saving of $6.47 m2

found with an activating temperature of 20 C. Com- the annual saving of $8.01 m2 by daily constant

activating top-soil temperature needs to be carefully in order to yield the optimal irrigation scheme usinge control in terms of the trade-off between water ands worth to mention that optimal activating soil tem-pends on meteorological conditions and thus can varyifferent seasons or different climatic zones. Analysisa yearly constant activating temperature serves as award optimizing irrigation schemes for building energyurther studies on a temporally varying activating soile are needed.

ing remarks

study, an advanced urban canopy model was usedthe environmental effect of urban irrigation for theetropolitan area. The performance of various uncon-

controlled irrigation schemes on mesic residentialis investigated, including (1) daily constant irrigation,isture-controlled irrigation, and (3) soil-temperature-irrigation. In general, irrigating mesic landscape ins cools the urban environment via enhanced evapo-n. Maximum cooling effect on canyon air temperaturere than 3 C in summer. Results show that the soil-e-controlled irrigation can reduce annual buildingsumption and the combined energy-water cost of then case by about 6%, which is the most efcient among

d schemes. By design, the soil-temperature-controlledivates irrigation during hot periods and helps to pre-r during cold seasons, thus optimizes the trade-offnergy and water use. Annual saving can be up to9 m2 wall area compared to the current irrigationaily constant) in Phoenix. The total saving of theature-controlled scheme requires a ne balance iner use. Site-specic analysis is therefore required tohe optimal activating soil temperatures.eworthy that estimated saving in this study providese rather than quantitative guidance for water-energy

urban irrigation, due to (i) the simplications made inthe building energy consumption, and (ii) the neglect


of the uncertainty inherent in model physics and the parameterspace. In addition, modeling results, especially those for the dailyconstant irrigation scheme, are based on the monthly availablein situ measurement of irrigation over mesic residential landscapes.Timing, duration and amount of actual irrigation vary from neigh-bor to neighbor thus the results cannot be directly up-scaled toextract monetary saving for the entire city. Having more detaileddata availability in other study areas will help to validate andimprove the proposed irrigation schemes in this study. In addi-tion, the numerical model can be further improved by incorporatingdynamic representation of mesic vegetation (e.g., soil water uptake,growth and withering, and root zone dynamics) and advanced sim-ulation of building energy use (e.g., latent load, internal thermalload, and windows). Nevertheless the current study deepens ourinsight into the trade-off between energy and water use and facil-itates a development of new paradigm for urban irrigation.

Acknowledgements

This worunder graneddy-covarNSF under g

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