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Urban Water Balance 2. Results From a Suburb of Vancouver, British Columbia. Grimmond, C. S. B.; Oke, Timothy R. 1986

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WATER RESOURCES RESEARCH, VOL. 22, NO. 10, PAGES 1404-1412, SEPTEMBER  Urban o  Water  1986  Balance  Results From a Suburb of Vancouver, British Columbia C. S. B. GRIMMOND  AND T. R. OKE  Departmentof Geo•lraphy,The Universityof British Columbia,Vancouver,British Columbia,Canada The paper demonstratesthe use of the C. S. B. Grimmond et al. (this issue)water balancemodel. It is usedto calculatethe daily, monthly, and annual water balancecomponentsfor a suburbancatchmentin Vancouver,British Columbia.The budgetresultsfor one completeyear are presentedand where possible are comparedwith thosefrom other cities.The balanceis also comparedwith that for a rural area in the region, thereby illustrating the effectsof suburbandevelopment.In interpretingthe resultsspecialconsiderationis directedtoward elucidatingthe role of irrigation(mainlygardensprinkling)in the suburban water balance.The temporal pattern of external water useis relatedto prevailingweatherconditions.In particular,it is shownto be closelyrelatedto evapotranspiration. The relationshipis a complexfeedback systeminvolving human as well as biophysicalcontrols. The model is run both with, and without, an irrigation input to gauge its impact on the water budget. Together the resultsprovide both quantitative and qualitativesupportfor the idea that irrigation is the sourceof water supportingthe relativelylarge ratesof suburbanevapotranspirationreportedin energybalancestudies.  INTRODUCTION  This paper appliesthe urban water balancemodel of Grimmondet al [this issue]to a suburbancatchment.The purpose of the study is to discoverthe daily, monthly, seasonal,and annual patterns of water exchangein an environment where human modification to hydrologicand climatologicprocesses, amongst others, is pronounced [Lazaro, 1979; Landsberg, 1981; Douglas, 1983]. These changescan be viewed in the simple unifying frameworks of the water and energy balancesof urbanized terrain [e.g., Oke, 1978a]: p+ I=r  + E + AS  Q* + Ql• = Qt• + Q• + AQs  (1)  terms for a suburban area, the land use which occupiesthe largest fraction of the area of most cities. Special attention is given to the piped water supplyfor two reasons.First, residential domestic  uses often  account  for more  than  half of total  water use in many cities [Douglas, 1983]; therefore better knowledge of this term may be helpful in designingmanagement strategies.Second, energy balance measurementsin an area near the catchment used for this study suggestthat evapotranspiration plays a more important role in (2) than might have been expected JOke, 1978b, 1979; Kalanda et al., 1980; Oke and McCaughey, 1983] and that the piped supply, via garden irrigation, may also play an important part. Water balance data can help to establishif the reported energy exchangesare consistentwith thoseof water.  (2)  where p is precipitation,I is the piped-in water supply,r is net runoff, E is evapotranspiration, AS is net water storage change,Q* is net all-wave radiation, QF is anthropogenicheat released by combustion, Qn and QE are the turbulent heat fluxes of sensibleand latent heat, respectively,and AQs is net heat storagechange.The balanceseffectivelyshare a common term, sinceQE is the latent energyinvolved in the massflux of water E. A priori it is usually suggestedthat urbanization leads to an increasein the water inputs p and I but a decrease in both E and AS due to partial waterproofingand removal of vegetation. In response,urban runoff is usually greatly enhanced,leading to the need to engineerthe orderly removal of large volumes of water following precipitation. The parallel view of urban effects on the energy balance holds that the urban inputs Q* and QF combine to heat the city at least as strongly as its environs, and since the evapotranspiration output is likely to be reduced,the sensibleheating of the air Qn and urban material AQs is boosted,giving the well-known heat island JOke, 1982]. In this paper application of the Grimmondet al. [this issue] model helps elucidate the relative roles of the water balance  METHODS  Sites and Instrumentation  The Grimmondet al. [this issue]urban water balancemodel was applied to a residential suburb of Vancouver, British Columbia. The input data were derived from measurements  conductedat threesites(Figure 1). The primary site,to which the model resultsapply, is the Oakridge catchment.This is a 21-ha test site set up by the City of VancouverEngineering Department (Water Works Division). The catchment is definedby a linked networkof water supplypipes.It is situated on a slight (less than 1ø) southfacingslope. There are no streamsflowing through the area, nor evidencethat there were beforeurbanization[Proctor,1978]. For budgetpurposesthe catchmentmay be expectedto exhibit no net long-term changein storage. There are approximately420 residentsliving in 191 one- or two-storey,single-familydwellings[StatisticsCanada, Census Division, 1983]. Analysis of aerial photographsshows the catchmentto have 60% perviousand 40% imperviousland cover.Note that lessthan 1% of the surfacecoverwas open water (swimmingpools, etc.). Observationsin the catchment includeddaily totals of water use by the householdsfrom a meter installedon the feederpipe, and of precipitationbased  Copyright 1986by the AmericanGeophysicalUnion.  on the catchesof four 100-mmrain gaugesmountedwith their orifices0.3 m abovethe groundat differentlocations.  Paper number 5W4318. 0043-1397/86/005W-4318505.00  Soil moisture measurements were made once a week at a 1404  GRIMMOND AND OKE: URBAN WATER BALANCE,2  1405  ] and parks, recreational, agricultural undeveloped areas  •  •ndustrial, commercial and institutional areas  ':-"•• residential areas  0  I  1  I  Kilometres  English Bay  2  I  •  Fig. 1. Distributionof land usesin the Vancouverarea and the location of the study catchmentand climatological stations.  secondsite 0.3 km north of the Oakridge catchment (Figure 1). The average dry weight moisture content of the upper 0.2-m layer was determinedusing the gravimctric sampling technique[Gardner, 1965]. The third site, the Kcrrisdale climate station, is 0.8 km northwestof the Oakridgecatchment(Figure 1). A pneumatic,  energyand water balancesof the VancouverregionseeHare and Thomas[1979].) The footnotesto Table 1 draw attention to the most significant deviations of the study year from normal conditions.  Consideringthe contentof the presentstudythe most important anomaliesare that the May-June period was drier, July telescopic mastsetup in a backgardensupportedinstruments wetter and lesssunny,and January 1983 much warmer, than at a heightof 9 m abovethe groundto sensenet radiation(net normal. These featuresare only of interestin the context of of the studyperiod.They presentno special pyrradiometer), wind speed(sensitivethreecup anemometer), representativeness relative humidity (electricalcapacitance),and air temperature problemsto the useof the model.During the studyyear virtually all of the precipitationfell as rain. No similardata exist (thermistor).Precipitationwasmonitoredusinga 200-mmdiameterorificetipping bucketrain gaugemountedon a garage to judgethe normalcyof the wateruseand soil moisturedata but no specialconditionsare to be anticipated. roof at 5.7 m. All signalswere recordedhourly on a microThe anthropogenicheat flux input to the urban systemwas loggerand storedon cassettetapes. Observation  Period  not enteredas a separateterm. To do so would risk "double counting"thisheat sourcebecause it is alreadyrepresented in  Observations were conducted at the three sites for one comthe measurednet radiation, air temperature,and humidity plete year: January22, 1982,to January21, 1983,inclusive; data. General estimatesfor the total city of Vancouver have this will be referredto as the study year. It is perhapshelpful beengivenby Yap [1973]as 15, 19,and 23 W m-2 for the to considerhow representativethis year was in comparison summer,winter, and annualperiods,respectively. The heat storageparameterizationschemeused here is with climatic normals.This can be accomplishedwith the aid that strictlyshouldonly be of Table 1, which lists the mean monthly and annual climatic basedon empiricalrelationships normals of five important meteorologicalvariablesfor the usedin the warmer half of the year. Their use in the winter VancouverInternational Airport (Figure 1) togetherwith the leadsto a nonzerostorageover the annual period. To account for this the winter data were adjustedaccordingto a simple correspondingdata for the studyperiod. Vancouver experiencesa modified marine westcoastcli- linear correction scheme with a maximum correction of 18.6 mate. Winters are mild, cloudy,and wet with frequentcyclonic W m- 2 on the date of the winter solstice. activity.Summersare coolbut sunnywith moderatehumidity. Thus for about half of the year input of water is high and Model Input Values The valuesof the Oakridge catchmentparametersused to radiant energyis low, whereasin the summerthe reverseis true. Through much of the winter the soil storageis at field run the Grimmondet al. [this issue]water balancemodel for capacityor is saturatedso that mostof the abundantrainfall the studyperiodare listedin Table 2. As noted,the areasof  is lost as runoff, but in late summer it is common for there to  be a water deficit and mild drought.(For detailsof the general  imperviousand perviousland coverwere determinedfrom aerialphotographs. It wasassumed, basedon generalobserva-  1406  GRIMMOND ANDOKE.'URBANWATERBALANCE, 2 TABLE 1. Comparisonof Normal (1951-1980)and 1982-1983MeasuredValuesof Climatic Variables for VancouverInternationalAirport Mean  Mean Air  Precipitation,  Temperature,  mm  Month  Normal  January* February'• March April May• June Julyô August September October November December{} Total  JanuaryII  153.8 116.6 101.0 59.6 51.6 45.2 32.0  øC_  Bright  Relative  Sunshine,  Humidity,  hours  1982  Normal  1982  Normal  238.6 224.6 64.9 89.3 22.6 29.2 67.0  2.5 4.4 5.8 8.8 12.2 15.1 17.3  1.9 4.2 5.5 7.7 12.1 16.7 17.0  53.5 92.5 129.3 180.5 246.1 238.4 307.1  41.1  37.2  17.1  16.8  256.2  67.1 114.0 150.1 182.4 1112.6  44.2 118.0 174.9 149.7 1260.2 1983  14.2 10.0 5.9 3.9 9.8  14.6 10.2 4.2 4.1 9.6 1983  183.1 121.0 69.3 47.9 1919.6  153.8  172.3  2.5  6.3  53.5  %  Mean Wind  Speed, km h- •  1982  Normal  1982  Normal  1982  23.6 69.1 151.9 226.2 251.5 285.1 210.3 219.9 168.4 126.7 89.9 71.8 1894.4  87 85 82 75 74 76 74  85 95 78 69 70 72 75  78  77  80 87 88 89 82  82 83 84 83 80 1983  12.2 12.1 13.5 13.3 11.8 11.5 11.4 10.6 10.6 11.2 12.2 13.0 12.0  12.4 13.8 11.4 14.7 12.2 11.3 12.8 12.3 10.7 12.9 10.2 12.2 12.2  87  86  1983  49.8  1983  12.2  11.8  Data fromEnvironment Canada[1982;1983]andHay andOke[1976]. *Sunshine second lowest on record.  •'One of the wettest. $Third driest on record. ôLowest sunshineand fifth wettest on record. {}Sunshine third highest. IlWarmest on record.  tion, that 50% of the pervious cover was in receipt of irrigation water. The values of the wind profile parameters(the displacementand roughnesslengths) used were those calculated by Steyn [1980] for a similar residential area to the east of the Oakridge site (at sunset,see Figure 1). Estimateswere based on a detailed descriptionof the site geometry and the algorithmscitedin the work by Grimmondet al. [this issue]. Values of the surfaceretention capacity were assignedfollowing a review of the literature (Table 2) and weighted according to the fraction of the catchmentarea coveredby the surfaces involved. The soil storage capacity was similarly chosen.Storagewas assumedto be full at the initiation of the model; this is reasonable, since the model was initiated at the  time of midwinter, a very wet period in Vancouver(Table 1). The soil field capacity was determined from field and laboratory tests. The infiltration rates for the pervious areas were assumed to be greater than the precipitation rates. This is based on measurementsof infiltration rates (J. de Vries, personal communication, 1982; see Grimmond [1983]) and precipitation intensity records. The mean daily winter water use lw, which is used in the ß  model to separatethe internalfrom the externalwater supply l e, was calculatedto be the averageof the metereddata distributed  across the  November-March  total  catchment  area  for  the months  inclusive. WATER BALANCE RESULTS  The Grimmondet al. [this issue]urban water balancemodel was run for the Oakridge catchmentfor the study year. The output givesthe water balancesof both the internal and external hydrologicsystems(asdefinedin the work by Grimmondet al. [this issue]and illustratedin their Figure 1). Thesewill be treated separatelyhere; for water engineeringand other purposesit would be acceptableto sum them to form one integrated suburban system.  Internal System  The internal balance simply consistsof an equality between the input of piped water for domestic purposes within the home and the output of waste water via the sanitary drain and sewer system. At Oakridge the input was almost constant throughout the year. On a mean daily basis each household used 827 L of water, or about 376 L for each resident, which  when averagedover the catchmentis equivalentto a depth of 0.76 mm of water. The mean monthly quantities are therefore about 22 mm. These figuresdo not account for water losses from the pipe system. In relation to the total (internal plus external) runoff from the catchment, the output from the internal systemis most important in the summer.At that time there is the least rainfall to shed,and the soil moisture storageis capableof accepting some infiltration. In fact, internal runoff (rw _• 22 mm) exceededthat in the external water budget re for 4 of the summer months, and was of the same magnitude in 2 more (Figure 2). In winter the external runoff is almost an order of magnitude larger. External System  The external suburban water balance for the study catchment is given as annual and seasonaltotals in Table 3, and the monthly totals are plotted in Figure 2. This is the "peoplemodified" hydrologic system(including the effectsof surface cover change such as removal of vegetation,paving over the  soil, artificial drainage networks, etc.; the results of piped water use such as garden and other irrigation, filling swimming pools,streetcleaning,storage pondsand reservoirs,etc., and any changesin precipitationclimatology due to urban  modificationof the atmospheric boundarylayer).Whereasthe internal system is self-contained(except for leaks) and is routed by engineeredpathways,the external systemmeshes  GRIMMONDAND OKE: URBAN WATER BALANCE,2  1407  TABLE 2. ValuesUsed to Describethe Oakridge Catchmentfor 1982-1983 Water BalanceCalculations Catchment  Parameters  Impervious, A •  Value  Source  PhysicalLand Cover 0.40  Pervious  unirrigated, A 2 irrigated,A 3 Displacementlength,d Roughnesslength  0.30 0.30 3.5 m  photo analysis  vapor,Zoo momentum,Zorn  52 mm 0.52m  Brutsaert [1982] Steyn [1980]  Measurementheight of wind, z, vapor, zo  Steyn [1980]  9m 9m  HydrologicPropertiesof the Surfaceand Subsurface Imperviousretentioncapacity,S•* 0.59 mm Brater [1968] Pervious retention capacity* Wright and McLaughlin unirrigated,S2 2.11 mm [1969] irrigated,S3 2.11 mm Soil storagecapacity,S,• 150 mm Hare and Thomas[1979]  Fieldcapacity,0s Water  Mean daily winter water use, Impervious area receiving external piped water,/5  0.55  measured  Use 0.76 mm 1.0  measured assumed  Initial Storage Conditions Soil  150 mm  Retention, impervious Retention, pervious,unirrigated Retention, pervious,irrigated  assumed  0 mm 0 mm 0 mm  Calculationsare determinedusingdata from Grimmond[this issue]. *Capacity is the capacityfor surfacetype x fraction of area.  suchnetworksv•ithnatural•rocesses and environments to form a complex hydroclimatologicalfeedbacksystem. Water inputs to the external systemare strongly seasonal. In the winter, precipitation is the only significantinput, but in summer external water use plays an equal role and in some months is the primary source.Thus summerirrigation helpsto maintain the possibility of water availability in the Oakridge catchmentthroughout the year. How general this availability is dependson the spatial pattern of irrigation. Outputs from the system show a similar seasonalbias. In the winter, when energy for evapotranspirationis limited and the soil moisture store is well stocked, 90% of the lossesare as runoff. However, in the summer the primary output is to the atmosphere(81%). The change of soil (and other) moisture storage in the systemis remarkably small. The systemis between field capacity and saturation during the winter so there is little change.The greatestactivity is in the spring when precipitation and runoff both drop but evapotranspirationis growing rapidly in responseto increasingenergyavailability. Soils are still moist, vegetationhas no difficulty in acquiringwater for growth processes,and there is a significantdraw down of the soil moisturestore. By early summerthe residentsperceivea  needto supplementthe precipitationsupplyby irrigation.The linkagebetweenthis perception,the amount of water applied, and the status of the moisture  store is close and will be dealt  250  .-. 200 I-  P  150  100  I1 '..iSll \,  E  /,,% /  100  •  j  t  E  :D150 ]  o  ••,//•'\\•>  200  /h re 250 I  I  I  I  I  I  I  I  I  I  I  I  I  J  F  M  A  M  J  J  A  S  O  N  D  J  with in a later section. Figure 2 shows that the inputs are TIME (months) equal to, or slightly exceed, the outputs so that storage changesare small. By November the soil storesare restocked Fig. 2. External monthly waterbalance fortheOakridge suburban catchment for 1982. again.  1408  GRIMMONDAND OKE.' URBAN WATER BALANCE,2  TABLE 3. Seasonaland Annual Water Balancesfor the Oakridge Catchment  Period  p  +  Ie  =  E  +  AS  +  re  input (p + le) was lost to the air leading to a draw down of the soil moisture store. The extreme conditions underlying the 1980 results have been discussedby Oke and McCaughey •1983] and Cleughand Oke •1986].  Summer  mm %  298.0 50.1  296.2 49.9  483.0 81.3  - 18.1 - 3.0  129.2 21.7  SUBURBAN-RURAL  HYDROCLIMATOLOGICAL  PARAMETERS  Hare and Thomas [1979] provide water balance data for the Vancouver area using Thornthwaite climatological budget techniquesand the VancouverAirport climatologicalnormals. Year* Here we will characterizetheseestimatesas being for a "rural" mm 1214.7 301.7 577.7 3.4 935.4 site and will compare them with the 1982 suburban set. To % 80.1 19.9 38.1 0.2 61.7 minimize some of the problems associatedwith comparing Resultsare for the externalwater systemonly and are expressed normals versusa singleyear, and a natural budget versusone both as depth of water (mm) and as percentageof the total water supplementedby irrigation, the data have been nondimensioninput (%). alized by expressingall values as ratios of the total water *The studyyear. SeeTable 4 for the corresponding wholesystem input to the system. For example, the rural-runoff ratio is (internal and external)annual water balance. Winter  mm %  916.7 99.4  5.5 0.6  94.6 10.3  21.5 2.3  806.1 87.4  defined as is usual:  Cr '-- rr/Pr  COMPARISON WITH OTH]ER URBAN BALANCES  Table 4 is a summarylist of attempts to evaluate the water balanceof citiesor urbanizedcatchments.Direct comparison of the results from these studiesis not straightforwardfor many reasons.Foremost amongst which are the geographic differencesbetween cities (e.g., climate, physiography,vegetation, soils, urban form and urban function, especially degreeof industrialization),the definition of the area represented(e.g.,whole or part of a city or an amalgamof urbanized territoriessuchasin the caseof Sweden,Table 4), and not least the emphasisof the original study and the techniques used to estimate or measure the terms in the balance. In con-  structingTable 4 somelicensehas been taken in interpreting the original studiesand, in order to aid comparison,the balanceshave been standardizedto a whole (external and internal) basisand to the annual period. It is evident that the piped water supply representsfrom 14 to 40% of the total water input of these cities and that the Oakridge valuesare closeto average.The piped input can be partitioned betweendomestic,industrial,municipal,and agricultural uses,plus leakage.In many urban areasdomesticuse is the largest category (e.g., Sydney 59%, Sweden 55%, and Mexico City 53%). In old water distribution systems,especiallywhere frost heaveis a problem,leakage"losses"from the water mains may approach50% [Douglas,1983]. On an annual basischangein storageis approximatelyzero so the output side of the budgetis sharedby evapotranspiration and runoff. The exact breakdowndependson many factors including the degree of surface waterproofing by built uses,the natural and engineereddrainagenet, irrigation practices,generalc!imate,etc. The rangein Table 4 showseachof theseterms can accountfor 30-70% of the output. Perhaps the least expectedresult for most hydrologistsis to find that  but the correspondingsuburbanratio is Cs = rs/(p + I e)  Similar ratios are definedfor evapotranspirationF and water storagechangeG for both environments.Finally, to illustrate  the degreeto whichthe suburbanbalanceis supplemented by irrigationwe havean irrigationratio (Hs = le/(P + Ie)). In most respectsboth environmentsbehave in a similar fashionduring the precipitation-dominated, colderhalf of the year (Figure3). In/he otherhalf very markedrural-suburban differencesoccur,especiallyin F and G. This is coincidentwith the period of irrigation when H s = 0.78 (June)there is 3.5  timesmorewater•input to thesuburban system thanwithout irrigation. The rural and suburban ratios show a similar variation  throughthe year(Figure3a).Overall,the suburban environment shedsa greaterproportionof its input via runoff.On an annualbasis,Cr = 0.5 and Cs = 0.62.This is probablyattributable to the greaterwaterproofingof the suburbansurfaceand to the summerstockingof soil moistureby irrigation.The latter factor reducesthe needto rechargesoil moisturein the autumnand hastensthe arrival of the time wheninput generates runoff. Figure 3a also includes measured data from a  rural catchmenteastof Vancouver.The ratioswerecomputed usingthe gaugedstreamflowfrom West Creek (44 km to the west of the study site) and precipitationfrom the Whalley ForestNurseryclimatestation(22 km to the westof the study site).In general,the agreementis reasonablebut the higher summervaluesat West Creek are probably due to groundwater affectingthe baseflow.Groundwater was not considered to be important in the calculationsfor the other two cases.  The summerevaporationand storagechangeratios(Figure  The rural patternshowsevapotranspievapotranspiration frommid-andhigh-latitude cities canap- 3b)areveryinteresting. proach 60% Olaan annual basis, and as demonstrated earlier, can be greater than 80% in the summer months for the external budget. The external budget resultsfor Oakridge compare very fa-  vorably with the resultsof an analysisconductedby S. M. Loudon and T. R. Oke (unpublishedmanuscript,1986)in the same Oakridge catchment for two summer months in 1980  (Table 5). The 1980studyutilizeda very much simplercomputational scheme.The proportion of the water input derived from piped water was the same in both years, as was that contributing to runoff. The primary diffferenceis the role of evapotranspiration.In 1980 an amount equivalent to all the  rationto greatlyexceedthe waterinput(i.e.,Fr > 1)from May to September. This resultsin a dramaticdrop in waterstorage for the sameperiod.The suburbancasebeginsthe sameway in May but thereaftershiftsto a quite different mode: from June to Septemberevapotranspirationstayssmallerthan the water supply(Fs < 1). Indeed, Fs almost seemsto be tied to Hs, raisingthe possibilitythat irrigation plays a controlling role in suburbanevapotranspiration. Equally remarkableis  the lack of storagechangeduringthe summer.SinceCsis not a major componentat this time, this impliessomedegreeof off-setting,feedback,or balancebetweenF s and Hs. These questionsare the focus of the sectionwhich follows.  GRIMMOND AND OKE: URBAN WATER BALANCE,2  TABLE 4.  1409  Examplesof Urban Water Balance Studies(in Order of DecreasingArea)  Author  Comments (Location, Period, Date, Area) Total urbanized area of Sweden: annual, circa 1970' 4024 km 2  Lindh [1978]  Purpose:part of theInternationalHydrological Decade researchprogram Techniquesused:p, I, W, E, and r estimatedbut method  not stated  p+I-E+r+AW 701 + 235 = 360 + 576 + 0, mm 75 + 25 = 38 + 62 + 0, %  Campbell [1982]  Mexico City, Mexico' annual, 1980' area not defined  Purpose' part of a study of Mexico City as an ecosystem  Techniquesused:p, !, E, and r estimatedbut method  not stated  p+l=E+r 86 + 14-  71 + 29, %  Hong Kong' annual,1971' 1046km2  Aston [ 1977]  Purpose' prediction of future water requirements Techniquesused' p measured,E measured(pan), r residual,and W measured p+l+W=E+r 1912 + 1310 + 64 - 1128 + 2158, mm 58 + 40 + 2 - 34 + 66, % (this includes groundwater replenishment)  Sydney,Australia'annual,1962-1971'1035km2 Purpose'predictiono[ futurerequirements for  Bell [1972]  disposalof sewageeffluent Techniquesused' p measured,I, W estimated but method not stated,E modeled (Penman basis), and r modeled (empirical) p+l+W=E+r 1150 + 333 + 16 = 736 + 763, mm 77 + 22 + 1 = 49 + 51, % L'vovich  Moscow,USSR' Annual' 879 km 2  and  Chernogayeva [1977]  Purpose: To determine the influence of urbanization Techniquesused:p measured,E residual,and r modeled  p=E+r 700 = 400 + 300, mm 100 = 57 + 43, % S. M. Loudon T. R. Oke  and  (unpublished manuscript, 1986)  Oakridge, Vancouver, Canada: daily, 2 months,  July-August1980;0.21km2 Purpose: to determine the summer suburban water balance  Techniquesused' p and I measured,E measured (Bowen ratio) or modeled(regression),and r and AS estimated  p+I=E+AS+r 90 + 172 = 190- 25 + 97, mm (2 summermonths) 34 + 66 = 73 - 10 + 37, %  This study  Oakridge, Vancouver, Canada: daily, monthly, seasonal,annual, 1982' 0.21 km 2 Purpose' to investigatethe relative importance of the componentsof the suburbanwater balance Techniquesused' p and I measured,r and AS modeled, and E modeled (combination method) p+I=E+AS+r 917 + 141 = 95 + 22 + 941, mm (winter) 87 + 13 - 9 + 2 + 89, %  298 + 436 = 482 - 18 + 269, mm (summer) 41 + 59 = 66-  3 + 37, %  1215+ 576 - 578 + 3 + 1210,mm (year) 68 + 32 = 32 + 0 + 68, % Here, W, groundwater;AW, changeof groundwaterstorage. ROLE OF SUBURBAN IRRIGATION  SuburbanIrrigation Patterns  The Oakridge residentialarea is generallya prosperousone  with largerthan averagelots (670 m2) that are generallyas-  sociatedwith well-tendedgardens.The water supplyis based on a flat rate of payment and use is unrestricted. On an  annual basis, 52% of the piped water is used for garden sprinkling. Two water use patterns are evident at Oakridge. First, in  1410  GRIMMONDAND OKE' URBAN WATERBALANCE,2 JULIAN  TABLE 5. Comparisonof the Oakridge External Water Budgetin July-August 1982 With That in 1980 for the Same Months Year  p  +  Ie  =  E  +  AS  +  r  103 45  + +  128 55  = =  182 79  + +  16 7  + +  33 14  90 46  + +  106 54  = =  190 97  25 13  + +  31 16  22  •  4  DAY  32  42  52  62  72  82  92  ,  i  i  i  I  i  ,  i  i  i  i  i  i  i  102  __., ,•, 2  1982  mm %  ._.  60/0  1980  mm %  After S. M. Loudon and T. R. Oke (unpublishedmanuscript,1986).  the colderhalf of the year (e.g.,Figure 4 (top))the total water useI - I,• is almostconstantat approximately0.76 mm d- x and shows no dependenceupon weather characteristics. Second,in the warmerhalf of the year(Figure4 (bottom))the amountsare often greater(up to a maximumof 7.7 mm d-•, of which approximately7 mm d- x is attributableto external uses)and the patternis very much more variable.Even visually it is evident that variability is related to weatherevents. 1.2  •  1.0  -  0.8  rr  0.6  u_  0.4  z  •  i  i  i  i  i  i  i  I  i  I  z  40  2O  0  22  1  11 21 FEB  JAN  3  13 MAR  23  2  12 APR  TIME  JULIAN 123 8  133 .......  143  153  DAY  163  173  %households t  183  193  203 100  PipedWater(I)  ß  50m•  60/0  0.2  I  i  I  I  I  I  I  30/0  40  20  20  10  0.0 '  '  I  I  I  I  I  I  I  I  I  I  J  F  M  A  M  J  J  A  S  0  N  D  i  ,  i  ,  i  ,  ,  ,  ,  2.4  0  2.2 (b)  0  3  13  23  2  12  MAY  2.0  22  2  JUN  12  22  JUL  TIME  Fr  1.8  Fig. 4. Variationof daily totalsand averagesof total wateruseI and climatological characteristicsat Oakridge in the periods January-March 1982(top) and May-July 1982(bottom).  1.6  1.4  1.2  1.0 0.8  Precipitation leads to a drop in external use; on the other hand, increasesof air temperature or solar radiation (not shown)give rise to an increasein water use (for sprinkling, swimmingpools,car washing,etc.). The importance of sprinkler irrigation was confirmed through a surveyof Oakridge residentsin May-August 1982. During this period approximately 12% of the households  0.6 0.4  0.2  0.0 -0.2  -  xx• •  tt  -0.8  age of surveyhouseholdsreporting external use on each day are plotted in Figure 4 (bottom), and show a remarkablecorrespondencewith the piped water record from the meter. It  tt  ,  -0.6  maintained a dailydiaryof wateruseactivities. Thepercent-  tt  t  -O.4  l  'tl  -1.0  shows that suburban  / Gr ß  ß  t  -1.2  J  I  I  I  I  I  I  I  I  I.  I  I  F  M  A  M  J  J  A  S  O  N  D  i  water use variations  are the result of  more or less people watering rather than the same number using greater or lesseramounts of water. Sprinkling was the overwhelminglyreported use. The association between summer water use and weather can  be quantified statistically.Simple linear regressionbetween I and air temperaturealone explains 66% of the variance and for the July-August period this rises to 72%. S. M. Loudon Fig. 3. Annual variation of hydroclimatologicalparameters for rural (Vancouver Airport) and suburban (Oakridge) environments, and •T. R. Oke (unpublishedmanuscript,1986) found a 66% where C is the runoff ratio, F is the evapotranspirationratio, G is the explanation for the same variable in July-August of 1980. water storage change ratio, H is the irrigation ratio, subscripts is Cohen[1985] found similarresultsfor Metropolitan Toronto suburban,and subscriptr is rural. Seetext for definition of ratios and data sources. during the summer.If additional variablessuchas the number TIME  (months)  GRIMMONDAND OKE.' URBANWATERBALANCE,2  152  •  10  162  172  JULIAN  DAY  182  192  202  212  222  ......  1411  controlledby a variety of biological,hydrological,and climatologicalprocesses. A similarlystrikingrelationshipwas obtained by S. M. Loudon and T. R. Oke (unpublishedmanuscript, 1986) for the same site in 1980 and included measured œfrom energybalanceobservations.  The magnitudeof mostof the maximumIe and E valuesare  in therange5-6 mmd-1 duringthismidsummer period.Why theyshouldagreeso well is not clear.It may be purelyfortu-  25/0 20  15  lO  ,,..,I. ........,  5 o:  o  1  10  20  30  11  21  31  10  itous,or it is conceivable that irrigationtoppingup of the soil moistureis maintainedat just the rate necessary to matchthe lossestherebykeepingAS approximatelyzero. However,such action would also have to take into account the contribution  to storage by precipitation. There is some evidence of this.  Notice that whereasthe irrigationresponseto precipitation input is virtuallyimmediate,the recoveryfollowingthe event revealsan inertia comparedto the evapotranspiration (e.g., Fig. 5. Daily totalsof externalwateruseI e and evapotranspiration July 4-10, July 15-18). By contrast the recovery following a E at Oakridgeduringa midsummerperiodof 1982. cloudy(but not rainy)periodis moreimmediate(e..g,June15, of days sinceprecipitation, soil moisture and net radiation are August6). This suggests that the perceptionof soil moisture included in a stepwisemultiple regressionanalysisthe expla- status(probablyto supporta greenlawn) is the motivating nation of variation can be raised to 85%. This high level of factor. Woolmingtonand Burgess[1983] also considerthat explanation is possible despite the influence of nonweather wateringis stronglyrelatedto the perceptionof heat and the factors (e.g., day of week, holidays) and is thus interesting consequent desireto engagein water play. They alsonote that becausethe processesinvolved are not simply part of a physi- much of the irrigationin Canberrais careless, leadingto cal cause-and-effectsystem.The sequenceinvolveshuman de- runoff, as water findsits way onto footpaths,driveways,and JUN  JUL  AUG  TIME  cision making and action (to sprinkle or not) which is only indirectly linked to weather eventsthrough the perceptionand  road gutters,and then to streamsor sewers.A similar observation appliedto the Oakridgecatchment.  assessment of the need for water.  The systemillustratedin Figure 5 is probablya complex feedbackloopsothat it is difficultto identifythe drivingforce. Certainly,the removalof waterby evapotranspiration creates the requirement for irrigation,but as a consequence, evapotranspirationis enhanceddue to the greateravailabilityof water.Nevertheless, the closeness of the linkagerevealedby Figure 5, and the corresponding one of S. M. Loudon and T. R. Oke (unpublished manuscript,1986)is unexpected. The availabilityof the Grimmond et al. [this issue]model makesit possibleto simulatethe effectof irrigationupon the  SuburbanIrrigation and Et)apotranspiration As was noted in the introduction, energy balance studiesat the Sunset site (Figure 1) have raised basic questions concerning relatively large rates of evapotranspiration from suburban areas. Four of the most important queriesare as follows.  1. Are the latent heat flux measurements, via Bowen ratio-  energy balance and eddy correlation-energy balance techniques,consistentwith the availability of water ? 2. Could irrigation sprinklingbe the sourceof the water? 3. Are the temporal patterns of irrigation and evapotranspiration linked in such a way as to explain the observed "rebound" of E during a period with no precipitation? The rebound referred to follows a sequence:precipitation causes an increasein the role of latent heat in the energybalance(the Bowen ratio decreases),over the next few days apparent drying out causes a steady decline in the role, but this is followedby an unexpectedrecovery. 4. What processesare capable of maintaining relatively large evapotranspirationrates in a suburbanarea? Despite the surfaceat Sunsetbeing 36% impervious,and probably half of the remaining area receivingno irrigation, observationsshow evapotranspirationproceedingat closeto the equilibrium rate with Bowen ratios usually lessthan 2, and typically around unity, even many days after precipitation. A number of answersto thesequestionsare provided or are impliedby the contentsof Figure 5. It includesthe daily totals of external water use le and evapotranspirationE from the Oakridge site during a period in the summer.Given the agreement between the two traces it is worth reiterating that they are obtainedcompletelyindependently,I e comesfrom the metered water supply data (after subtraction of 1,•) and is governed bY human decisionsregardingthe need for water. E is calculated from climatological station data using the Grimmond et al. [this issue] evapotranspiration submodel and is  suburban water balance. Figure 6 shows the differencebe-  tweenthe monthlyexternalbudgetcomponents for Oakridge in 1982when le is excluded.All other inputsremainthe same as before.Duringthe summerirrigationseasonevapotranspiration losses are considerably reduced. Summed over the 120  ,  ,  ,  ,  ,  ,  ,  ,  ,  ,  ,  ,  J  F  M  A  M  J  J  A  S  0  N  D  100 80  40  o  -20 -60 -80 -100  TIME  (months)  Fig. 6. Differencesin monthlyexternalwater budgettotalsdue to the exclusionof irrigation.  1412  GRIMMOND AND OKE.' URBAN WATER BALANCE,2  period April-August E is decreasedby 24% if no irrigation is applied. It can be noted that the biggestimpact on the summer balance would be to create a soil water deficit (despite lowered E) becauseof the reduced ability to restock. Even greaterdelayedeffectsoccurin the October-December period. When precipitationinputs start to dominate, the irrigation casegeneratesmuchlessrunoff becausea muchlargerproportion of the input is usedto stockthe soil moisturestore.The corollaryof this is that irrigationmay lead to increasedrisk of suburbanstorm floodingin the autumn.This is an exampleof wherethe proposedmodel would be a usefulpart of stormwa-  son in summer in Vancouver, B.C., BoundaryLayer Meteorol., in press,1986. Cohen, S. J. Effects of climatic variations on water withdrawals in  metropolitan Toronto, Can. Geogr.,29, 113-122, 1985. Douglas, I., The Urban Environment, 229 pp., Edward Arnold, London, 1983.  Environment Canada, Atmospheric Environment Service, Monthly Meteorological Summary, January-December 1982, Vancouver International Airport, B.C., 1982. Environment Canada, Atmospheric Environment Service, Monthly Meteorological Summary, January 1983, Vancouver International Airport, B.C., 1983. Gardner, W. H., Water content,in Methodsof Soil Analysis,Physical and Mineralogical Properties,Mongr. 9, edited by C. A. Black, pp. ter modeling. 92-93, AmericanSocietyof Agronomy,Madison, Wisc., 1965. Returningto the questionsposedat the start of this section, Grimmond, C. S. B., The suburban water balance: Daily, monthly and annual resultsfrom Vancouver, M.S. Thesis, 172 pp., Dep. of the present study enables us to make the following conGeogr., Univ. of B.C., Vancouver,1983. clusions. Grimmond, C. S. B., T. R. Oke, and D. G. Steyn, Urban water bal1. Based on the external balance data in Figures 2 and 5 ance,1, A model for daily totals, Water Resour.Res.,this issue. there seems no doubt that there is sufficient water available in Hare, F. K., and M. K. Thomas, Climate Canada (2nd ed.), 230 pp., Wiley, Toronto, Ont., 1979. the suburban environment to support the energy balance estiHay, J. E., and T. R. Oke, The Climateof Vancouver,49 pp., Tantalus mates of evapotranspiration.  2. The same data clearly support the possibilitythat irrigation is the sourceof the water. 3. The temporal patterns of I e and E (Figure 5) appear to be strongly connected.Following precipitation there is a delayed recoveryof the I e term during which period drying out starts to occur, and E may decreasein importance in the energy balance. After about 4-6 days I e recoversto approximately match E, and the reboundof E is possible.Indeed, the largestdaily total E value was registered11 days after the last rainfall (seeJune 16, Figure 5), and high valuescontinuedfor up to 19 days after rain in the samesequence. 4. Although the preceding considerationsshow enough water is available from sprinkling to support the relatively high rates of evapotranspirationobserved,appropriate processesare needed to explain how a patchy, wet-dry surface achievesthe end result. We suggestthat two scalesof advective effectmay be involved.The first is the microscaleprocess of "oasis"-typeadvectiondemonstratedby Oke [1979] for an irrigated suburban lawn. In this case an isolated patch of moist, cool vegetationloseswater at an enhancedrate due to the provision of sensibleheat from surroundingdry, hot surfaces.The secondis the mesoscaleprocessof advection due to entrainment of drier air down into the daytime planetary boundary layer [McNaughton and Jarvis, 1983]. This may be a common feature over urban areas where strong thermal and mechanical turbulence results in vigorous surface-boundary layer coupling [Hildebrandand Ackerrnan,1984]. Acknowledgments.The researchwas supportedby a grant from the Natural Sciencesand EngineeringResearchCouncil of Canada.  Specialthanksare due to Dr. and Mrs. J. L. Knox for makingtheir propertyavailableand to the VancouverCity EngineeringDepartment for access to the water meter. Field assistance and constructive  discussion wasgenerously givenby Drs. M. Church,J. Hay, D. Steyn, and a reviewer.  Research Ltd., Vancouver, B.C., 1976. Hildebrand, P. H., and B. Ackerman, Urban effects on the convective boundary layer, J. Atmos.Sci.,41, 76-91, 1984. Kalanda, B. D., T. R. Oke, and D. L. Spittlehouse,Suburban energy balance estimates for Vancouver, B.C., using the Bowen ratio-  energybalanceapproach,J. Appl.Meteorol.,19, 791-802, 1980. Landsberg,H. E., The Urban Climate, 275 pp., Academic,Orlando, Fla., 1981.  Lazaro, T. R., Urban Hydrology: A Multidiscipline Perspective,249 pp., Butterworths,Stoneham,Mass., 1979. Lindh, G., Urban hydrologicalmodelling and catchmentresearchin Sweden,in Researchon Urban Hydrology,edited by B. McPherson, vol. 2, pp. 229-265, UNESCO, Paris, 1978. L'vovich, M. I., and G. M. Chernogayeva,Transformation of the water balancewithin the city of Moscow, Soy. Geogr.,18, 302-312, 1977.  McNaughton,K. G., and P. G. Jarvis,Predictingeffectsof vegetation changeson transpirationand evaporation, Water Deficits Plant Growth, 11, 1-47, 1983.  Oke, T. R., BoundaryLayer Climates,327 pp., Methuen, London, 1978a.  Oke, T. R., Surface heat flux and the urban boundary layer, ProceedingsWMO Symposiumon Boundary-LayerPhysicsApplied  to SpecificProblemsof Air Pollution,WMO Publ.510, pp. 63-69, World Meteorol. Org., Geneva, 1978b. Oke, T. R., Advectively-assisted evapotranspirationfrom irrigated urban vegetation,BoundaryLayer Meteorol., 16, 167-174, 1979. Oke, T. R., The energeticbasis of the urban heat island, Q. J. R. Meteorol. Soc., 108, 1-24, 1982.  Oke, T. R., and J. H. McCaughey, Suburban-rural energy balance  comparisonsfor Vancouver B.C.: An extreme case? Boundary Layer Meteorol.,26, 337-354, 1983. Proctor, S. J., Map Of Vancouver old streams,Water J. Vancouver Aquarium,3, 4, 1978. Statistics Canada, Census Division, Census of Canada 1981, Enumeration area data on microfiche, Ottawa, Canada, 1983.  Steyn,D. G., Turbulent diffusionand the daytime mixed layer depth over a coastalcity, Ph.D. thesis,161 pp., Dep. of Geogr.,Univ. of B.C., Vancouver, 1980.  Woolmington,E., and J. S. Burgess,Hedonisticwater use and lowflow runoff in Australia's national capital, Urban œcol.,7, 215-227, 1983.  REFERENCES  Aston, A., Water resourcesand consumptionin Hong Kong, Urban Ecol., 2, 327-353, 1977.  Bell, F. C., The acquisition,consumptionand eliminationof waterby Sydneyurban system,Proc.Ecol.Soc.Aust.,7, 160-176,1972. Brater, E. F., Stepstoward a better understandingof urban runoff processes, Water Resour.Res.,4, 335-347, 1968. Brutsaert,W., Evaporationin to the Atmosphere: Theory,Historyand Applications, 299 pp.,D. Reidel,Hingham,Mass.,1982. Campbell,T., La Ciudad de Mexico comoecosistema, CienciasUrbanas, 1, 28-35, 1982.  Cleugh,H., and T. R. Oke, Suburban-ruralenergybalancecompari-  Wright-McLaughlin Engineers Ltd., Urban storm drainage criteria manual, Denver Regional Council of Governments,Denver, Colo., 1969.  Yap, D., Sensibleheat fluxesin and near Vancouver,Ph.D. thesis,177 pp., Dep. of Geogr., Univ. of B.C., Vancouver, 1973. C. S. B. Grimmond and T. R. Oke, Department of Geography, The University of British Columbia, Vancouver, British Columbia, V6T 1W5, Canada.  (ReceivedOctober 3, 1985; revisedApril 22, 1986; acceptedMay 5, 1986.)  

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