evapotranspiration in global climate models

Adv. SpaceRes.Vol. 7, No. 11, pp. (11)17—(11)26,1987 0273—1177/87$0.0O+.50Printedin GreatBritain. All rights reserved. Copyright© 1987 COSPAR

EVAPOTRANSPIRATIONIN GLOBALCLIMATE MODELS

RobertE. Dickinson

NationalCenterfor AtmosphericResearch,* P.O. Box3000, Boulder,CO 80307—3000,U.S.A.

ABSTRACT

Evapotranspirationis an importantcomponentof thehydrologicalcycleoverland. It returnswatertotheatmosphereandsuppliesrunoff. GlobalClimate Models include evapotranspirationbut in thepasttheir treatmenthasbeenquite crude. It is now becomingincreasinglyrealistic. Current treatmentsassignlandcoverandsoil propertiesto eachmodelgrid square.Fromtheseassignmentsareinferredthephysicalandphysiologicalpropertiesof theoverlyingvegetationthataffect surfaceenergybalanceandevapotranspiration.Themodelsincludedescriptionsof root wateruptake,stomatalresistance,responseto waterstress,canopyradiation,boundary—layerresistanceof canopyelements,aerodynamicresistanceof soil andcanopy,interceptionof rainfall, andsoil hydrology including runoff. Thefirst suchmodel,proposedin Dickinson /1/, is comparedwith the treatmentof Sellers et a!. /2/. The latter is morerealistic in severalfeaturesbut with acost of additional complexity. An exampleof a questionsuchmodelscan usefully addressis the climate changeexpectedwith tropical deforestation. Preliminaryresultsfrom suchasimulationdo not showanyextremechangesin rainfall, but do indicatean increasein flood runoffwith concomitantdrying of soil.

INTRODUCTION

Evapotranspirationclosesthe loop in thehydrological cycle over land by returningwaterto the at-mosphereand, correspondingly,returnsa largeamountof energyto theatmosphere—equalto about20% of thesolar energyabsorbedin theatmosphereplus surfaceoverland. This waterreturnedto theatmospherehelpsdrive circulation patterns(in modelsand reality) directly throughreleaseof latentheatandindirectly throughthemajorrole of watervaporandcloudsin atmosphericradiativetransfer(Figure 1). Without this evapotranspiration,circulation andrainfall patternswould be quite different/3,4/.

Of equal importanceis the control by evapotranspirationof thedisposition of rainfall, that is, the

amountof runoff availablefrom asurface.Yield andcontentof runoff from agiven areaarerelatedtothevegetationcoverof thearea.Forestsusually tend to reducethepeakamountsof runofffrom heavyrains,but they mayalso helpto maintainrunoff duringdrierseasonsby enhancinggroundwaterstorage.

Evapotranspirationis inextricably linked to other aspectsof land—surfaceprocesses,in particular,theabsorptionanddispositionof solar radiation,net energyloss by thermal infrared,height of vegetation,surfacetemperaturesandwinds, thedistribution andmovementof moisturewithin the soil, amountsandphaseof precipitation,andthemovementof waterthroughplants,especiallyroots andleaves.

Global Climate Models (GCMs) now attemptto synthesizethedetailsof climate throughcoupling ofmodelsof theatmospherewith modelsof the land andoceansurface(Figure 1). For theatmosphere,thesemodelssolvethethree—dimensionalfluid conservationequationsfor heatandmomentum,as drivenby atmosphericradiation,latentheatrelease,andsubgrid—scaleeddyfluxes of heatandmomentum,andoveroceansarelinkedto comparabledynamicmodelsfor oceanprocesses.Traditionally,muchlessdetailhasbeenattemptedover land.

* The NationalCenterfor AtmosphericResearchis sponsoredby theNationalScienceFoundation.

(11)17

(11)18 R. E. Dickinson

THERMO-

WATER

ATMOSPHERE

RADIATION SENSIBLE ANDLATENT FLUXES

SURFACES

N W COVE

,/~/ ICE OCEAN

Fig. 1. Schematicof componentsof an atmosphericmodel andits couplingto land andwatersurface.

Manabe/5/ first introducedland hydrology into a GCM, usinga schemedevelopedby Budyko /6/.He assumedthat evapotranspirationE dependedon four quantities—apotentialevaporationE

0, theamountof waterin the soil W, a field capacityfor waterWFC = 0.15, and a limiting wateramountWK = O.

7&WFC, andthat

E=E0, W>Wk, (1)

E = E0W/Wk, W ~ Wk. (2)

Thesoil waterW correspondsto thatof a “bucket” filled by precipitationP andemptiedby evapotran-spirationandrunoffR (Figure 2) accordingto

W<WFC, (3)

dWW�WFC, (4)

with runoff given by

R=0, W<WFC, (5)

R=P—E, W>WFC. (6)

In manyareasof science,complexsystemshaveinitially beenrepresentedby suchsimple box modelsto representthe important quantitiesandtime scales. In thepresentcase,suchamodel is constrainedto give at leastsomewhatreasonableresults, i.e., netenergyis in balanceat thesurfaceaccordingtoequationsI omit here.Landsurfaceshaverelativelylow heatcapacitiesso that thermalfluxes into or outof the soil arenegligiblefor unfrozensoil on time scaleslongerthana day. Processesof energybalancedeterminesurfacetemperaturewhich helpsdetermineE0. Evapotranspirationgoesto zero as the soildriesout andcorrespondsto that fromsomekind of a wet surfacewhenthesoil is moist. Furthermore,thetime integralof evaporationmustbalancetheprecipitationstoredin thesoil.

Evapotranspiration (11)19

However,theactualprocessesofevapotranspirationandrunoff overlandsurfacesaremuchmorecomplexthan thesystemequations(1)—(6), whichfail to describesomeof themostbasicprocessesinvolved andso presumablyare too inaccuratefor manypurposes.Indeed,as reviewedby Carson/7/, many morecomplexformulationsofevapotranspirationandrunoffhavebeenattemptedin GCMs but notnecessarilywith more reality or validation than thatof equations(1)—(6).

BUDYKO BUCKET MODEL PLANETARY BOUNDARY LAYER

RAIN

q, ~ T~

GROUND ~ rd rpiant: ~~ q CT

9 ) )9&.. SOIL T9

RUNOFF ~ ,,

o r~01i9 rSOII

1Prg 1Prc

Fig. 2. Budyko bucketmodel. Fig. 3. Sellers GCM parameterizationfor landsurface.

Somecurrentefforts, i.e., /1, 2/, e.g., Figure 3, are intendedto include thebiophysicalprocessesofcanopiesrepresentedby uniform surfacepropertiesover a model grid square. Such an approachisconceptuallymuchmore attractivethan arepastapproachesthat ignoredpropertiesof thevegetationresponsiblefor muchof theevapotranspiration.However,eventhesenewapproachesare likely to beonly partially successfuluntil we learnproceduresfor treatingthespatialvariability on variousscalesof soils, terrain,vegetationcover, andrainfall, etc. Remotesensingby satellite, togetherwith surfacemappingof land cover, synthesizedinto global datasets, anddevelopmentof appropriateaveragingmethodologiesarethusessentialfor makingfuther progresson this question.

There is a close synergismbetweenthe treatmentof physicalprocessesof land surfacein GCMs andtheir observationby remotesensing. For example,area—averagedsoil moistureis calculatednot onlyby GCMs but also through10—izm windowchannelradiancesgiving diurnalor daytimemaximumskintemperatures,e.g., /8, 9, 10/. The sameor a comparablemodel of evapotranspirationis neededforboth objectives,andit hasonly recentlybeenrecognizedthatcomparablebiophysicallybasedmodelsofplant transpirationareneededfor both questions.

WHAT DOES A BIOPHYSICAL PARAMETERIZATION OF LAND-SURFACEPROCESSESCONSISTOF?

To illustratethecontentof currentland—surfaceparameterizationsthat incorporatetheroleof vegetation,I briefly summarizethedescription/1, 11/ of theformulationnow usedon an experimentalbasis in theCommunityClimateModel (CCM) at NCAR. Theformulationconsistsof thefollowing:

(1) Assignmentof acharacteristiclandcovertype(e.g., tropical forest,or grassland,or drylandagricul-ture, or irrigatedagriculture,or tundra,etc.,18 typesin all), soil properties(i.e., albedo,porosity,drainagecharacteristicsfrom Wilson andHenderson—Sellers/12/). At present,depthof rootingzone,vertical distributionof roots,andaroot resistancefactor, vegetationalbedo,leaf areaindex,etc. areassignedaccordingto the covertype.

(11)20 R. B. Dickinson

(ii) A calculationoveradiurnal cycle of soil (or snow) surfacetemperaturein responseto net surfaceheating andallowing for shadingby vegetation,dependingon soil (or snow) heat capacity andthermalconductivity. Soil albedodependson aprescribedgrid squarevalueandon soil moisture.Fraction of areacoveredby snowdependson surfaceroughnessand mean snow depth. Snowalbedodependson solarzenithangleanda snowagefactor representingsnowcrystalgrowth andaccumulationof contaminants. Soil albedodependson a prescribedgrid squarevalue andsoilmoisture.

(iii) A calculationof soil moistureapproximatingthe vertical diffusion of waterthroughsoil, root ex-traction of water, surfaceevaporation,andrunoff. Subsurfacerunoff is obtainedfrom the waterthat percolatesthroughthesoil. A surfacerunoff treatmentis empirically tuned.

(iv) A specificationof details of vegetationcover from theprescribedland—surfacetype. We needinparticulara surfaceroughnessfactor, fraction of groundshaded,andrelativeareasof transpiringandnontranspiringplantsurfaces.

(v) Calculationof surfacedragcoefficientsfrom thesurfaceroughnessandbulk Richardsonnumber.

(vi) Calculationof foliagetemperaturein responseto energybalancerequirementsandconsequentfluxesof heatandmoisturefrom foliage to canopyair.

The last term above, thecalculation of foliage temperature,is the key to thedeterminationof thecanopyevapotranspirationas well as themost direct link to 10—tom window channelremotesensing.The commonly usedMonteith—Penman(M—P) approachfor determiningevapotranspirationactuallyeliminatescanopytemperaturesthroughits relationshipto thesaturationvapor pressureof wateratcanopytemperature.However, it cannotbereadily includedin GCM formulationswithout additionalquestionableparameterizations.In M—P, theresistanceto flux ofwaterfromleafsurfaceto top of canopyis aprescribedparameter,referredto as “canopyresistance.”Suchacanopyresistance,however,dependson a numberof variablefactors, someof which dependon leaf temperatureandsoil moisture. Moresophisticatedversionsof theM—P modelmaybe ableto includesomeadditionalfactors(e.g., ChoudhuryandMonteith /13/).

In thenow—currentone—layercanopymodels/1, 2/, canopyresistanceconsistsof the integratedeffect oftheleafstomataandleafboundarylayersin retardingwaterflux into thecanopyair, andof theresistancebetweenthecanopyair andthe air above the canopy. The lattercorrespondsapproximatelyto thestandardaerodynamicresistanceof thesurfaceusedin themomentumdepositioncalculation.Stomatalresistanceis summedin parallel, i.e., inverseconductancesare averaged.The stomatalresistanceofindividual leavesis relatedempiricallyto temperature,incidentvisible solar radiation,thedeficit of thewatervapormixing ratio from saturation,andtheexcessof leaf waterpotentialover that at wilting.The leaf waterpotential,in turn, dependson soil moistureandtheproductof evapotranspirationandthe resistancesto the diffusion of soil waterto andthroughtheplant roots; resistanceselsewherein theplant areneglected.

It is especially this dependenceof stomatalresistanceon soil moisture and root resistanceandthecommonoccurrenceof mid—daywaterstressthat precludessimpleapplicationof anM—P approachwitha prescribedcanopyresistance.In other words, the effectivecanopyresistanceandleaf temperaturearecalculatedjointly anditeratively. An additional complication is that the temperaturedifferencebetweencanopyairandoverlyingairdeterminesthestability factorthatcontributesto theaerodynamicresistancecalculation. Thus, aerodynamicresistanceis determinedjointly with foliage temperatures.The M—P approachusually dependson measurementof canopyresistanceto give reasonableresults.However, for reasonsof thedependencesdescribedabove,suchmeasurementsarenot readilygeneralizedwithout relating them to physiologicalandsoil waterprocesses,andM—P only applies precisely,afterthefact, i.e., after the implicit determinationof canopyandaerodynamicresistance.

Of thevariouscrucial processesthatenterinto theformulationof evapotranspiration,thedescriptionofroot resistanceis mostlikely to founderfor lackof datageneralizableto largeareas.Indeed,descriptionsof root resistancearesomewhatmeagreandspecieslimited, evenconsideringindividual plants.

COMPARISONSBETWEEN THE DICKINSON (1984) AND SELLERSET AL. (1986)FORMULATIONS FOR EVAPOTRANSPIRATION

It is informativeto comparetheDickinson/1/ formulation (dating backto a 1978draft manuscript,asreferencedin Washington/14/ andthat of Sellerset al. /2/ (also with at leasta five-year gestationperiod). Manyfeaturesof theSellersapproach(Figure3) describebetterthephysicsofcanopyprocesses,


hencearelikely improvementsovermy treatmentbut with somecostof additionalcomplexity,andotherfeaturesperhapsjust add complexity.

Root Model

Dickinson/1/ arguesthat physicaldetailsareunlikely to be available,so he representsroot resistancesimply by amaximumsustainablerateof transpirationmultiplied by adrynessfactordistributedamongthe soil layers(i.e., his equation(13)), accordingto someideaof root distribution. This drynessfactoris l.—WILT whereWILT is proportionalto the differencebetweenthe soil waterpotentialandthat atsaturation,andis scaledto go to 1. for asoil waterpotentialcorrespondingto that for leaf desiccation(wilting point potential).

Sellers et a!. /2/ usethemoredetailedmodelfrom which thatof Dickinsonwasparameterized.Thus,they require in principlefor each soil layeraroot resistanceper unit root length,aroot density, andavolume of root per unit volume of soil. In this case,I still prefer my approach,which is equivalenttousing theexpressionfor transpiration,equation(53) of Sellerset a!. /2/, assumingthat leaf potentialisfixed at its wilting potentialandthat root resistanceis largerthan theresistanceto waterflow throughthe soil (usuallythecase).

StomatalResistance(No WaterStress)

Stomatalresistanceis written (Dickinson/1/: equation(11)):

r8 = ramtn X Rt X St X Mt,(7)

~ ~amaz,

where ramjn = minimum resistance,Rt = a two—parameterdependenceon solar radiation(�1.) thatasymptotesto ramax/ramin, r8max = maximumresistancefor zero incident solar, St = temperaturedependentfactor (� 1.) implying stomatalclosurefor freezingtemperatures,Mt = waterstressfactordependingon leaf waterpotentialas discussedbelow.

It was arguedthat dependenceof r3 on watervapor deficit could not be includedbecauseof its widevariability betweenspecies,so that it waslumpedwith ramin.

Sellerset a!. /2/ (their equation38) usean approachsimilar to (7). Theroleof remmnx Rt is representedby threeparameters—a,b, c—andtheassumedfunctional form is essentiallythesamefor theirsas forours. Sellers does includea factor for vapor pressuredeficit, andthis andthe form of temperaturedependenceareadoptedfrom Jarvis/15/. TheDickinsontemperaturedependenceis S~tat T = 298Kanddecreasesas (298.0 —T)

2 to 0 at 273.0 and323 K. Sellersrequiresspecificationof optimum,upperlimit, andlower limit temperatures.His vaporpressuredeficit termgivesinverseofresistancedecreasinglinearly with vaporpressuredeficit, multiplied by a prescribedparameter.

StomatalResistanceDependenceon WaterStress

Dickinson /1/ arguesthat dependenceof stomatalresistanceon root resistanceis too complicatedtorepresentin a GCM, so he simply adjusts stomatalresistanceto matcha maximumrate of supplyfrom roots when demandexceedsthatsupply. However,Sellersintroducesaone—parametermodel fordependenceof stomatalresistanceon leaf waterpotentialandmodels leaf waterpotential in termsofplant, root, andsoil resistanceto flow. In practice,theDickinsonapproachcanbeviewedasasimplified,moreparameterized,versionof that of Sellers. The additional complexityof the latter cannotdo anydamageif its parametersareselectedreasonably,andit may leadto simulation improvements.

Soil WaterModel

Dickinson/1/ useda high—resolutionsoil—water—diffusionmodelto developatwo—layer soil model thatdependson hydraulicparameterscharacterizingthemultilayermodelandon empiricalconstants.Sellerset al. /2/ useathree-layersoil modelwith watermovementsimply definedby finite-differenceapprox-imations to thecontinuumequations.Both approacheshaveenoughgeneralityto applyover a widerangeof soil properties,in contrastto somepastparameterizationsthat weretunedto a specific soil.The Dickinson/1/ approachshould be more accuratefor thecircumstanceto which it wastuned, i.e.,diurnal drying overa long period. Under conditionsof soil waterincreaseas duringprecipitation,theSellersapproachmaybe morereliableandreadilyallows morelayersto be addedto increaseaccuracy.Milley andEagleson/16/ havedevelopedcoarse-resolutionsolutionsusingfinite elements.Mahrt andPan/17/ haveproposedanothertwo-layerapproach.

(11)22 R. E. Dickinson

Evaporationis obtainedby Dickinson/1/ as theminimum of either thepotentialevaporationrate orthe rate at whichwater canmoveupwardthroughthe soil to a dry surface. Sellers et a!. /2/ addanadditional resistanceterm to theequationfor soil evaporationand parameterizethis in termsof thewaterpotentialof their top layer. In principle, theDickinsonversion/1/ is againasimplified versionofthat of Sellerset al. /2/, andboth approachesshould give similar results.

RadiationModel

To calculatethedependenceof stomatalresistanceon solar radiation,Dickinson/1/ usesa two—layercanopy,assumingthat 75%of the incomingvisible solarradiationis incidenton theupperhalf of thecanopyfoliage and25% is incidenton thelower canopy. Effectsof leaf orientationareneglected. Foralbedo/11/, mean valuesarespecifiedfor visible andnear—infraredfluxes for eachland type, andasimpleempirical expressionis usedfor zenithdependencesbasedon Dickinson/18/. Albedosfor baresoil arelikewiseused,assumingno interactionbetweenvegetatedandbareregions.

Theseapproachesto albedocalculationshouldbe confirmedby referenceto remotesensing,e.g., /19/.Sellers et a!. /2/ develop the two-streamapproach/18/ and in calculatingalbedosallow for the in-teractionsbetweenthin canopiesandunderlyingsoils. Their approachis better for low leaf areaindexcanopies,providedalbedoinformationon theunderlyingsoil is available.They calculatecanopyaveragestomatalresistanceby assumingthattheincidentvisible solarradiationis exponentiallyattenuated,andso allow for thecontinuousdecreaseof radiationinto thecanopyaswell asleaforientation.Theresultingintegralsarecarriedout analyticallyfor a certainclassof leaf orientations.Thus, the Sellers et a!. /2/calculationof stomataldependenceon solarradiationis, in principle, a considerableimprovementoverthatof Dickinson/1/, providedthat the radiativepropertiesof thecanopyareadequatelycharacterizedas also thestomatalresponseto agivenlevel of solar radiation.

Leaf Boundary—LayerResistances

Heatandmoisturemustbe transportedfrom leaf surfacesto air within the canopy.Dickinson/1/ andSellerset a!. /2/ useessentiallythesameexpressionper unit leaf areaas theleafsurfaceconductances,proportionalto thesquareroot of themagnitudeof wind pastthe leaf, exceptthat Dickinsonincludesa dependenceon the inversesquareroot of the leaf dimension. To averageconductancesover all leafsurfacesto somesourceheight,Dickinson/1/ simply multiplies conductanceby leafareaindexandusessome“average”canopywind, inferredfrom an eddy diffusion modelof Brutsaert/20/ to bethefrictionvelocityu~timesafactorof orderunity. Sellerset al. /2/, on theotherhand,carryout theappropriateaveraging,usingawind profileconstructedanalyticallyfrom aneddydiffusion descriptionof momentumdepositionwithin thecanopyandanempirical“shelterfactor.” To this “forced” conductance,they adda “free convectionterm” of significanceunderconditionsof low wind speedandhigh radiation.

Thepositionof thecanopysourceheightis definedby Sellerset a!. /2/ as the level suchthathalfof thecontribution to leaf conductanceis from aboveandhalf below(centerof gravity).

CanopyAerodynamicResistances

Dickinson/1/ simply assumesthat aroughnessheightz0 for thecanopycan be usedto constructan

aerodynamicresistancefor thecanopy, using similarity theory, e.g., conductance= mean wind timesdragcoefficient(dragcoefficientfor neutralconditions= ic

2/1n2(z/z0),whereK = von Karmanconstant,

= roughnesslength, z = height of lowest model level 70 rn, for theNCAR CCM). In doing so,he neglectsthepossiblepresenceof a displacementheight h. Aerodynamicresistanceof unvegetatedground is calculatedseparatelyand linearly averagedwith that of vegetatedground. The averageaerodynamicresistanceis also assumedto governthetransferof heatandmoistureto thecanopysourceheight. Sellers et al. /2/ include many morephysicaldetails. They matchthesimilarity theoryeddydiffusion coefficient to a wind—dependenteddy diffusion coefficient profile within thecanopy,allowingfor an observedenhancementof eddy diffusion at the top of thecanopyover that simply inferred fromextrapolationof similarity theory to thecanopytop. Their theoryfor canopymomentumabsorptionyields adescriptionof z0 andh in termsof canopydensity.

Soil AerodynamicResistance

For transferof heatandmoisturefrom bare soil surfaces,Dickinson /1/ usesthesameaerodynamicresistancefor canopyas for theunderlyingsoil, but usesan averageof in—canopyandabove—canopywind, weightedaccordingto the fraction of areaassumedcoveredby vegetation. Sellers et a!. /2/assumethat the soil underliesthecanopyandconstructa conductancefrom their eddy diffusion andtheory.


Interception

Dickinson/1/ simply assumesthatall therainfall incidentoverthevegetatedfractionof thegrid squarefalls upon a canopyelement. Sellerset a!. /2/ allow for rainfalling throughcanopygapsas obtainedfromtheLAI andprescribedleaforientation. Dickinson/1/ assumesan interceptionreservoir’sWm~of0.2 mm per unit one-sidedprojectedareaof leavesor stems.Sellerset a!. /2/ asserttheir reservoirWmax

rangesbetween0.2 and0.5 mm, but what determinestheprecisevalueis not given. In both models,waterin excessof the reservoircapacitydrips throughthecanopy. For leaf waterlessthan capacity,both modelsassumepartial wetting of leaves. Dickinson/1/ assumestheareawettedis (W/Wmax)

2/3(Deardorif /21/), whereasSellersassumesthe areawetted is simply (W/Wmaz). Theseexpressionsarenot significantly different andneitheris likely to be correct in detail. A more realistic treatmentof partial wetting would involve considerationof the distribution andshapesof waterdropletson theleaves,dependingon thewaxinessof the cuticle andthespectrumof incident raindropsizes. Someconsiderations,suchas inferenceof plant diseaseincidence,may require thesedetails. Both authorsassumethewet anddry fractionof leavesto be at thesametemperature.

GroundCover

Sellers et a!. /2/ assumean additional vegetationlayerunderlyingtheir main canopy. This “groundcover” gives additional interceptionandalso hasa stomatalresistancebut remainsat thesametem-peratureas thesoil. Dickinson/1/ did not includethis feature. Neithermodelincludesalayer of deadleaves,but the Sellerset a!. /2/ groundcovercould be usedto representsuchdeadleaves.

Overall Comparison

The Sellers et a!. /2/ formulation appearsto improveespeciallythe formulation of overall canopypropertiesas inferred from propertiesof individual canopyelementsandallows a distinction betweensparseanddensecanopiesnot possiblewith theDickinson(1984)formulation.

Sellers et a!. /2/ allow for not only a fractional coverof vegetationbut also penetrationof canopygapsto baresoil, whereasDickinson/1/ allows only for the former. However,in both models,the soilunderlyingthecanopyis coupled thermally to the soil outsidethecanopy. Thus, neithertreatmentisrealisticfor agrid squarewith vegetationandnon—vegetatedareasdistinguishedby separatelargeareas.

Many further sensitivity studies will be neededwith theseand simpler approachesto establishwhatparametersaremost important. For example,McNaughtonandSpriggs/22/ find evapotranspirationismostsensitiveto surfaceenergybalance,also quite sensitiveto canopyresistance,andratherinsensitiveto mostother resistances.Future effortsmay haveto include effectsof surfacevariability overa gridsquare.For example,Wetzel /23/ finds thatevapotranspirationwith realistic soil moisture variabilityis quite differentfrom thatwith uniformsoil moisture.

A DEFORESTATIONSENSITIVITY STUDY

Meteorologistsareinclinedto askhowadetailedbiophysicaltreatmentof evapotranspirationwill improvemodelsimulationsof atmosphericclimate. This questionis not readilyanswered.We know that drasticchanges(e.g., from a wet to a dry surface,Shukla and Mintz /3/) can inducemajor climate changes.However,simple improvementsin treatmentof land evapotranspirationmaynot haveeffects that arelargecomparedto thoseof themany other defectsof current models (e.g., poor rainfall simulation)and will not necessarilyappearas improvedclimate simulationsbecauseof the possiblepresenceofcompensatingerrors. (Recentsensitivitystudiesarereportedby Laval et a!., /24/ andSud andSmith/25/.) The primaryvalue of thebiophysicaltreatments,I believe,is to expandthe rangeof importantquestionsthat we canaskwith globalclimate models. Many interestingissuesinvolve thequestionofchangeof climatewith land—coverchange,andcanonly be addressedif we can model therole of landcoverin theclimate system. Onesuchexampleis thequestionof theclimate effectsof the removaloftropical forests, e.g., in theAmazon.

A suspicion that forestsmay modify rainfall datesback many hundredsof yearsand was a majorargumentin the 19th century crusadefor forest conservation. Now at a time when tropical forestsarebeingrapidly depleted,our modeling tools, i.e., our biophysicaltreatmentsof forest/atmosphericinteraction,may be reachinga point wherewe can usefullyexplorethequestionof thepossibleclimateimpactsof tropical forest removal.

Precisedescriptionsarenot now possiblefor threereasons:(i) therearetoo manyuncertaintiesin themodeling, (2) forest processesimportantfor climate arestill inadequatelydescribed,and(3) we haveaninadequatedescriptionof changesto be expectedin tropical forests.


However, sensitivity studiesshould be informative. We havebeencarryingout one suchstudywitha version of the NCAR CCM with diurnal andseasonalcycle (collaboratorsA. Henderson—Sellers,M. Wilson, and P. Kennedy). After simulating severalseasonalcycles as a control, we repeatedthelast yearwith theAmazonforest convertedto grassland.We arestudyingseparatelytheeffectson theAmazonnorthversussouthof theequatorandthesemi—aridregionof northeastBrazil (Figure4).

EQ- J~L3)~

Fig. 4~Regionsof Amazonregionanalyzedfor

L4 deforestation.GCM experiment:Li—northern

___________ - Brazil, L4—southernBrazil. Hatching indicates25S - ~j_)~~irested Amazon,L2—southernAmazon,L3—northeastwheretheforest wasremoved.

I?7’

18 I

16 - U Deforestation Run

14 XCentral Station/ • Control Run

12

I0

>‘ 2 ered;; L28640

0 I IE18 11111111111

~l6X~ ~

~I41-12 Fig. 5. Rainfall for GCM controlanddeforesta-I— 10

w 6 ~ tion simulationscomparedwith observation(obsLI = grid squareaverage;Alto Tapajós,PuertoAy-C-) acuchoareindividual meteorologialstationsin~ .—~.. /

centerof grid square).//2- ,X

I I I I I I I I012 i I 1 I I I I

2~~-

.J FMAM J JAS 0 ND J

MONTH


Figure 5 showsthemodel—simulatedcycleof rainfall overtheseareasfor controlandwithout forest,andcomparedto observations.The modelapparentlydoesa reasonablejob of simulatingtheseasonalcycleand spatialdistribution of rainfall overtropical South America. Furthermore,theredoes not seemtobe any obviousmajor overall effectof thedeforestationof therainfall distribution. Further integrationis requiredto see anysuchchange. However,we do find major changesin thesurfacehydrology. Inparticular,surfacerunoff is enhancedandevapotranspirationreduced.

REFERENCES

1. R. E. Dickinson, Modeling evapotranspirationfor three-dimensionalglobal climate models, in:Climate Proce8aesand Climate Sensitivity,GeophysicalMonograph29, MauriceEwing Volume 5,AmericanGeophysicalUnion, Washington,DC 1984, 58—72.

2. P. J.Sellers,Y. C. Sud,andA. Dalcher,The designof asimplebiospheremodel (SiB) for usewithingeneralcirculationmodels,J. Atmo~.Sci. 43, 505—531 (1986).

3. J. ShuklaandY. Mintz, Influenceof land—surfaceevapotranspirationon theearth’sclimate,Science215,1498—1501 (1982).

4. Y. Mintz, The sensitivity of numericallysimulatedclimatesto land—surfaceboundaryconditions,in: The Globa! Change,ed. J. T. Houghton,CambridgeUniversity Press,Cambridge1984, 79—105.

5. S. Manabe,Climateandoceancirculation: I. The atmosphericcirculationandthehydrologyof theearth’ssurface,Mon. Wea. Rev. 97, 739—774 (1969).

6. M. I. Budyko, HeatBalance at the Earth’s Surface, Gidrometeoizdat,Leningrad1956, 225 pp., inRussian.

7. D. J. Carson,Currentparameterizationsof land—surfaceprocessesin atmosphericgeneralcirculationmodels,in: Land—SurfaceProce8sesin AtmosphericGenera!Circulation Mode!8, ed. P.S.Eagleson,CambridgeUniversity Press1982, 560 pp.

8. T. N. Carlson,J.K. Dodd, S. G. Benjamin,andJ. N. Cooper,Satelliteestimationof thesurfaceenergybalance,moistureavailability andthermalinertia,J. Clim. App!. Meteor. 20, 67—87 (1981).

9. P. J. Wetzel,D. Atlas,andR. H. Woodward,Determiningsoil moisturefrom geosynchronoussatelliteinfrared data:A feasibility study,J. C!im. App!. Meteor. 23, 375—391 (1984).

10. 0. Taconet,R. Bernard,andD. Vidal—Madjar,Evapotranspirationoveranagriculturalregionusinga surfaceflux/temperaturemodel basedon NOAA—AVHRR data, J. Clim. App!. Meteor. 25,284—307 (1986).

11. R. E. Dickinson, A. Henderson—Sellers,P. J. Kennedy,andM. F. Wilson, Biosphere/Atmo8phereTransfer Scheme(BATS) for the NCAR Community Climate Model, NCAR Technical NoteTNXXX, NationalCenterfor AtmosphericResearch,Boulder, CO, 80307(1986).

12. M. F. Wilson andA. Henderson—Sellers,A global archiveof land coverandsoils datafor useingeneralcirculationclimatemodels,J. Climato!. 5, 119—143 (1986).

13, B. J. ChoudhuryandJ. L. Monteith, Implicationsof stomatalresponseto saturationdeficit for theheatbalanceof vegetation,Agric. Fore8tMeteor. 36, 215—225 (1986).

14. W. W. Washington,R. Dickinson,V. Ramanathan,T. Mayer, D. Williamson, G. Williamson, andR. Wolski, Preliminaryatmosphericsimulationswith thethird—generationNCAR generalcirculationmodel,Reportof theJOCStudyConferenceon ClimateModels: Performance,Intercomparison,andSensitivityStudies, Washington,DC, 3—7 April 1978, ed. L. Gates,CARP PublicationSeriesNo.22 (1979).

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