Water use by a groundwater dependent maize in a semi-arid region of Inner Mongolia: evapotranspiration partitioning and capillary rise

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ContentslistsavailableatScienceDirect

Agricultural

Water

Management

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a te / a g w a t

Water

use

by

a

groundwater

dependent

maize

in

a

semi-arid

region

of

Inner

Mongolia:

Evapotranspiration

partitioning

and

capillary

rise

Yao

Wu

a

_,

_Tingxi

_Liu

a,∗

_,

_Paula

_Paredes

b

_,

_Limin

_Duan

a

_,

_Luis

_S.

_Pereira

b

a_College_of_Water_Conservancy_and_Civil_Engineering,_Inner_Mongolia_Agricultural_University,_Hohhot_010018,_China

b_LEAF_-_Landscape,_Environment,_Agriculture_and_Food,_Institute_of_Agronomy,_University_of_Lisbon,_Tapada_da_Ajuda,_1349-017_Lisbon,_Portugal

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22September2014

Accepted20January2015

Availableonline11February2015

Keywords:

SIMDualKcmodel

Groundwatercontribution

Dualcropcoefﬁcients

Croptranspiration

Soilevaporation

Groundwaterdependentecosystems

a

b

s

t

r

a

c

t

Thisstudyaimedatassessingthesoilwaterbalance,groundwatercontribution,croptranspirationand soilevaporationofarainfedmaizecropinHorqinsandyarea,north-easternInnerMongolia,China. TwoyearsofﬁelddatafromtheAgulasitewereused,2008withrelativelyhighrainfall(363mm)and highwatertable,and2009withlowrainfall(125mm)andlowerwatertable.TheSIMDualKcwater balancemodelwascalibratedwithobservedsoilwatercontentdataof2008andvalidatedwithdata of2009.Themodelusesthedualcropcoefﬁcientapproachforevapotranspiration(ET)partitioning, andparametricfunctionsforcomputingcapillaryrise.Therespectivemodellingresultsshowthatthe groundwatercontributionrepresentedca.50%ofcropETinbothyears.Estimationerrorsaresmall, withrootmeansquareerrorsof0.007and0.008cm3_cm−3_respectively_in₂₀₀₈_and_2009._The_Nash_and

Sutcliffemodellingefﬁciencywerehigh,0.93inbothyears,whichindicatesalowvarianceofresiduals. ThecalibratedbasalcropcoefﬁcientKcbmid=0.95denotesalowdensityofthecropbecauseitismuch

lowerthancommonpotentialvalues.Soilevaporationwasrelativelylow,23%ofETinthewetyearand 17%inthedryyear,becausecapillaryrisedoesnotcontributetosoilevaporationbuttorootsextraction only.Resultsshowthatcapillaryriseplaysamainroleinsupplyingthevegetationthroughouttheseason, henceastrongdependenceofvegetationupongroundwater.

1. Introduction

TheHorqinsandyland(hereafterHorqin)isthelargestofthe foursandylandswithactivedunesofInnerMongolia.It hasan areaof12,500km2_and_is_located_in_eastern_Inner_Mongolia._It_is_an

ecologicallyfragilearea,wherewinderosionisveryactive(Lietal., 2004)andtheprocessofdesertiﬁcationisimportant(Zhaoetal., 2006).Thesesandylandareasareuniqueeco-systemsconsidering climate,geo-hydrologicalconditions,physicalgeographyandland andwateruse(Duanetal.,2011a).

Research hasbeendeveloped in Horqinrelative to the eco-hydrologicalprocessesandtheirrelationswithvegetationandsoil (Duanetal.,2011a,b;Zhaoetal.,2009;Zuoetal.,2009).The veg-etationinHorqinconsistsofpioneerspeciesinthemobilesandy dunes,shrubsandherbaceousvegetationinsemi-fixedandfixed sandydunes,forestsinsandysoilsoflowlands,andcroplandsand grasslandsinmoreflatlandswithsandyloamsoils.Vegetation eco-typesvarywithsoilmoistureanddepthoftheshallowgroundwater

∗ Correspondingauthor.Tel.:+8613947116156.

E-mailaddresses:[email protected](Y.Wu),[email protected](T.Liu).

(MaandLiu,2007;Shietal.,2007).Zhengetal.(2012)assessed theimportanceofgroundwaterforvarioustypesofvegetationand theroleofvegetationtypesongroundwaterdecline.However,the observationsofthetime variabilityof thesoil watercontentat varioussoildepthsandfordiversevegetationtypeswerenot fol-lowedbyanadequatesoilwater balance.Therefore,despitethe recognizedimportanceofsoilwaterandgroundwaterforthe vari-ousvegetationtypes,knowledgeonthevariouscomponentsofthe waterbalanceandestimatesoftheshallowgroundwater contribu-tion(GWC)toevapotranspiration(ET)arelacking.

Zhangetal.(2009)observedthatfarmlandexpansionisamajor cause of steppe loss in northern China. In Horqin farmland is increasingwithanaverageannualrateof1.03%,mainly replac-ingsteppegrasslandbymaize.Baganetal.(2010)reportedthat theareaofcroplanddoubledoverthelastthreedecades.In par-allel,otherstudiesreferthedeclineoftheshallowgroundwater associatedwithlandusechanges,includingirrigatedcrops(Zhang andZhao,2003;Zhaoetal.,2009;Zhengetal.,2012).InHorqin, maizewater usestudiesreferonlytoirrigation(Lietal.,2003; Tanget al., 2011).No studies have beenperformed onrainfed maizesupplied byshallowgroundwater.Therefore, thereisthe needforbetterunderstandingtheprocessesrelativetowateruse http://dx.doi.org/10.1016/j.agwat.2015.01.016

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byagroundwater-fedmaizecrop,thusperformingawaterbalance studyfocusedontheroleoftheshallowgroundwaterandonthe cropevapotranspirationdynamics.

Theimportanceofshallowgroundwaterfor sustaining vege-tationhas beenassessed in severalwater balancestudies with lysimetersthatevidencedthedependencyofvariousecosystems andcropsystemsfromshallowgroundwater(Kahlownetal.,2005; Kowalik,2006;Gowingetal.,2009;LiuandLuo,2011).However, modelswerenotusedinthesestudies.Differently,various deter-ministicmodelshavebeenusedtoassesstheGWCbysimulating theupwardﬂuxesfromthewatertable,e.g.,RZWQM(Stulinaetal., 2005),WAVE(Liuetal.,2006), HYDRUS-1D(Soyluetal.,2011), andSWAP(Xuetal.,2013).Stochasticapproacheswerereported, amongothers,byVervoortandVanderZee(2008)andTameaetal. (2009).Modelsreferredaboveareabletoproducegoodestimates ofGWCbutmaybehighlyexigentintermsofsoilwaterdata,and requireappropriateobservationofthesoilwatercontentandof thesoilwaterpotentialfortheircalibrationandtesting.Differently, whenusingasoilwaterbalancemodelonlythebasicsoilhydraulic propertiesarerequiredandthemodelmaybecalibratedusingonly soilwatercontentobservationdata.Thisisthecaseoftheapproach developedbyLiuetal.(2006)thatderivedfromtheWAVEmodela setofparametricequationsforsimulatingupwardanddownward ﬂuxesthroughthebottomoftherootzoneofacroppedsoil.These parametricequationswerethenintegratedinthewaterbalance modelISAREGthatproducedgoodestimatesofcropETinpresence ofshallowwatertables(Pereiraetal.,2007;Cholpankulovetal., 2008).Thoseparametricequationsarepresentlyintegratedinthe waterbalancemodelSIMDualKc,formerlytestedwithamaizecrop inpresenceofashallowwatertable(Rosaetal.,2012b).

TheSIMDualKc(Rosaetal.,2012a)isnotonlyabletosimulate waterusebyacropinpresenceofashallowwatertable,buthasalso theadvantageofpartitioningcropET(ETc)intoactualcrop

transpi-ration(Ta)andsoilevaporation(Es)usingthedualcropcoefﬁcient

approachproposedinFAO56(Allenetal.,1998).Modellingusing

thisapproachorforthequantiﬁcationofthegroundwater contri-butiontoEThasnotbeenappliedinInnerMongolia.Therefore, consideringtheimportance ofbetterunderstandingcropwater useinthefragileHorqinsandlandand,inparticular,theneedfor improvedknowledgeontheinteractionsbetweenvegetationand shallowgroundwater,thatmaysupportabetterpolicyonwater andlandscapemanagementinHorqin,theobjectivesofthisstudy consistof:(a)calibrationandvalidationoftheSIMDualKcmodelto assesswaterusecomponentsofrainfedmaize;(b)evaluatingthe groundwatercontributiontocropET;and(c)assessingimpactsof changesinwatertablelevelsonthedynamicsofmaizewateruse, transpiration,soilevaporationandgroundwatercontribution. 2. Materialsandmethods

2.1. Siteandcropcharacteristics

Thestudyareaislocatedin thesouthern Horqinsandyarea (43◦1848 to 43◦2124 N, 122◦3300 to122◦4100 E),in the Agulaeco-hydrologicalstudyarea,easternInnerMongolia(Fig.1). AsmappedinFig.1,themainvegetationtypebetweendunesis presentlycroplandfollowedbygrasslandmeadows, groundwater-fed(Duanetal.,2011a).Theduneshavevarioustypesofshrubs vegetation.The croplandis mainly occupiedwithmaize; other cropsaresunﬂower,blackbeansandmillet.Themaizefarmﬁeld usedinthisstudywaslocatedwithinalargemaizecroparea.

Theclimateissemi-aridcontinentalwithcoldanddrywinter; accordingtotheKöppenclassiﬁcation,theclimateisaBsk,cold aridsteppe(Kotteketal.,2006).Theaverageannualprecipitation is389mm,whichmainlyoccursduringthesummermonsoon.The temperaturevariesfrom−13◦_C_in_January_to₂₄◦_C_in_July₍_Duan

etal.,2011a).Anautomaticmeteorologicalstationisinstalledat about200mfromthemaize studysite. Observedweatherdata includedprecipitation(mm),airtemperature(◦C),relative humid-ity(%),windspeed(ms−1)withtheanemometerinstalledat2m

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aboveground,andsunshineduration(h).Thesedatawereusedto computethereferenceevapotranspiration(ETo,mmd−1)usingthe

FAO-PMmethod(Allenetal.,1998).

Theclimatein theareais dryand windy.Minimum relative humiditydataforbothcropseasonsof2008and2009showthat atmosphericdrynesswasparticularlyimportantbythespringand late summer(Fig. 2a). Windspeed was alsohigher during the sameperiods(Fig.2b).Duringsummer,duetomonsooneffects,air humidityishigherandwindspeedislower.Thus,becauseETois

highlyinﬂuencedbytheatmosphericdrynessandhighwindspeed, thehighestETovalues(Fig.2c)wereobservedduringspringandlate

summer,thusoutoftherainyperiods.Consideringthemaizecrop season,ETowashigherduringthedrieryearof2009and

precipita-tion(Fig.2c)wasmuchhigherin2008thanin2009,thusthestudy wasperformedundercontrastingclimateconditions.

Thesoilisasandyloamwhichmainsoilhydraulicpropertiesare summarizedinTable1.Thesoilwatercontent(SWC,cm3_cm−3₎

wassurveyed witha neutronprobe previously calibrated for a widerangeofsoilwaterdata;measurementswereperformedwith ﬁve-dayintervalsandthreereplications.Readingswereperformed forevery0.10muntilthewatertabledepth.LimitationsofSWC

observationsusing neutronprobesarediscussedby Allenetal. (2011)relativetosoildataacquisitionrequirementsandaccuracy, whichweresatisfiedinthecurrentstudyrelativetoETestimation. Thegroundwater depthwasobserved every 5–10days at a wellinstalledinthecentreofthefieldusingawaterlevel trans-ducer.ResultsinFig.3showthatin2008precipitationwasenough abundanttopercolatetothewatertableandkeepithigh.Onthe contrary,precipitationin2009wasinsufficientandthewatertable declinedthroughoutthecropseasonduetoplantrootsextraction. Springmaize(Zeamaysvar.Zhengdan958)wasplantedby10th ofMayonbothyearsandharvestedby1stand5thOctober, respec-tivelyin2008and 2009.Table2presentsthedatesofthecrop growthstages for both experimentalyears. Maindifferences in lengthsofcropstagesrelatetoclimateconditions.

Plantdensitywas52,500plantsha−1.Noirrigationwasapplied. Plantheight(h,m),leafareaindex(LAI)andfractionofgroundcover (fc)weresurveyedina1m2sampleareaevery10days;surveys

includedobservationsoftheeffectiverootdepth(Zr,m),whichdid

notexceed0.70m.Thus,modellingwasdevelopedconsideringa rootdepthof0.70m.Resultsforcropheightandrootdepthsare giveninTable3andthefccurvesareshowninFig.4.

Fig.2. Climaticcharacteristicsrelativetothecropseasonsof2008(left)and2009(right):(a)minimumrelativehumidity(%)( ),(b)windspeed(ms−1₎₍ _),_(c)_daily

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Table1

SoilhydraulicpropertiesoftheAgulamaizesite.

Layer Depth(cm) Layerthickness(cm) Fieldcapacity(cm3_cm−3₎ _Wilting_point_(cm3_cm−3₎

1 0–30 30 0.22 0.08

2 30–90 60 0.20 0.07

Table2

Datesofthemaizegrowthstagesforeachseason. Season Cropgrowthstages

Initial Rapidgrowth Mid-season Late-season

2008 10May–09June 10June–14July 15July–08September 09September–01October 2009 10May–19June 20June–19July 20July–09September 10September–05October

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 05/05 25/05 14/06 04/07 24/07 13/08 02/09 22/09 12/10 Gr ound wa ter dep th (m)

Fig.3.Groundwaterdepthvariationduringthemaizecropseasonsof2008(–•–) and2009(––).

Table3

Cropheight(h)androotdepth(Zr)throughoutthecropgrowthstages. Initiation Startrapid

growth Start mid-season Start Late-season Harvest 2008 h(m) 0.01 0.50 1.70 1.95 1.90 Zr(m) 0.20 0.25 0.70 0.70 0.70 2009 h(m) 0.01 0.50 1.65 1.85 1.80 Zr(m) 0.20 0.30 0.70 0.70 0.70 2.2. SIMDualKcmodel

The SIMDualKc model (Rosa et al., 2012a) performs the soil water balance at ﬁeld scale with a daily time step. For

evapotranspirationcomputations,itappliesthedualcrop coefﬁ-cientapproach(Allenetal.,1998,2005),thuspartitioningETcinto

soilevaporation(Es),andactualcroptranspiration(Ta).Themodel

hasbeenpreviouslycalibratedandvalidatedformaizeinpresence ofashallowwatertableinPortugal(Rosaetal.,2012b),for mon-soonconditionsintheNorthChinaPlain(Zhaoetal.,2013),and forsprinklerandtrickleirrigationinBrazilandPortugal(Martins etal.,2013;Paredesetal.,2014).ThepartitionofETcwastestedin

previousstudies,namelyforsoilevaporationbyZhaoetal.(2013) andfortranspirationbyPac¸oetal.(2014).

Thedailysoilwaterbalanceoftherootzonemaybedescribed as:

Dr,i=Dr,i−1−(P−RO)i−Ii−CRi+ETc,i+DPi (1)

whereDr,iandDr,i−1aretherootzonedepletion[mm]attheend

ofdayiandi−1,respectively,Piisprecipitation,ROiisrunoff,Ii

isnetirrigationdepth,CRiiscapillaryrisefromthegroundwater

table,ETc,iiscropevapotranspiration,andDPiisdeeppercolation,

allreferring to dayiexpressedinmm. In this applicationIi=0

becausethecropwasnotirrigatedandROi=0becausethelandis

ﬂatandrunoffwasnotobserved.Themodeloutputsconsistofthe dailytermsofthesoilwaterbalance,thusET,netirrigation,crop transpiration,soilevaporation,deeppercolationandgroundwater contribution.

WhentheSWCintherootzonefallsbelowthesoilwater thresh-old SWCp correspondingtothedepletionfraction fornostress

(p),i.e.,SWC<SWCp,thentheactualevapotranspiration(ETcact)

issmallerthanETcandisestimatedas:

ETc act=(KsKcb+Ke)ETo (2)

where ETcact is the actual crop ET (mmd−1), Kcb is the basal

cropcoefﬁcient,Ke is the soilevaporationcoefﬁcient, Ks is the

waterstressreductioncoefﬁcient,andEToisthereference

evapo-transpiration(mmd−1).Ksiscomputeddailyfromcomparingthe

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availablesoilwaterwithSWCp,thusKs=1.0ifSWC≥SWCp,

oth-erwiseKs<1.0dependinguponthewaterdeﬁcitinthesoil(Allen

etal.,1998;Rosaetal.,2012a).TheproductKsKcbrepresentsthe

actualKcbrelativetotheactualsoilwaterandenvironmental

con-ditions(Kcbact).Thus,theactualcroptranspirationisTa=KsKcbETo

andsoilevaporationisEs=KeETo.

Kcbandpareinputparametersthataresubmittedtocalibration.

TheinputKcbisadjustedtoclimateconditions(minimumrelative

humidityandwindspeed)followingAllenetal.(1998)andtocrop densityasafunctionofthefractionofgroundcoveredbythecrop (fc,Fig.3)andcropheight(h,Table3)usingtheprocedureproposed

byAllenandPereira(2009).

Keiscomputedafteradailywaterbalanceoftheevaporative

layerofthesoil,takingintoconsiderationfcandtheevaporative

characteristicsof the soil (Allen et al., 1998,2005; Rosa et al., 2012a).Thetwo-phasesRitchie’s model(Ritchie,1972)is used, whichassumesaﬁrstphasewhereevaporationisenergylimited andasecondphasewhereevaporationislimitedbytheavailable soilwater.Thecumulatedevaporationattheendoftheﬁrstphase isnamedreadilyevaporablewater(REW,mm),whilethatattained attheendofthesecondphaseisthetotalevaporablewater(TEW, mm).REWandTEW,aswellasthethicknessoftheevaporative layer(Ze,m),areobjectofcalibrationanddependupon thesoil

hydraulicproperties(Allenetal.,1998,2005).Furtherdescriptions ofthemodelareprovidedbyRosaetal.(2012a).

Theparametricequationsusedforestimationofthedaily cap-illaryriseanddeeppercolationﬂuxesarethoseproposedbyLiu etal.(2006).TheactualCRﬂuxes(mmd−1)areestimatedusing dataontheactualwater tabledepth(Dw,m),actualsoil water

storage(Wa,mm),potentialevapotranspiration(ETc,mmd−1)and

potential(maximum)capillary rise(CRmax,mmd−1).Theactual

dailyCRﬂuxiscomputedproportionallytoCRmaxdependingupon

ETc,Dw and Wa (mm). CR reducesrelative to CRmax when the

rootzonestorageWaishigh,orthedepthDwofthewatertable

increases,and/orETcdecreases.Theparametersrelativetowater

storage(a1,b1,a2,b2),totherelationshipsbetweenDw andETc

(a3,b3),andtotheCRmaxcalculation(a4,b4)havetobecalibrated

takingintoconsideration thelocalsoil and watertable charac-teristics(Liuetal.,2006).Setsof parametervalues forsilty,silt loamand sandyloamsoilswere proposedbyLiu et al.(2006), whichmaybeusedasdefaultvaluestoinitiatetheparameters’ calibration.TheDPﬂux(mmd−1)iscomputedusingatimedecay functionrelatingthesoilwaterstoragenearsaturationwiththe timeaftertheoccurrenceofaheavyrainorirrigation(Liuetal., 2006). The corresponding parameters (aD, bD) also need to be

calibrated.

TheinputdatarequiredbySIMDualKcconsistsof:

(a)Meteorologicaldailydataonprecipitationandreference evapo-transpirationETo,minimumrelativehumidity(RHmin,%),and

windspeedat2mheight(u2,ms−1).

(b)Soilwatercontentdataatﬁeldcapacityandwiltingpointfor thevarioussoillayersoftherootzone(Table1).

(c)Cropdataondatesofcropgrowthstages–initial,crop develop-ment,mid-season,lateseasonandendseason(Table2);basal cropcoefﬁcients (Kcb)andsoil waterdepletionfractions for

non-stress(р)fortheinitial,mid-seasonandend-seasonstages, whichneedtobecalibrated;rootdepths(Zr,m),plantheight

(h,m)andfractionofgroundcover(fc)alongthecropseason

(Table3andFig.4).

(d)SoilevaporationdataonREWandTEW(mm),anddepthZe

[m] oftheevaporablelayerasdeﬁnedabove,whichrequire calibration.

(e)Groundwatercontributionanddeeppercolationdatarelative towatertabledepths(Fig.3)andLAIvaluesatvariousdates

alongthecropseason,andtheCRandDPparameterspreviously mentioned,thatrequirecalibration.

Whenusedforirrigation,themodelrequiresdataonirrigation depthsanddates,orrelativetovariousirrigationoptions.Ifrunoffis likelytooccur,thecurvenumberCN(Allenetal.,2007)isrequired forcomputingRO(mm).

2.3. Modelcalibration,validationandperformanceassessment Simulationofannualcropsstartsonthedayofplantingand restartsattheplantingdayeveryyear.Themodelisinitializedevery yearofthecropseasonwithselected/calibratedparametersand, forthatinitialday,withanestimated/observedinitialvalueofthe soilwatercontentandofthewatertabledepth.Theparameters usedtoinitializethemodelconsistoftheinitial(default)setwhen startingwiththecalibration,orthecalibratedsetwhenperforming validationorusingthemodelinanyapplicationstudyorpractice, e.g.,simulatingalternativewatermanagementoptions.

The calibration procedure of the SIMDualKc model aims at obtainingcropparameters(Kcbandpforeachcropgrowthstage),

soilevaporationparameters (Ze,TEWandREW) andCRandDP

parameters (asdescribed before) that minimizethedifferences betweenobservedandsimulatedsoilwatercontentintheentire rootzone.AsforotherreferredSIMDualKcapplications(e.g.,Rosa etal.,2012b;Martinsetal.,2013;Zhaoetal.,2013;Paredesetal., 2014),thecalibrationprocedurestartedbyselectinganinitialset ofcrop,soilevaporation,CRandDPparametersthatcorrespondto thecharacteristicsofthecropandsoilunderconsideration.An iter-ativetrialanderrorsearchprocedure(T&E)wasdevelopedaimed atprogressivelyreducingtheresidualerrorsofsimulation.TheT&E procedurewasusedtoﬁrstsearchforimprovedvaluesforKcband

p.Whenerrorsbecamesmall,T&EwasappliedtotheCRandDP parametersuntilanacceptablematchofobservedandsimulated SWCvalueswasobtainedalongwithreducederrors.Thenthe pro-cedurewasappliedtothesoilevaporationparametersandlaterto reﬁnetheestimationoftheKcbvalues.

Thecalibrationwasperformedwithdataof2008andthe vali-dation,usingthepreviouslycalibratedparameters, withdataof 2009.Twoyearsofdataprovidefortwoindependentdatasets,one forcalibrationandtheotherforvalidation.Theadequatenessof calibratingandvalidatingSIMDualKcwithonlytwoyearsdatawas discussedbyMartinsetal.(2013)inlightofrecommendationsby SinclairandSeligman(2000)oncropmodels.AbiSaabetal.(2015) haveshownthatcalibrating acropwater modelforanyamong three years of observations led to similar model performance, whichsupportsusingoneyearforcalibrationandthesecondfor validation.Inaddition,recommendationsaboutdatarequirements (Allenetal.,2011)werefulﬁlledandthemodelcalibrationwas initiatedwithadequatedefaultparametersaspreviouslyanalyzed (Rosaetal.,2012b).Twoyearsofdatawerethereforeconsidered sufﬁcientinthisapplication,particularlytakingintoconsideration thatthesetwoyearshadcontrastingclimateandwatertabledepths (Figs. 2c and 3), thus allowing to assess maize water use and groundwatercontributionforawiderrangeofobserveddata.

Toassessthegoodness-of-fitofthemodellingapproachseveral indicatorswereselectedasappliedanddefinedin former stud-ies(Rosaetal.,2012b;Martinsetal.,2013;Zhaoetal.,2013).The followinggoodness-of-fitindicatorswereadopted:

(a)The parameters of the linear regression forced throughthe origin relating observed and simulated SWC data, i.e., the corresponding regression coefﬁcient (b) and determination coefﬁcient(R2_)._b_close_to₁_means_that_the_predicted_values

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1.0indicatesthatmostofthevarianceoftheobservedvaluesis explainedbythemodel.

(b)Therootmeansquareerror(RMSE),theaverageabsoluteerror (AAE),theaveragerelativeerror(ARE),andthemaximumerror (Emax),whicharestatisticalmeasuresoftheestimationerrors

(unitsasforthevariableunderﬁtting).

(c)Themodellingefﬁciency(EF,non-dimensional),proposedby NashandSutcliffe(1970),thatdeterminestherelative magni-tudeoftheresidualvariancecomparedtothemeasureddata variance.EFcloseto1.0indicatesthatthevarianceof residu-alsismuchsmallerthanthemeasureddatavariance,contrarily whenEFis≤0(Moriasietal.,2007).

(d)The Willmott (1981) index of agreement (dIA,

non-dimensional), that represents the ratio between the mean square error and the “potential error”. dIA=1.0 indicates

perfect agreement and dIA=0 indicatesnoagreement atall

between the observed and predicted values (Moriasi et al., 2007).

3. Resultsanddiscussion

3.1. Modelcalibrationandvalidation

Asdescribedabove,thecalibrationoftheSIMDualKcmodelfor rainfedspringmaizewasperformedbyminimizingthedifferences betweenobservedandsimulatedSWCdatafrom2008crop sea-sonwhilethevalidationwasperformedwith2009data.Resultsof comparingtheobservedandsimulatedSWCshownobiasinthe simulations(Fig.5).

Table4showstheinitial/defaultandcalibratedparameters rel-ativetothecrop(Kcbandp)andtosoilevaporation(TEW,REW,

a)

b)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 SW C ( cm 3cm -3) θFC θp θWP 0.00 0.05 0.10 0.15 0.20 0.25 0.30 SW C ( cm 3cm -3) θp θFC θWP

Fig.5.Simulatedandobservedsoilwatercontent(SWC)fortherainfedmaizecrop

in(a)2008(calibration)and(b)2009(validation).LinesFC,WPandprefertoSWC

atﬁeldcapacity,wiltingpointandwhendepletionequalsthedepletionfractionfor

nostress,p.

Ze).TheinitialandcalibratedCRandDPparametersarepresented

inTable5.Itcanbeobservedthattheinitialsetsofcropandsoil evaporationparameterswereclosetothecalibratedones. Differ-encesbetweeninitialandﬁnalvaluesoftheCRandDPparameters arenotverylargeduetoagooduseofinformationproposedbyLiu etal.(2006).

Table 6 presents the goodness of fit indicators when the model was used with calibrated and initial/default parameters (Tables 4 and 5). When using calibrated parameters, results showregressioncoefficientscloseto1.0andhighdetermination coefficients of 0.93 for both the calibrationand the validation. Thevariationoftheobservedvaluesisthereforelargelyexplained by the model. Errors of estimate are small, with RMSE of 0.007cm3_cm−3_for_the_calibration_and_0.008_cm3_cm−3_for_the

vali-dation,whileAREdidnotexceed4.0%.TheNashandSutcliffeEFare high,0.93forboththecalibrationandthevalidation,which indi-catesthattheresidualsvarianceismuchsmallerthanthemeasured datavariance,i.e.,themodelisagoodpredictorofthesoilmoisture dynamics.TheWillmotindicesofagreementareveryhigh,0.98, thusindicating thatthemeansquareerrorisquiteclosetothe potentialerrorduetomodelling.Goodness-of-ﬁtresultsindicate thatSIMDualKcwasappropriatelycalibratedandtheparameter valuesmaybeusedwithconﬁdenceinfurthercroplandstudies inHorqin.Sinceparameterswereobtainedfortwoyearshaving contrastingclimateandwatertabledepths,theapplicabilityofthe calibratedparametersislikelygood.

Results using the default parameters are also presented in Table 6. They show that the model performance using default parameterswereacceptable.Thisindicatesthatthemodelmaybe appliedtoconditionswhendataisinsufﬁcientforcalibrationas alreadynoticedinpreviousapplications(Rosaetal.,2012b). How-everresultsdependupontheappropriateselectionofthedefault parameters.

3.2. Cropcoefﬁcients

ThecurvesofthecropandevaporationcoefﬁcientsKcb,Kcbact,

KcactandKe(Eq.(2))arepresentedinFig.6togetherwiththedaily

precipitationdepths.KcbvaluesrepresentedinFig.6areslightly

differentfromthoseinTable4becausethelatterarethoseused asinputtothemodel,whilethoseinFig.6arealreadyadjustedto climateandtocropdensityandheight.

Theactualbasalcropcoefﬁcient(Kcbact)isbelowthepotential

Kcbcurve(Fig.6)fortheinitialperiodinbothseasonsbecausethere

wasinsufﬁcientrainandthecapillaryrisewasverysmallsince rootswerenotyetdevelopedandETwasverysmall.Thereforethe cropwaswaterstressed.CRincreasedonlyafterrootsdeveloped andtheirdistancetothewatertablebecamesmaller.In2008,the Kcbcurvecoincideswiththepotentialonebythemidofthecrop

developmentstage(Fig.6a)whentherewasenoughraintodevelop themaizecropandtorisethewatertable(Fig.3).Fromthereon bothrainfallandCRweresufﬁcienttosustaincroptranspirationat itspotentiallevelandKcbactwasequaltothepotentialKcb.

Differ-ently,in2009,maizewasstressedfrommid-seasontoharvesting becauserainwasinsufﬁcientandthewatertabledepthincreased (Fig.3), thus makingCR alsoinsufﬁcient. Therefore, Kcbact<Kcb

duringmostofmid-season andthelateseasonuntilharvesting (Fig.6b).

Thesoilevaporationcoefﬁcient(Ke)washighduringthe

ini-tialstageinbothseasons(Fig.6),whenfcwassmall(Fig.4)anda

largefractionofsoilwasexposedtoradiation,thusfavouringsoil waterevaporationwhenthesoilwaswettedbyrain.Kedecreased

asthecropdevelopedandthefractionfcincreased;thusKerapidly

droppedandkeptlowduringthemid-season.Itslightlyincreased againbytheendofthelateseasonwhenleafsenescenceoccurred, fcslightlydecreasedandmoreenergybecameavailableatthesoil

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Table4

TheinitialandcalibratedbasalcropcoefﬁcientsKcb,pdepletionfractionsfornostressandsoilevaporationparameters.

Cropparameters Soilevaporation

Kcbini Kcbmid Kcbend pini pmid pend Ze(m) REW(mm) TEW(mm)

Initial/default 0.15 1.15 0.30 0.55 0.55 0.55 0.10 8 15

Calibrated 0.15 0.95 0.20 0.50 0.50 0.50 0.10 10 16

Table5

Initialandcalibratedparametersofthecapillaryriseanddeeppercolationparametricequations.

Capillaryrise Deeppercolation

a1 b1 a2 b2 a3 b3 a4 b4 aD bD

Initialvalues 220 −0.17 160 −0.27 −1.3 6.2 3.0 −2.5 200 −0.0173

Calibratedvalues 320 −0.17 270 −0.27 −1.3 6.2 3.0 −2.5 230 −0.02

Table6

Indicatorsof“goodness-of-ﬁt”oftheSIMDualKcmodelappliedtogroundwaterfedmaizewhenusingcalibratedanddefaultparameters.

Parameters Cropseason b R2 _RMSE_(cm3_cm−3₎ _AAE_(cm3_cm−3₎ _ARE_(%) _I_max_(cm3_cm−3₎ _EF _d_IA

Calibrated 2008 0.99 0.93 0.007 0.006 3.2 0.016 0.93 0.98

2009 1.01 0.93 0.008 0.006 3.9 0.017 0.93 0.98

Initial/default 2008 0.97 0.97 0.009 0.007 4.2 0.019 0.88 0.97

2009 0.91 0.87 0.018 0.016 10.5 0.031 0.61 0.90

surfaceforevaporation.Differencesbetween2008and2009are visibleforKebecause,duetolessandsmallerwettingsin2009,soil evaporationwasthensmaller(Fig.6b).TobenotedthatKeisnull

whenthecropwasfedbycapillaryrisebecauseCRdoesnotsupply moisturetotheuppersoillayerbutonlytorootsextraction.Fig.6 alsoshowstheKcactcurvethatrepresentsthedailysumofKcbact

andKe.ThedailyKcactcurveshowsasequenceofpeaksduetosoil

Fig.6.Kcb( ),Kcbact( ),Ke( ),andKcact( )curvesaswellasdaily

precipitation( )inthewetyearof2008(a)andinthedryyearof2009(b).

evaporationwhenwettingeventsoccur,thusbeingverydifferent fromthetypicallineartimeaveragedKccurveproposedinFAO56

(Allenetal.,1998).

TheKcbiniandpvaluesarewithinthoseproposedbyAllenetal.

(1998).However,themid-seasonKcbmid=0.95islowerthan

cur-rentlyobservedpotentialKcbmidvaluesformaize(e.g.,Allenetal.,

1998;Zhaoetal.,2013).Thisisduetolowplantingdensity,that leadstolowfc,andtolowcropheight(Fig.4andTable3).As

ana-lyzedbyAllenandPereira(2009),theKcbandKcvaluesincrease

fordenseandhighcrops.LowKcbmidvalueswereobtainedwith

formermaizevarieties(e.g.,Wright,1982;Pereiraetal.,2003).The resultingKcbmid=0.95mustthereforebeconsideredasthe

poten-tialvaluefor alow densitygroundwaterdependentmaize crop adaptedtoHorqinareabutnottobeusedwhencroppingfor max-imumyield.TheKcbendvaluesarelowerthanusualduetoleaves

pickingforanimalfodderduringlateseason,whichistypicalfor maizecultivatedasfoddercropinthisregion.

3.3. Croptranspiration,soilevaporationandgroundwater contribution

Theactualevapotranspirationcomputedbythemodelwas434 and360mmrespectivelyin 2008and2009,andcrop transpira-tionwasthen333and300mmrespectively(Table7).Differences betweentheseyearsareduetocontrastingrainfallandwatertable depths,where2009wasaparticularlydryyear,causingquiteheavy waterstressfrommid-seasontoharvest(Fig.6b).Thesimulatedsoil evaporationwas23%and17%ofETcactduringthecropseasonsof

2008and2009,respectively,withlargesoilwettingsbyrainfallin 2008(Table7).Theseresultshighlycontrastwiththosereported byZhaoetal.(2013)for maizeintheNorthChinaPlain,where soilevaporationwas37–45%ofETcact,henceaboutdoublethat

observedinHorqinarea.Thisdifferenceresultsfromthefactthat themaizecropreferredbyZhaoetal.(2013)wascroppedunder fullsupplementalirrigationandrainfallwasmuchmoreabundant thaninHorqinwherethesupplyofcroptranspirationwaslargely duetocapillaryrise(Fig.7andTable7),whichdoesnotcontribute towettheuppersoilevaporationlayer.

Fig. 7 presents the daily simulated soil evaporation, actual croptranspirationandcapillaryrisetogetherwiththe precipita-tionobservedalongbothcropseasons.Maindifferencesbetween

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Fig.7. Soilevaporation(Es, ),actualcroptranspiration(Ta, ),and

cap-illaryrise(CR, )ﬂuxes,aswellasprecipitation( )duringthecropseasonsof

2008(a)and2009(b).

seasonsrefertorainfall,moreabundantin2008,andtothe ground-watercontributionsincethewatertablewashigherin2008(Fig.3). TheEspeaksintheinitialstageofthecropcorrespondto

precipi-tationevents,moreabundantandlargerin2008.Duringthisstage, soilevaporationwasmuchlargerthantranspirationbecausethe soilwasnotcoveredbyvegetation(smallfc)andradiationenergy

washighlyavailableatthegroundsurfaceforevaporation.Results forEsandTainTable7showthatEsaccountedfor84%and67%of

actualevapotranspirationduringtheinitialstagein2008and2009, respectively.CRwasthennegligible(Fig.7)becausecroprootswere notyetdeveloped.

ThesizeofEs peaksdecreasedduringthecropdevelopment

stage.Whenthecropdeveloped,transpirationrapidlyincreased and,becausefcincreased(Fig.4),lessenergywasavailableforsoil

evaporation.Thus,Es/ETcactreducedto33%and13%respectively

in2008and2009(Table7),withasmallerratioin2009dueto lessrainfalleventswhiletranspirationbecamethemain compo-nentofET.Duringthis cropdevelopmentstagerootsdeveloped andcapillaryrisestartedincreasingtoattainmaximumvaluesby

themid-seasonstage.Duringmid-season,CRﬂuxesweresmallerin 2008becauseprecipitationwaslargeandextractionbyrootswas thenpredominantlyfromsoilwater.

Inthemid-seasonstageCRkepthigh,thusbecomingthemain contributortoET,essentiallytoTa.Naturally,theratioEs/ETcact

decreasedto7%and3%respectivelyin2008and2009,whenthe cropfullydevelopedandfcincreasedfurther.Thoseratiosarequite

lowbecauseETwasmainlysuppliedbycapillaryrise,whichdoes notcontributetoEs.Duringthelateseason,duetosenescenceof

leavesandtheirpickingforanimal’sfodder,fcdecreasedtogether

withtranspirationresultinginanincreaseoftheratioEs/ETcact,

particularlyin2009whenthecropwashighlystressed(Fig.6b), andtranspirationhighlyreduced(Table7).Consideringthecrop season,resultsevidencethatahigherEs/ETcact wasobservedin

2008becauseprecipitationeventswerelargerandmorefrequent. Differencesbetweenbothyearsareevidencedduringthe mid-and late-season(Fig.7).Inthewet2008,rainfallfavouredcrop vigourandcontributedtoraisethewatertable,thustokeepGWC highthroughoutthatperiod.Differently,inthedry2009,rainfall wasinsufﬁcientandthewatertablesteadilydeclinedalong the mid-andlate-season.Hencethecropcouldnotkeepan appropri-aterateofextractionofwaterfromthewatertable.Theevidence of water stress during that period is given by the decreasing Kcbact curveshown in Fig.6b indicating that thecrop

progres-sivelydecreasedtranspirationwhiletheupwardﬂuxesfromthe water tablealso progressively decreased (Fig.7b).At the late-seasonthecropwasmaintainedwithgroundwatercontribution only,howeverquitereduced.Nevertheless,GWCin2008and2009 representedsimilarfractionsoftheETcact,respectively50%and

51%(Table8).TheseﬁguresevidencetheimportanceofGWCto cropsintheHorqinsandyarea.

3.4. Soilwaterbalanceandgroundwatercontribution

Themonthlyresultsofthesoilwaterbalanceofthemaizeﬁeld show thatprecipitation in both years wasinsufﬁcient tocover maize ETcact (Table8), which evidencestherole of

groundwa-tercontribution.GWCin2008was216mm,whichcorresponds to50%ofETcactof436mm.Resultsweresimilarin2009,where

GWCwas185mmcorrespondingto51%ofETcactof360mm.As

referredbefore,differencesinGWCrelatetothewatertablelevel (Fig.3),thatwashigherin2008duringmid-seasonandlate sea-son,whencroprootswerelargerandcropETcwashigher,i.e.,when

conditionsfavouredhighercapillaryrise.

During theinitialstage ofthe crop, byMay,GWC wasvery low.Inbothyears,itincreasedto20%inJunebecausefcincreased

and rootsdeveloped.The largestvalues wereobservedfor July, 76and85mmrespectivelyin2008and2009,butcorresponding toquitedifferentpercentagesofETcact,respectively53%and67%.

Thisbehaviourreﬂectstherelativelyabundantrainin2008,when a largeamountdidpercolate tothegroundwaterfollowing the stormsofJuly31andAugust1,totaling121mm.Thelackofrainin AugustandSeptemberledtoloweringthewatertableandtoGWC muchsmallerin2009thanin2008.

Table7

ActualmaizeevapotranspirationETcactanditspartitionintosoilevaporation(Es)andpotentialandactualcroptranspiration(Tc,Ta)foreachcropdevelopmentstagein2008

and2009.

Cropgrowthstages Es(mm) Tc(mm) Ta(mm) ETcact(mm) Es/ETcact(%)

2008 2009 2008 2009 2008 2009 2008 2009 2008 2009

Initial 48 39 21 33 9 19 57 58 84 67

Cropdevelopment 32 12 80 80 65 80 97 93 33 13

Mid-season 15 5 201 207 201 172 216 177 7 3

Lateseason 7 3 58 58 58 29 64 32 10 9

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Table8

Monthlysoilwaterbalancecomponentsforthemaizecropseasonsof2008and2009.

Period P(mm) ETc act(mm) Es(mm) Ta(mm) DP(mm) SWC(mm) GWC(mm) GWC/ETc act(%)

2008 May 19 36 29 7 0 15 1 3 June 55 54 33 21 0 −12 11 20 July 179 144 28 116 7 −103 76 53 August 76 119 6 113 93a ₆₀ ₇₆ ₆₄ September 34 85 8 77 0 0 51 60 Season 363 434 102 333 100 −41 216 50 2009 May 2 27 17 10 0 27 2 7 June 55 53 27 26 0 −14 11 21 July 49 127 11 116 0 −7 85 67 August 12 107 2 105 0 45 49 46 September 7 46 3 43 0 0 38 83 Season 125 360 60 300 0 47 185 51

P–precipitation,ETcact–actualevapotranspiration,Es.–soilevaporation,Ta–actualtranspiration,GWC–groundwatercontribution,DP–deeppercolation,SWC,variation

ofthesoilwatercontent.

a_This_high_percolation_results_from_{precipitations}_totaling₁₂₁_mm_in_days₃₁_July_and₀₁_August.

Theuseofstoredsoilwater(SWC)wasrelativelyimportant inbothyearsduringtheinitialstage,inMay,particularlyin2009 whenrainfallwasverylow.Then,whenmonsoonrainsstart,SWC resultedincreasedattheendofJuneandJuly,particularlyin2008. Thestored SWCwas againcontributing toET inAugust but in SeptemberthevariationofSWCwasnull.BythenGWCwasquite important,representing60%ofETcact,in2008and83%in2009,

whensoilwaterwaslargelydepleted(Fig.5b).

ResultsaboveshowthatcroppinginHorqinandsimilarareas isonlypossiblewithgroundwatercontributionorwithirrigation usingpumpedgroundwater.However,irrigationwillinducea pro-gressivedeclineofthewatertablethatwillaffectallnaturaland man-madeecosystemsandwilloriginateaprogressiveincreaseof irrigationrequirements.Thus,inthefollowingsectionimpactsof alternativescenariosofgroundwaterdepthsareanalyzedaiming atshowingtheconsequencesofgroundwaterwithdrawal. 3.5. Assessingimpactsofalternativegroundwatertabledepth scenarios

Groundwatertabledepthvariationsarelikelyoccurringandits declineiscontinuing(Zhaoetal.,2009;Zhengetal.,2012;Yan etal.,2014).Therefore,severalalternativegroundwatertablelevel (WTL)scenariosweresimulatedwiththeSIMDualKcmodelaimed atassessingrelatedimpactsonGWCandonwaterusebyrainfed maizeascroppedinHorqin.Scenarioswerebuiltuponthe obser-vationsof2008(highrainfallandhighwatertable)and2009(low rainfallandlowwatertable)andconsistofchangingtheWTLas follows:

WTL+10–observedWTLincreasedby10cm. WTL−10–observedWTLdecreasedby10cm. WTL−20–observedWTLdecreasedby20cm.

Simulations were performed using all data observed in both years and the calibrated parameters, with the WTL data changed from observations as referred above. Results relative

togroundwatercontribution,ETcact,croptranspirationand soil

evaporation are presented in Table 9. For scenarios builtfrom 2008data,i.e.,highrainfallandhighWTL,changesinwatertable depthindicatesmall impacts onboth ETcact and GWCbecause

precipitation is enough to favour crop vigour and to recharge thegroundwater.Inotherwords,theconditionrelativeto2008 correspondstoawellbalancedcondition.

Forscenariosbuiltfrom2009data,i.e.,withlowrainfalland lowwatertable,resultsshowthatWTLdecliningleadtoagreat decreaseofGWC,from185to115mmwhenWTLisreducedby 10cmandto101mmifa20cmloweringisconsidered(Table9). DecreasesofETcactarealsolarge,from360to295mmiftheWTL

lowersby10cmorto283mmifa20cmloweringisconsidered. Differencesbetweenresultsreferringto10and20cmdeclining are small becausethe cropis already highly stressed withthe presentscenarioand thereforeresultsforboth deﬁcitscenarios reﬂectmainlytheperiodsofAugustandSeptemberwhenstress isalreadyhigh.Theactualtranspirationwoulddecreaseby65or 76mm(Table9)forthescenarioswithWTLdecreasingby10or 20cm respectively,i.e.,22% and25%. TheseTa decreaseswould

producehighyieldlossessinceyieldisproportionaltoTa.Results

agreewiththosepresentedbyXuetal.(2013).Soilevaporation remainsunchangedbecauseitdependsonlyfromrainfallanddoes notdependuponupwardﬂuxes,whichdonotsupplythe evap-orablesoillayer.Differently,raisingthewatertableby10cminthe dryyearcouldhaveimportantbeneﬁtstoGWC,resultingthatETcact

andTawouldincrease by19 and20mmrespectively.However,

becauserainfallisverylow,beneﬁtsarequitesmall.

Results of assessment of impacts of increased or decreased watertabledepthsrelativetopresentindicatethatan appropri-ateregionalmanagementofthewatertableandprotectionofthe groundwatertoavoid its decliningare paramountfor the con-servationoftheHorqinlandscapeandecosystems,andtofurther combatdesertiﬁcationinthisfragilesandyarea.Moreover,because thedecliningofthewatertablemayinducefarmerstoirrigateby pumpingfromthegroundwater,theonlysourceofwaterinthe area,itisimportanttounderlinethatpumpingwillcauseafurther

Table9

Impactsofchangesingroundwatertablelevel(WTL)ontheseasonalgroundwatercontribution,actualevapotranspiration,actualtranspirationandsoilevaporation.

Groundwatertabledepthscenarios GWC(mm) ETcact(mm) Ta(mm) Es(mm)

Highrainfall(363mm) ObservedWTL 216 434 334 104

WTL−10 210 437 333 104

WTL−20 189 436 332 104

Lowrainfall(125mm) ObservedWTL 185 360 300 60

WTL+10 208 379 320 60

WTL−10 115 295 235 60

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loweringofthegroundwatertable,thusincreasingthedegradation oftheecosystemsandtherisksofdesertiﬁcation.

4. Conclusions

The SIMDualKcmodel, that adoptsthedual cropcoefﬁcient approach,wassuccessfullycalibratedandvalidatedforsimulating capillaryrisefromthegroundwatertableusingtwocontrasting maizecropseasons,withhighandlowrainfallandhighandlow watertabledepths.Hence,thestudyprovidedforaproperanalysis ofcropwateruse,soilwaterbalanceandgroundwatercontribution towaterusebyagroundwaterdependentmaize.Resultsshowthat usingonlytwocontrastingseasondatasetswasenoughtoproperly performthedesiredGWCassessmentandtoappropriately com-putetheactualtranspirationandsoilevaporationusingtheFAO56 dualcropcoefﬁcientapproach.

Agoodsetofbasalcropcoefﬁcientsandevaporationand cap-illary rise parameters were then calibrated that allow further exploringthemodel toassess croplandwater useand ground-watercontributioninHorqinSandyArea.Simulationresultsalso showthatusingappropriatelyselecteddefaultparametersallows extendingtheuseofSIMDualKctoothersimilar areasin Inner Mongolia.

Resultsforbothcropseasonshavingdifferentrainfalland con-trastingwatertabledepthsshowthatgroundwatercontributionis extremelyimportant,exceeding50%ofactualcrop evapotranspira-tionovertheseasonand,inparticular,duringmid-seasonandlate seasonwhenyielddevelopmentandmaturationoccurs.

Itwasobservedthattheratioofsoilevaporationto evapotrans-piration(Es/ETcact)wasrelativelysmall,23%and17%respectively forthewater abundantand waterdeﬁcitseasons, withthesoil evaporationamountdependingonrainfallbecausecapillaryrise doesnotcontributetoEs.Naturally, theamountofGWChighly relateswithactualtranspirationandbeinggenerallyabove50% duringmid-andlate-season.

Resultsindicatethatcroppingintheareahasahighfailurerisk whenrainfallislowandthewatertableisrelativelydeep.Toverify thisassumption,simulationswereperformedforwatertable sce-nariosbuiltfromthoseobserved.Forthecaseofhighrainfalland highwatertable,itwasveriﬁedthatchangesinwatertablelevel producedrelativelysmallchangesinGWC,ETcact,andTa. Differ-ently,loweringtheWTLforthedryyearhavingalowwatertableled tolargedecreasesinGWCand,consequentlyinETcact,andTa.The feasibilityofcroppinginHorqinifthewatertabledeclinesbecomes very low, particularlyifrainfall is low. Therisk of degradation oftheecosystemsandlandscapeofHorqinwouldthenincrease. Therefore,appropriateregionalmanagementofthegroundwater, includingavoidanceofpumpingforirrigation,isrequiredfor con-servationoftheHorqinSandyLandlandscapeandtofurthercombat desertiﬁcationinthearea.

Acknowledgements

ThisstudywassupportedbytheNationalNaturalScience Foun-dation of China,Contract #51139002, #51069005; theChinese MinistryofScienceandTechnology,contract#2010DFA71460;the InnerMongoliaAgriculturalUniversityGrantNDTD2010-6;andthe InnerMongoliaIndustrialInnovationTeam,aswellasLEAF- Land-scape,Environment,AgricultureandFood,UniversityofLisbon,and theChina–Portugalcooperativeresearchprogramme.

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