309chAPTeR 13. Forests and Water

GraemeLockaby,ChelseaNagy,JamesM.Vose, ChelcyR.Ford,GeSun,SteveMcNulty, PeteCaldwell,ErikaCohen, andJenniferMooreMyers1

key FiNDiNGS

• Forestconversiontoagricultureorurbanuseconsistentlycausesincreaseddischarge,peakflow,andvelocityofstreams.Subregionaldifferencesinhydrologicresponsestourbanizationaresubstantial.

• Sediment,waterchemistryindices,pathogens,andothersubstancesoftenbecomemoreconcentratedafterforestconversion.Iftheconversionistoanurbanuse,theresultingadditionalincreasesindischargeandconcentrationswillproduceevenhigherloads.

•Althoughphysiographiccharacteristicssuchasslopeandsoiltextureplaykeyrolesinhydrologicandsedimentresponsestolanduseconversion,landuse(ratherthanphysiography)istheprimarydriverofwaterchemistryresponses.

•Conversionofforestlandtourbanusesmaydecreasethesupplyofwateravailableforhumanconsumptionandincreasepotentialthreatstohumanhealth.

• Increasesinurbanizationby2060intheAppalachians,Piedmont,andCoastalPlainwillincreaseimperviousnessandfurtherreducehydrologicstabilityandwaterqualityindicesintheheadwatersofseveralmajorriverbasinsandinsmallwatershedsalongtheAtlanticOceanandGulfofMexico.

1GraemeLockabyisaco-leadauthorandAssociateDeanandProfessor,AuburnUniversitySchoolofForestryandWildlifeSciences,Auburn,AL36849.ChelseaNagyisaGraduateResearchAssistant,BrownUniversity,DepartmentofEcologyandEvolutionaryBiology,Providence,RI02912.JamesM.Voseisaco-leadauthorandProjectLeader,U.S.DepartmentofAgricultureForestService,SouthernResearchStation,CenterforIntegratedForestScience,CollegeofNaturalResources,NorthCarolinaStateUniversity,Raleigh,NC27695.ChelcyR.FordisanEcologist,U.S.DepartmentofAgricultureForestService,SouthernResearchStation,CoweetaHydrologicLaboratory,Otto,NC28763.GeSunisaResearchHydrologist,U.S.DepartmentofAgricultureForestService,SouthernResearchStation,EasternForestEnvironmentalThreatsAssessmentCenter,Raleigh,NC27606.SteveMcNultyisaSupervisoryEcologist,U.S.DepartmentofAgricultureForestService,SouthernResearchStation,EasternForestEnvironmentalThreatsAssessmentCenter,Raleigh,NC27606.PeteCaldwellisaResearchHydrologist,U.S.DepartmentofAgricultureForestService,SouthernResearchStation,EasternForestEnvironmentalThreatsAssessmentCenter,Raleigh,NC27606.ErikaCohenisaResourceInformationSpecialist,U.S.DepartmentofAgricultureForestService,SouthernResearchStation,EasternForestEnvironmentalThreatsAssessmentCenter,Raleigh,NC27606.JenniferMooreMeyersisaResourceInformationSpecialist,U.S.DepartmentofAgricultureForestService,SouthernResearchStation,EasternForestEnvironmentalThreatsAssessmentCenter,Raleigh,NC27606.

•Onaverage,watersupplymodelprojectionsindicatethatwaterstressduetothecombinedeffectsofpopulationandlandusechangewillincreaseintheSouthby10percentby2050.

•Waterstresswilllikelyincreasesignificantlyby2050underallfourclimatechangescenarios,largelybecausehighertemperatureswillresultinmorewaterlossbyevapotranspirationandbecauseofdecreasedprecipitationinsomeareas.

•Approximately5,000milesofsoutherncoastlinearehighlyvulnerabletosealevelrise.

iNTRoDucTioN

Comparedtoallotherlanduses,southernforestsprovidethecleanestandmoststablewatersuppliesfordrinkingwater,recreation,powergeneration,aquatichabitat,andgroundwaterrecharge(Brownandothers2008,Jacksonandothers2004,Sunandothers2004).Forestsareuniqueamonglandcoversbecausetheyarelong-livedandrelativelystable.However,theyaresubjecttosubstantialstructuralandfunctionalalterationsbymanagementpracticesand/ornaturaldisturbances,theintensityofwhichdetermineswhetheralterationsareshortorlong-term.WaterresourcesintheSouthareatriskofdegradationfromagrowingpopulation,continuedconversionofforeststootherlanduses,andclimatechange.Urbanandagriculturallandscanimpairwaterresourcesbyintroducingnutrients,sediment,bacteria,andotherpollutantstostreams.Additionally,alteredhydrology—includinghigherpeakflows,andlowerbaseflowsandhydroperiods(Amatyaandothers2006)—iscommonwithforestconversiontootherlanduses.Together,thesechangescanmodifythehabitatandconsequentlythecompositionofaquatic(andriparian)communities.

HistoricallandusepracticeshavedramaticallychangedthelandscapeoftheSouth.Soilerosionandsedimentationwereprevalentthroughouttheregionduringtheperiodofagriculturalexpansioninthe18thand19thcenturies.Evidencecanstillbeseentodayinthesedimentdepositsoffloodplains.ThismassivetopsoilerosionanddepletedsoilproductivitywasfollowedbyaperiodofagriculturalabandonmentandreforestationthroughoutmuchoftheSouth.Todaymajorlandusesincludeforestry,agriculture,

Chapter 13. Forests and Water

313chAPTeR 13. Forests and Water

HadCM3B2.Itshouldbenotedthatclimatemodelsareoftencalibratedtobestaddressclimatewithinaspecificgeographicregion(e.g.,MIROC32A1BwasspecificallydevelopedforJapan).Theapplicationofanygeneralcirculationmodel(GCM)willnotbeuniversallyreliable(i.e.,accuratelyabletopredictfutureclimate).InthisstudytheMIROC32A1BpredictsclimaticconditionsfortheSouthernUnitedStatesthatareextremeformostpartsoftheglobe.Othermodelscouldbeconsideredmoremoderateintheirpredictionsoffutureclimate.TheresultsofallformodelpredictionsarepresentedinthischapterbutfurtherdiscussionoftheGCMcanbefoundinchapter3.

Population and land use change data—TheU.S.CensusBureau(2001)recordsindicatethatpopulationincreasedabout30percentfrom1980to2000.Populationprojectionsatthecensusblocklevelwereaggregatedtothe8-digitHUCwatershedscaleforeachyearfrom1967to2050(NPADataServicesInc.1999).FortheWaSSImodelsimulation,weselectedthelandusedata(chapter4)belongingtoCornerstoneA(IntergovernmentalPanelonClimateChangestorylineA1B,levelcropprices,hightimberprices).Formodelsimulations,thelanduseclassesdescribedinchapter4weregroupedintoeightcategoriesas:crop,deciduousforest,evergreenforest,mixedforest,grassland,shrubland,savanna,andwater/urban/barren.Inaddition,thelandusedatawererescaledfromthecountytothe8-digitHUCwatershedformodelinput.Therepresentativeyearwas2000forhistoricbaselinemodelsimulations,and2050wasselectedforfuturemodelsimulations.

Sea level Rise

WeusedtheanalysisofTitusandRichman(2001)toidentifylandarea1.5m,1.5to3.5m,and>3.5mabovesealevelandgenerated66maps(basedon1-degreedigitalelevationmodels)tooutlinecoastalareasontheGulfofMexicoandthesouthernAtlanticStatesfromVirginiatoFlorida.Thevulnerabilityofcoastalregions(coastalvulnerabilityindex,orCVI)wascalculatedusingtheanalysesofHammar-KloseandThieler(2001),whoincorporatedgeomorphology,coastalslope,rateofrelativesea-levelrise(mmyr-1),shorelineerosionandaccretionrates(myr-1),meantidalrange(m),andmeanwaveheight(m)intocalculationsofCVIfortheAtlantic,Pacific,andGulfofMexicocoasts.Arankingwasappliedtoeachvariablefor~5kmsegments(or3minutes)ofcoastlineandthencombinedtoformanindexofriskusingthefollowingequation:

CVI=

Whereaisgeomorphology,biscoastalslope,cisrelativesea-levelrise,disshorelineerosion/accretionrate,eismeantiderange,andfismeanwaveheight,andthefinalCVIwas

brokenintotofourriskcategories:low(<8.7),moderate(8.7to15.6),high(15.6to20.0),andveryhigh(>20.0).

DatasourcesusedtocalculateCVIincludedstategeologicmapsand1:250,000-scaletopographicmapstodeterminegeomorphology;acombinationofU.S.NavyETOPO5andNationalGeophysicalDataCenterdigitaltopographicandbathymetricelevationdatabasestodeterminecoastalslope;NationalOceanServicedatatodeterminerelativesea-levelriseandmeantidalrange;acombinationoftheMayandothers(1982)CoastalErosionInformationSystemdatasetandmorerecentStateandlocalregionalstudiestoestimateshorelineerosionandaccretionrates;andtheU.S.ArmyCorpsofEngineersWaveInformationStudytoestimatemeanwaveheight(Hammar-KloseandThieler2001).Afteralldatawerecompiledandrescaledtoa5kmgrid,eachvariablewasrankedfrom1(verylowvulnerabilitytosealevelrise)to5(veryhighvulnerabilitytosealevelrise).

ReSulTS

Physical environment of the Southern Region

Thesouthernclimateispredominantlyhumidsubtropical;however,thewesternmostareas,suchasTexasandOklahoma,aresemi-arid.Theaverageannualtemperaturerangeis15to21°Candtheprecipitationrangeis1010to1520mmyr-1(Bailey1980).Ultisols,thepredominantsoilorderoftheSouth,arestronglyleachedandnutrientpoorwithasubsurfaceaccumulationofclay(Bailey1980,USDANaturalResourceConservationService2009).ThereliefismostlylevelalongmuchoftheAtlanticandGulfofMexico,however,theupperCoastalPlainofAlabamaandMississippiismoderatelytogentlyrolling(MartinandBoyce1993).CoastalPlainsoilsaresandy.ThePiedmonthasgentlyrollingtosteepterrainwithclayeysurfaceandsubsurfacesoils.Consequently,thepotentialforerosionishighthroughoutthePiedmontandevenhighertowardtheBlueRidgesubregionoftheSouthernAppalachians(Trimble2008).TheSouthernAppalachianshavesteeptopographyandelevationrangesfrom225to900m.ThethreemajorriverbasinsoftheSoutharetheMobile,Tennessee,andCumberland(WorldWildlifeFund2010).Southernstreamssupportadiversityoffreshwaterspeciesandarethusahighconservationpriority(WorldWildlifeFund2010).Physiographicsubregions,andthelandscapecomponentsofwatershedswithinthem,areconnectedthroughtheflowofenergyandmaterials,themovementofspecies,andthemovementofinsects,disease,andotherdisturbanceagents.Unlikemanyoftheexchanges,themovementofwateracrossmostoftheSouthisfairlypredictablebecausewaterfollowshydrologicflowpathsthatareprimarilydrivenbyelevationgradients.ExceptionsoccurinthelowerCoastalPlainandother

√ a*b*c*d*e*f6( )

314The Southern Forest Futures Project

systemswherehydrologyisdominatedbygroundwaterhydrology.Understandinghowchanginglandscapeswillalterthequantity,quality,andvalueofsurfacewaterandgroundwaterrequiresanalysesatexpandingspatialscalestoexaminehowrapidurbanizationaffectsforestpracticessuchascutting,roadbuilding,anddrainage.

Functions of Forested Wetlands and Riparian Forests

Forestedwetlandscanbedescribedbyhydrogeomorphicconsiderationssuchaslandscapeposition,watersource,andhydrodynamicsaredominantprocessregulators(Ainslie2002).Thethreemostcommonclassesofsouthernforestedwetlandsareriverine,depressional,andflatwithmineralororganicsoil(table13.3).Ingeneral,forestsandhydrologicalcyclesareconnectedthroughtheprocessesofevapotranspiration(Amatyaandothers2008).Hydrologicalfunctionsofsouthernforestedwetlandsmayincludefloodmitigationorshort-termsurfacewaterstorage;andtoalesserextentthanforestedwetlandsinotherregionsoftheUnitedStates,theyabatestormsandrechargegroundwater(NationalResearchCouncil1995,Walbridge1993).Biogeochemicalfunctionsofwetlands,includingcyclingofelementsandretentionandremovalofdissolvedsubstances,servetoimprovesurface,subsurface,andgroundwaterquality(Blevins2004,NationalResearchCouncil1995).Regardlessoftype,allforestedwetlandscontributetofoodwebmaintenancebyprovidinghabitatforplantsandanimals(Faulkner2004,Walbridge1993).Someforestedwetlandsmayprovideuniquefunctionsbasedontheirdistinctivecharacteristicsandstructure.Forexample,CarolinaBaysmaycontainrareandendangeredplantsandalsoprovidedesirablebreedingsitesandhabitatforbirdsandwildlife

(Ainslie2002).Andmineralsoilflatscanhaveveryhighherbaceousspeciesrichnessinpartbecauseoftheiruniquefireregime(Ainslie2002).

Riparianforestsalsoprovidehydrological,biogeochemical,andhabitatfunctions.Manystudieshaveshownthatriparianforestshelptostabilizestreambanksandtrappollutantssuchassediment,nutrients,bacteria,fertilizers,andpesticidesfromrunoff(AndersonandMasters1992,BinkleyandBrown1993,delaCrétazandBarten2007,KlapprothandJohnson2000,Naimanandothers2005,USDANationalAgroforestryCenter2008,Vellidis1999).Inparticular,Naimanandothers2005foundthat“Thehydraulicconnectivityofriparianzoneswithstreamsanduplands,coupledwithenhancedinternalbiogeochemicalprocessingandplantuptake,makeriparianzoneseffectivebuffersagainsthighlevelsofdissolvednutrientsfromuplandsandstreams,whilegeomorphologyandplantstructuremakethemeffectiveattrappingsediments.”However,anintactripariancorridordoesnotensurestreamprotectionasthisrelationshipisdependentonotherfactorsincludingresidencetimeofpollutantsinthebuffer,depthandvariationofwatertable,uplandlandusepractices,climate,andwatershedcharacteristicssuchastopography,hydrology,soils,andvegetation(delaCrétazandBarten2007,Groffmanandothers2003,Tomerandothers2005,Walshandothers2005).

Habitatfunctionsprovidedbyriparianforestsincludelowerwatertemperaturesforaquaticanimalsduetoshadingfromtrees,alongwithshelterforbirdsandwildlife(AndersonandMasters1992,BinkleyandBrown1993,Naimanandothers2005,Vellidis1999).Ripariaaresourcesoflargewoodydebris,whichcreateshabitatheterogeneity,actsasasubstrateforcolonization,andprovidesnutrientstotheaquatic(and

Table 13.3—Southern wetland types and characteristics

Wetland type characteristics Sources

Riverine (bottomland hardwood wetlands)

Occurring in floodplains or riparian corridorsAinslie 2002, Brinson 1993, Palmer 1994, Childers and Gosselink 1990, National Research Council 1995, Naiman and others 2005, Walbridge 1993, Meyer 1992, Dennis 1988

Many connections between wetland and stream channel (overbank flow and subsurface connections)Including cypress stands, sloughs, and hardwood swamps associated with brown-, black-, and red-water streams in Atlantic and Gulf Coastal Plains

Depressional

Named for their depressional topography which promotes surface water accumulation

Ainslie 2002, Brinson 1993, Duryea and Hermansen 1997, Dennis 1988Cypress trees predominant

Including cypress domes (Gulf Coastal Plain) and Carolina bays (Atlantic Coastal Plain)

Wet flats

Can have either organic soils (pocosins) on plateaus or mineral soils in the Atlantic Coastal Plain between rivers or floodplain terraces Ainslie 2002, Brinson 1993,

Gresham 1989Pocosins characterized by dense evergreen shrub vegetation Mineral flats characterized by a closed canopy of hardwoods or an open savannah with some pines

315chAPTeR 13. Forests and Water

riparian)community(Naimanandothers2005).Inputsoforganicmatterfromriparianforestssupplyanallochthonousenergysourcetostreamecosystems,therebylinkingtheriparianandaquaticfoodwebs.Additionally,ifBestManagementPracticesareimplementedappropriately,riparianforestscanalsoprovidewoodproducts,pastureforlivestock,andrecreationalopportunities(AndersonandMasters1992).

hydrologic effects of Forest conversion to other land uses

Harvestingforestsreducesevapotranspirationandinfiltration;creatingimpervioussurfacesincreasesoverlandflow(PaulandMeyer2001).Similarly,forestconversiontoagriculturallandmaycompactsoils,reduceevapotranspirationandinfiltration,andincreaseoverlandflow.Regardlessofpost-harvestinguse,characteristicchangesinhydrologyfollowingforestremovalincludegreaterstreamflowandpeakflows(BoschandHewlett1982,Crim2007,delaCrétazandBarten2007,Hibbert1967,McMahonandothers2003,Schoonoverandothers2006).Representativehydrographsfortypicalforested,agricultural,andurbanwatershedsareshowninfigures13.2to13.4.Astudyofthe13SouthernStatesshowedthatstreamflowincreasedby69to210mmyr-1followingforestharvesting(Grace2005).Stednick(1996)foundthata20percentchangeinforestcoverproducesaquantifiablechangeinwateryieldintheAppalachians,butthatthethresholdisabout25percenthigherforthePiedmontandCoastalPlain.Sincethe1970sinHouston,impervioussurfaceswereresponsiblefor32percentofthe159percentincreaseinpeakflows,and77percentofthe146percentincreaseinannualrunoff(OliveraandDeFee2007).Similarlysincethe1960sintheWhiteRockCreekwatershedofnortheasternTexas,peakflowsincreasedby20to118percentwithvaryingprecipitationintensitiesinresponsetodramaticincreasesinimperviouscover(Vicars-GroeningandWilliams2007).Streamhydrographsofurbanwatershedsreflectaflashyhydrologywithgreaterpulsesandfasterattainmentofpeakflowsduringstormevents(Beighleyandothers2003,BoggsandSun2011,Calhounandothers2003,Crim2007,Schoonoverandothers2006).However,asaridregionsnaturallyhaveflashyhydrologyduetoinherentprecipitationregimes,urbaneffectsmaybeobscuredonthehydrographsofstreamsinthosepartsoftheSouth(Grimmandothers2004).Flow-durationcurvesalsodepictchangesinhydrologybydisplayingthepercentageoftimestreamflowequalsorexceedsaparticularvalue.Thepre-urbanizationflowdurationcurveexhibitsmoregradualvariationswhilethepost-urbanizationcurveismuchsteeper(fig.13.5).

Inurbanandagriculturalwatersheds,decreasedinfiltrationproduceslessgroundwaterrecharge,possiblyreducingbaseflows(Calhounandothers2003,RoseandPeters2001,Wangandothers2001).Asanexample,intributariesof

Figure13.2—Representativehydrographofaforestedwatershed(Crim2007).Dischargeunitsareliterspersecondperha.

Figure13.3—Representativehydrographofapastoralwatershed(Crim2007).Dischargeunitsareliterspersecondperha.

Figure13.4—Representativehydrographofanurbanwatershed(Crim2007).Dischargeunitsareliterspersecondperha.

316The Southern Forest Futures Project

theupperChattahoocheeRiver,Calhounandothers(2003)estimatedthatevery1percentincreaseinimpervioussurfacereducesbaseflowby2percent.However,thisisnotalwaysthecaseasillustratedbythelackofbaseflowresponsetoincreasesinimpervioussurfaceintheFloridaPanhandle(Nagyandothers2012);andbyhighermedianbaseflowinpastoralwatershedscomparedtoforestedwatershedsintheGeorgiaPiedmont(Schoonoverandothers2006).ThelessresponsivebaseflowintheCoastalPlainmaybeexplainedbydifferencesbetweentheextentofsurfacewaterandbaseflowrechargezonesinveryflatterrainswherebaseflowzonesmayextendbeyondsurfacecatchmentboundaries.Inthecaseofthepastoralvs.forestedwatershedsinthePiedmont,apparentlyreducedETandadequatesurfaceinfiltrationratesinthepasturesaccountedforthehigherbaseflowsthere.

AlthoughhistoricallytheSouthhasnotexperiencedagreatdeficiencyofwatersupplycomparedtootherregionsintheUnitedStates,withcontinuedforestlossandexpandingurbanization,watersupplymaybecomeamorepressingissueinthisregion.Whenmodelinglanduseeffectsonly,Sunandothers(2008)predictedreducedwaterdeficitsduetoincreasedwateryieldfollowingconversionofforesttourbanlanduses.However,theyfoundthatwaterresourceswouldlikelybeundergreaterpressureinthefuturewhentheeffectsofclimatechangeandpopulationgrowthare

alsotakenintoaccount.Additionally,itshouldbenotedthatdespiteincreasedstreamflowfollowingforestremoval,availablesurfacewatermightdeclineduetounstableflowregimes.AfterincreasingwithdrawalratesfromAlabamastreamsnearBirmingham,surfacewateravailableforhumanuserangedfromabout20to45percentofdischargeforurbanwatershedsand20to60percentforforestedwatersheds;andathigherwithdrawalrateswateravailabilitywassignificantlyhigherinforestedthanurbanwatersheds.2Therefore,althoughtotalwateryieldisoftenreducedinforestedwatershedscomparedtourbanwatersheds,forestedwatershedsmayhaveagreaterpercentageofwateravailableforuse,suggestingthatincreasingurbanizationcontributestogreaterstress.Lastly,degradationofwaterqualityfrompoint-andnon-pointsourcepollutioncanalsoreducetheamountofavailablewater(Sunandothers2008).

effects of Forest conversion on Sediment

Forestsstabilizesoils(Jacksonandothers2004);thereforesoilismorereadilyerodedfollowingremovalofvegetation,andistransportedassedimentintofloodplainsandotherareasoflowertopography(Jacksonandothers2005a,Trimble2008)and/ordirectlyintostreamchannels.Theeffectsofhistoricalagriculturaluse,inparticularrow-cropagriculture,onsoilerosionandsubsequentsedimentdepositionthroughouttheSouthwereprofound(Casarim2009,Jacksonandothers2005a,Trimble2008).Forexample,intheGeorgiaPiedmont,sedimentdepositionfromhistoricalagriculturewasasmuchas1.6mintheMurderCreekfloodplain(Jacksonandothers2005a)andaveraged1.8minBonhamCreekandSallyBranchwatersheds(Casarim2009).Itcanbedifficulttodifferentiatesedimentcontributionsfromcurrentlanduseversushistoricalagriculturalusewithinawatershedbecausethelegacyeffectsofhistoricallandusecanbeobserveddecadeslater.Jacksonandothers(2005a)estimatedthat25to30percentofthesedimentloadofMurderCreekconsistedofre-suspendedlegacysediment.ThismeansthatlanduseconversionsinthePiedmonthavethepotentialtore-suspendlegacysedimentthataccumulatedinthestreambedsdecadesagoinadditiontogeneratingsedimentexportfromterrestrialsources.

Thecombinedeffectsofalteredhydrology,removalofvegetation,andanincreaseinimpervioussurfaceoftencauseurbanwatershedstoexhibitstreamsedimentconcentrationsmuchhigherthanforestedwatersheds(ClintonandVose2006,LenatandCrawford1994,Schoonoverandothers2005).IntheSouthernAppalachians,totalsuspendedsolidsconcentrationswere4to5timesgreaterinanurban

2L.Kalin,unpublisheddata;AuburnUniversity,SchoolofForestryandWildlifeSciences,602DuncanDr.,Auburn,Alabama,36849;kalinla@auburn.edu.

Figure13.5—Streamflow-durationcurvesincubicfeetpersecondbeforeandafterurbanization.(Source:L.Kalin,unpublisheddata;AuburnUniversity,SchoolofForestryandWildlifeSciences,Auburn,AL,36849;latif@auburn.edu)

317chAPTeR 13. Forests and Water

comparedtoareferencestream(ClintonandVose2006).IntheGeorgiaPiedmont,totaldissolvedsolidsconcentrationsweretwiceashighinurbanstreamscomparedtoforestedstreams(Crim2007,Schoonoverandothers2005),buttotalsuspendedsolidsdidnotincreasesignificantlyunderurbancover.IntheCoastalPlain,Wahlandothers(1997)foundatwofoldincreaseintotalsuspendedsolidsinurbancomparedtoforestedstreams(upto200mg/Lduringstormflowintheurbanstream).Erosionassociatedwithurbanlandusescanbeparticularlyhighatconstructionsitesandareasofnewdevelopment(Novotny2003,PaulandMeyer2001).Forexample100-to10,000-foldincreasesovernondevelopedconditionswerereportedbyPaulandMeyer(2001),especiallyinareaswithgreatertopographicvariationsuchastheSouthernAppalachians.

effects of Forest conversion on Water chemistry

Undisturbedforestedwatershedsaregenerallyassociatedwithlowstream-waterconcentrationsofmostions.Sincemostforestsaredeficientinoneormoreelements,forestedsystemsaregenerallyeffectiveinretaininginputsofnutrients.Consequently,netexportofmacronutrients,ornutrientsrequiredinlargequantitiessuchasN,P,andK,fromundisturbedforestedcatchmentsisoftennegative,indicatinganaccretionofforestbiomass(LikensandBormann1995,SwankandDouglass1977).Asanexample,the3600haTableRockReservoirwatershedintheSouthernAppalachiansofSouthCarolinahasbeenhighlyrestrictedintermsofanyhumanactivitysince1930,andwaterqualitythereremainsunchangedsincethattime.3

Increasednutrientconcentrationsandloadshavebeenobservedinurbanandagriculturalstreamscomparedtoforestedstreams.Excessnutrientsmayarisefromfertilizers,wastewatereffluent,andindustrialwasteinurbanareas;andfromanimalwasteandfertilizersinagriculturalareas.ConcentrationsofNO3

-,Cl-,Na+,K+,Ca2+,andMg2+wereallhigherinanurbanstreamthanareferencestreamintheSouthernAppalachiansduringbaseflowandstormflowboth(ClintonandVose2006).Nitrateshadthemostpronouncedincreasewithmeanconcentrationsofabout0.01mg/Linthereferencestreamand0.7mg/Lintheurbanstream(ClintonandVose2006).However,ammoniumconcentrationswerehigherintheforestedstreamduringstormflow.BolstadandSwank(1997),workinginthesamephysiographicsubregion,foundnoincreasesinnitrate,ammonium,orphosphateconcentrationsduringbaseflowasurbanizationindicesincreased.However,duringstormflow,slightincreaseswerenotedfornitrate(from0.05to0.07mg/Lwithasignificantregressionrelationship)andammonium.

3Okun,D.A.1992.PropertiesoftheTableRockandPoinsettReservoirs:theirfuture.Unpublishedreport.24p.Onfilewith:theGreenvilleWatershedsStudyCommittee,P.O.Box728,Greenville,SC29602.

IntheGeorgiaPiedmont,SchoonoverandLockaby(2006)foundresultssimilartothoseofClintonandVose(2006)fordissolvedorganiccarbon,NO3

-,Cl-,andK+,aswellasSO4

2-whencomparingstreamswith<5percentand>24percentimpervioussurface.Theconcentrationsinstreamsofwatershedswith>24percentimpervioussurfaceweregenerallytwo-to-fourtimeshigherthanthoseoflessdevelopedcatchments.Forinstance,medianbaseflownitrateconcentrationswere0.61mg/Lforstreamswith<5percentimpervioussurfaceand1.64mg/Lforstreamswith>24percentimpervioussurface;stormwaterconcentrationswere0.36and1.93mg/Lrespectivelyforthesamecomparison(SchoonoverandLockaby2006).Althoughammoniumconcentrationshavebeenreportedtobehigherinforestedthaninurbanstreams(ClintonandVose2006,Tuffordandothers2003),twoPiedmontstudiesproduceddifferentresults(Crim2007,SchoonoverandLockaby2006).Similarly,dissolvedorganiccarbonconcentrationsareoftenhigherinforestedwatershedsthaninurbanstreams(Wahlandothers1997)althoughthereareexceptions(SchoonoverandLockaby2006).

Unlikemoststudiesofland-use/land-coverimpactsonwaterquality,whichhavesubstitutedspacefortime,Westonandothers(2009)evaluatedwaterqualitychangeswithintheAltamahaRiverBasinoftheGeorgiaPiedmontformorethan30years.Theincreasesinpopulationduringthatperiodexceeded100percentinsomeofthebasin’swatersheds.Duringthatperiod,agriculturallandusedeclinedaspopulationsrose,producingdecreasesinstreamammoniumandorganiccarbonconcentrationsbutincreasesintotalnitrogenandnitrogenoxideconcentrationsandloads.Phosphorusconcentrationsdidnotincreasewithurbanization,whichtheauthorssuggestmayreflecttheeliminationofphosphatesindetergentsafter1972.TheyalsosuggestthatforthePiedmont,elevatedtotalnitrogenandnitrogenoxidesmayserveaswaterqualitysignaturesofurbanizationandelevatedammonium,andthatorganiccarbonmaybeassociatedwithagriculture.Ammoniumandorganiccarbonarealsooftenlinkedwithforestcoverbuttheeffectsofchangesinforestcoverwerebeyondthescopeofthischapter.

Therearefewerstudiesofland-use/land-coverassociatedwiththeCoastalPlainthanwiththePiedmont.Wahlandothers(1997)comparedwaterqualitywithintwocoastalwatersheds:onewithincreasingurbanization(18percentimpervioussurface)andtheotherwithpredominatelyforestcover(noimpervioussurface).Nitratewasconsistentlyhigherintheurbanstream:130ug/Linwinter(90ug/Linsummer)versus42ug/Linwinter(29ug/Linsummer).Ammoniumwashigherintheforestedstreamregardlessofseason(159ug/Lversus70ug/L)asweredissolvedorganiccarbonconcentrations(27ug/Lversus13ug/L).InastudywithintheFloridaPanhandle(Nagyandothers2012),

318The Southern Forest Futures Project

medianconcentrationsofnitrate,ammonium,calcium,potassium,andsulfatewerehigherinwatershedswithmoreurbanization(impervioussurfaceupto16percent)thanintheirforestedcounterparts.However,nitrateconcentrationsintheCoastalPlainwerewellbelowthosegenerallyobservedinsomestudiesofurbanwatershedsofthePiedmont(0.35mg/Lversus1.78mg/L).Medianconcentrationsoftotalphosphoruswerehighandincreasedfrom0.31mg/Linwatershedswith<5percentimpervioussurfaceto0.43mg/Linwatershedswith>10percentimpervioussurface.Similarly,totalsuspendedsolidsincreasedfrom1.50to2.40mg/Lforimpervious-surfacelevelsabove10percent.Incontrast,dissolvedorganiccarbondeclinedfrom36mg/Linwatershedswithlowimpervioussurfaceto30mg/Linwatershedswith>10percentimpervioussurface.Tuffordandothers(2003)foundthattotalphosphoruswassignificantlyhigherinurbanstreamsthanforestedstreamsintheCoastalPlain(concentrationsofroughly0.06mg/Lversus0.03mg/L).

Ingeneral,increasesinstreamconcentrationsofseveralelementswithinurbanizedwatershedsareverycommonalthoughthemagnitudeofincreaseandsometimestheparticularionsinvolvedvaryconsiderablywithinandamongphysiographicsubregions.Whilenitrateandpotassiumionscommonlyincrease(probablyduetotheirmobilityinwater),responsesoftotalpotassiumorphosphatearemuchmorevariableandmaynotoccuratall.Responsesoftheothermajorelementsfallinbetweenthoseofnitrateandphosphate.Sincedischargeusuallyincreaseswithurbanization,loadsincreaseaswellregardlessofphysiographicsubregion.Consequently,theredonotseemtobecleardistinctionsamongsubregionsintermsofstreamchemistryresponses,whichmayindicatethattheinfluenceoflanduseoverridesthatofphysiographyintheSouth.

Higherloadsofbasecations(K+,Ca2+,andMg2+),Cl-,andtotalnitrogenwerefoundinagriculturalstreamsthaninforestedstreamsintheCoastalPlain(Lowranceandothers1985).IncreasednitrateconcentrationsandloadsinagriculturalstreamscomparedtoforestedstreamsarecommonintheAppalachians(Hagenandothers2006),Piedmont(Crim2007),andCoastalPlain(Lehrter2006,Lowranceandothers1985).Forexample,nitrateloadswere1.5to4.4timeshigherinwatershedswithgreateragriculturallandthanwatershedswithlessagriculturalland(Lowranceandothers1985)andnitrateconcentrationswere2.1to4.4timeshigherinagriculturalversusforestedwatersheds(Hagenandothers2006).

effects of Forest conversion on human health

Urbanandagriculturallandusescontributetoincreasedbacterialconcentrationsinstreamwaters.Connectedstormwaterandseweroverflowsystemsorfailuresin

sewersystems(suchasbrokenpipes,mechanicalfailures,andblockagesfromtreeroots)candirectlyorindirectlyreleaserawsewageintosurfacewaters.Additionally,petandwildlifefecescanbetransportedinrunoffoverlawnsandimpervioussurfacesinurbanareas.FecalcoliformbacteriacountswerehigherinurbanthanforestedorreferencestreamsintheCoastalPlain(DiDonatoandothers2009,Hollandandothers2004,Mallinandothers2000),Piedmont(Crim2007,Schoonoverandothers2005),andSouthernAppalachians(ClintonandVose2006).Forexample,Mallinandothers(2000)reportthatfecalcoliformishighlycorrelatedwithimpervioussurface(r=0.975,p=0.005),percentdevelopment(r=0.945,p=0.015),andpopulation(r=0.922,p=0.026).Similarly,concentrationsofE.colicanbemuchgreaterinurbanwatershedsthanforestedwatersheds(Crim2007,Mallinandothers2000).UrbanwatershedsintheGeorgiaPiedmonthadthehighestmedianE.coliconcentrations(asmeasuredinMPNormostprobablenumber),rangingfrom135to1255MPN/100mL,comparedtomedianrangesof94to169MPN/100mLforpinecoveredwatershedsand59to170MPN/100mLforoak-pinecoveredwatersheds(Crim2007).Becker(2006)reportedthatresidentialareas(2.2percentofthewatershed)didnotappeartobeasourceofbacteriatoTravertineCreeksubbasininOklahoma,butattributedincreasedbacterialconcentrationsinthenearbyRockCreekbasintolivestockgrazingandsewageeffluentinperiodsofhighprecipitation.Andrewsandothers(2009)reportedfecalcoliformcountsashighas10,000colonies/100mLwiththehighestcountsinstormflowfortheportions(42.17percent)oftheIllinoisRiverbasininOklahomaandArkansasthatareinagriculturaluse(pasturesforcattleandconfinedfeedingoperationsforpoultryandswine);theyalsoobservedrapidurbandevelopmentontheupperportionofthisbasinthatmayfurtherimpairwaterresources.

Inadditiontothedangerofelevatedconcentrationsofbacteriainsurfacewaters,otherhealthrisksfromurbanandagriculturallandusesincludemetals,pesticides,andpersonalcareproducts(KlapprothandJohnson2000,PaulandMeyer2001).Tracemetalsedimentconcentrationscanbe2to10timeshigherinstreamsnearurbanandindustrialareasthaninforestedwatershedsorsuburbanwatersheds(Hollandandothers2004);theytendtoaccumulate,ratherthandegrade,overtimeinsedimentsandplantandanimaltissue(KlapprothandJohnson2000).Metalconcentrationsmaybeinverselyrelatedtosedimentparticlesize(PaulandMeyer2001)andthuswemightexpecthighconcentrationsofmetalstobemoreproblematicintheSouthernAppalachiansorPiedmontwithmoresiltyandclayeysoilsthanintheCoastalPlain.Pesticidesenterstreamsthroughrunofffromagriculturalandurbanareas(KlapprothandJohnson2000,PaulandMeyer2001)againunderscoringtheimportanceofriparianbuffersandotherforestedareasinslowingrunoffandenhancinginfiltrationbeforecontaminantscanreach

319chAPTeR 13. Forests and Water

thestream.Personalcareproductsincludingdeodorants,perfumes,andpharmaceuticalsmaynotberemovedbytraditionalwatertreatmentmethodsandarenotaswidelyregulatedasothersubstances(Kolpinandothers2002).

effects of Forest conversion on Aquatic communities

Alteredhydrologyandchannel-morphologyandhigherstreamtemperaturescausedbyforestconversioncandramaticallyaffectaquaticcommunities.Withincreasingurbanand/oragriculturaluses,speciesrichnessandabundanceoftendeclineasreportedforalgae(Sponsellerandothers2001),macroinvertebrates(Helms2008,LenatandCrawford1994,MaloneyandFeminella2006,PaulandMeyer2001,Royandothers2003),fish(Onoratoandothers1998,Walshandothers2005),andamphibians(HoulahanandFindlay2003,OrserandShure1972,Priceandothers2006,Wangandothers2001).Theseeffectsmaybeoffsetsomewhatforalgaebyadditionalstreamnutrients(Biggs1996,Chessmanandothers1999)andformacroinvertebratesbyperennialflows(Chadwickandothers2006).Musselshavevirtuallydisappearedfromsomesouthernstreamsasaresultofincreasedconversionoflandtourbanuses(GangloffandFeminella2007,Gilliesandothers2003).ThesedetrimentaleffectsonaquaticorganismsareparticularlyevidentintheSouthernAppalachians(Scottandothers2002,Waltersandothers2003)wherebothdiversityandendemismareveryhigh(Wallaceandothers1992).Cuffneyandothers(2010)foundahighcorrelationofmacroinvertebrateassemblageswithurbanmetricsineasternmetropolitanareas(includingRaleigh,NC;Atlanta;andBirmingham,AL),butnotincentralmetropolitanareassuchasDallas-FortWorth.Additionally,speciescompositionmayshiftassensitivespeciesarereplacedbymoretolerantspeciesorspeciesbettersuitedforthenewconditions(LenatandCrawford1994,Onoratoandothers1998,Priceandothers2006,Royandothers2005,Sutherlandandothers2002,Waltersandothers2003,WeaverandGarman1994).Forexample,inurbanstreamsofthewesternGeorgiaPiedmont,reptilespeciesrichnessincreasedatthesametimethattherichnessofsalamandersandotheramphibianspeciesdecreased(BarrettandGuyer2008).

Fishcommunitiesmayalsobestronglyaffectedbychangesinhydrologyandwaterqualitythatarederivedfromlandusechanges.Highervelocitiesanddischargeaswellasincreasedsedimentloadsmaydegradestreamhabitattoasignificantdegree(Nagyandothers2011).Inparticular,higherdepositionofsedimentinstreamchannelsmayreducediversityofhabitatwithnegativeimplicationsforsomefishspecies.Reducedabundanceofbenthicfeedersandlowerspawningsuccessingeneralmayaccompanysuchchanges(Nagyandothers2011).Inaddition,nearColumbus,GA,Helmsandothers(2005)notedindicatorsofreducedfish

healthsuchasoccurrenceoflesionsandtumorsinfishfromurbanstreamsandanegativecorrelationbetweenbioticintegrityoffishandtheproportionofimpervioussurfacewithinwatersheds.

implications of land use change Projections on Water Quality

Thedecreasesinforestcoverandincreasesinurbanizationthatareprojectedby2060carryimportantimplicationsforwaterresourcesintheregion.LossesofforestcoveracrossmuchofthePiedmontofNorthCarolina,SouthCarolina,Georgia,and,toalesserextent,Alabama(chapter5)implythatfurtherdegradationofwaterqualityanddestabilizationofsurfacewaterhydrologyarelikelyinlocalizedcatchmentswithinthatsubregion.Inaddition,itislikelythatthealterationsinthehydrologiccycleswithintheheadwatersofmajorriverbasins(fig.13.6)willaffectconditionsdownstream.Riverbasinsandwatershedswillundergoreductionsinevapotranspirationduetolowerleafareaindicesaswellasincreasesinimpervioussurfaces.Consequently,responsestodeforestation—reducedinfiltration,increasedrunoff,reducedbaseflow,andincreaseddischargeandvelocity—thatalreadyexistonthePiedmonttosomeextent(butaremoreoftenassociatedwithsteeperterrains)willlikelybeexacerbated.

Thesehydrologicresponses,combinedwiththeincreasedquantitiesofpotentialpollutantsinurbanizingwatersheds,willincreasestreamwaterconcentrationsand/orloadsofsediment,nutrients,pathogens,andvariouschemicals.Theresultcouldbesignificantdegradationofwaterqualitywithinriverbasinsandstreamsystems,andconcurrentnegativeeffectsondiversityofaquaticorganisms.Althoughresponseswillbemanifestedthroughoutriverbasins,cumulativeeffectswillmagnifythetrendsalongtheirlowerreaches.Ifstreamsremainconnectedtothefloodplainforeststhatliebelowthephysiographicfallline,somefractionofpollutantloadsmaybefilteredassedimentisdepositedbyspreadingfloodwaters.However,althoughuppercoastalforestsareprojectedtobeclearedtoalesserextentthanPiedmontforests,theyremainatsomeriskofconversionwithsubsequentreductionofpollutantfiltrationpotentialonfloodplains.Inaddition,theincreasedvelocityofstreamsandriversdraininghighlyurbanizedupperreacheswilltendtoincreasechannelincisement,therebyreducingthefilteringbenefitsofoverbankfloodingandsedimentdeposition.Althoughreservoirscreatedbydamsmaytrapsignificantamountsofsedimentandothersubstances,theyhaveafinitecapacityforsedimentfilling,whichisalreadybeingapproachedinsomeareas.Consequently,loadsofsediment,nutrients,andpathogenswilllikelyincreaseinlowerreachesofriverbasins,withexportsofthesematerialsexpectedtoelevatelevelsincoastalestuariesaswell.

321chAPTeR 13. Forests and Water

forestland(15to27millionacres)thatwouldberequiredtosubstantiallyreducenonpointsourcepollutantexports(Mitschandothers2001).IncreasedforestcoveragewillalsobecharacteristiconmanyinlandCoastalPlainareasofVirginia,NorthCarolina,SouthCarolina,Georgia,Alabama,Mississippi,andLouisiana(chapter5),atrendthatcouldprotectfloodplainforestsandmaintainwaterqualityofthosesystems.

AnotherlocationthatisanticipatedtoundergomajorincreasesinurbanizationisthesouthernhalfofFlorida,whereforestedwetlands(bothripariananddepressional)areprevalentandassociatedwaterqualityfunctionsareatrisk.Atatimewhenadditionalnonpointsourcepollutantexportsareoriginatingfromnewlyurbanizedlandscapes,aportionofthenaturalsystemswithpotentialtofilterpollutantloadswilldisappearwiththedemiseofforestedwetlands.

effects of expanded intensive Forest management on Water

TheestablishmentofpineplantationshasresultedinvastacreagesofintensivelymanagedpineforestsintheSouth.Plantationbasedforestry–usingpineandfastgrowinghardwoodspecies–islikelytoincreaseinthefuture(chapter9),anddemandfromashrinkinglandbaseandemergingwoodfibermarketsforbioenergyislikelytoincreasemanagementintensityonnewandestablishedplantations.ConsiderableinformationisavailableontheimpactsofforestmanagementonstreamflowthroughouttheUnitedStates(Brownandothers2005,JonesandPost2004).Forexample,

removingtheforestcanopyincreasesstreamflowforthefirstfewyears,butthemagnitude,timing,anddurationoftheresponsevariesconsiderablyamongecosystems.Insome,streamflowreturnstopreharvestlevelswithin10to20years;whereasinothers,streamflowremainshigherforseveraldecadesaftercutting,orcanevendroplowerthanpre-harvestlevels(Jacksonandothers2004).Thiswidevariationinresponsesisattributabletothecomplexinteractionsbetweenclimate,whichcanvaryconsiderablyfromdrytowetregimes,andvegetation,whichcanvaryinstructureandphenology(coniferousversusdeciduousforest).

Informationontherelationshipsbetweenspecificecosystemsorforesttypesandstreamflowcanbeinferredfromstudiesquantifyingannualevapotranspiration.Atannualtimescales,streamflowisapproximatedbythedifferencebetweenprecipitation(PPT)andevapotranspiration(ET),streamflow=PPT-ET.Therefore,foragivenamountofprecipitation,managementactionsthatalterevapotranspirationwillalsoalterstreamflow.Itiswellestablishedthatconiferousforests,withtheirgreatercapacityforinterceptionandtranspiration,havehigherevapotranspiration(andhencelowerstreamflow)thandeciduoushardwoodforests(Fordandothers2011,SwankandDouglass1974).Averagedacrossseveralclimateregimesandforesttypes,thedifferencebetweenconiferousandhardwoodforestsisabout55percentat1200mmyr-1precipitationandincreasingprecipitationwidensthisdifferenceinthetwoforesttypes.Evapotranspirationalsovariesconsiderablybetweenmanagedandunmanagedsouthernforests(table13.4).Thisvariationisimportantforevaluatingtheimplications

Table 13.4—Mean annual transpiration (mm yr-1) for southern forest types

Vegetation type Transpiration SourceLongleaf pine savanna 244 Ford and others 2008Old field 250 Stoy and others 2006Oak-pine-hickory forest 278 Oren and Pataki 2001Upland oak forest 313 Wullschleger and others 2001Mixed pine hardwood 355 Phillips and Oren 2001Mixed pine hardwood 442 Stoy and others 2006Planted loblolly pine 490 Stoy and others 2006Mixed pine hardwood 523 Schafer and others 2002

Slash pine flatwoodsEucalyptus hybrid plantation

563882

Powell and others 2005Estimated for Baker County, southwestern Georgia in 2006 for an average climate and rainfall yeara

Planted loblolly pine (early rotation) 328 Domec and others 2012; Sun and others 2010Planted loblolly pine (mid-rotation) 777 Domec and others 2012; Sun and others 2010

aDerived from a model that used data collected in 2006 by the Joseph W. Jones Ecological Research Center, 3988 Jones Center Drive, Newton, GA 39870. Model assumed no soil water limitation; all trees at age 5; 1,111 trees ha-1; and a leaf area index of 6 m2 m-2 (Mielke and others 1999).

322The Southern Forest Futures Project

ofincreasingpineplantationforestsintheSouthbecausethemagnitudeoftheeffectsonstreamflowdependsonthespecies,foresttype,orlandusebeingreplaced.Forexample,pineplantationsmayconsumenearlytwicethewaterconsumedbylongleafpinesavannas(table13.4).

Implicationsofincreasingmanagementintensityonwaterresourceswilldependonthespecificmanagementactivity.Increasingacreagesoffastgrowingspeciesforbioenergyproductionorcarbonsequestrationmayhavenegativeconsequencesforwateryield(Farleyandothers2005,Jacksonandothers2005b).Toillustrate,amatureeucalyptusplantation(age5,1,111treesha-1,leafareaindexof6m2m-2)growinginsouthwesternGeorgiacouldpotentiallyconsume882mmyr-1ofwater,exceedingotherforesttypesbyafactorof2.5(table13.4).Nitrogenfertilizationimprovesproductivityprimarilythoughincreasedleafarea(VoseandAllen1988),andevapotranspirationishighlycorrelatedwithleafareaindex(Sunandothersinpress).Shorteningrotationtimesusuallyincreasesstreamflowbydecreasingtheamountoftimethatthestandisatcanopyclosure,whenleafareaindexishighestandstreamflowislowest.Foranygivenleafarea,youngerorshortertreesalsohavehigherstomatalconductancethanolderortallertrees(Mooreandothers2004,Novickandothers2009,Schaferandothers2000).Althoughtranspirationperunitleafareaislessthanforyoungerforests,olderforestshavelargerleafareaandcaninterceptmorewater,andthereforehavegreaterevapotranspiration.Thismeansthatmanagingforolderforestsislikelytodecreasestreamflow.

ImpactsonwaterqualitywilldependonthetypeofmanagementactivityandtheeffectivenessofestablishedBestManagementPractices,whichwereoriginallydevelopedforlessintensivemanagement.Forexample,inreviewoftheimpactsofforestsfertilization,Foxandothers(2007)concludedthatcorrectlyappliedfertilizerrarelydegradeswaterquality.Incontrast,increasingthefrequencyofharvestforshorterrotationsmayhaveimpactsonsedimentyield,especiallyiftheharvestsresultingreatersoildisturbance(Ursic1986)orrequiremoreroadsandmorefrequentroadusage(Swift1988).

implications of climate change, land use change, and Population on Water Resources

Climate change impacts on water resources—Becauseofthecombinationofbiologicalandphysicalcontrolsonhydrologicprocesses,climatechangewillbothdirectlyandindirectlyimpactsouthernwaterresources(Brianandothers2004,Sunandothers2008).Thedirectimpactswilldependonhowtheamountandtimingofprecipitationarealteredandhowthisinfluencesbaseflow,stormflow,groundwaterrecharge,andflooding.Long-termU.S.GeologicalSurvey

streamflowdatasuggestthataverageannualstreamflowhasincreasedandthatthisincreasehasbeenlinkedtogreaterprecipitationineasternStatesoverthepast100years(IPCC2007,KarlandKnight1998,LinsandSlack1999);however,fewerthan66percentofallGeneralCirculationModelscanagreeonthedirectionofpredictedprecipitationchange,whetherwetterordrier(IPCC2007).Annualprecipitationwithinayearorfromoneyeartothenextisanaturalphenomenonrelatedtolarge-scaleglobalclimateteleconnections,suchasElNiñoSouthernOscillation,PacificDecadalOscillation,andNorthAtlanticOscillationcycles.ManyregionsoftheUnitedStateshaveexperiencedanincreasedfrequencyofprecipitationextremesoverthelast50years(Easterlingandothers2000,Huntington,2006,IPCC2007).AstheclimatewarmsinmostGeneralCirculationModels,thefrequencyofextremeprecipitationeventsincreasesacrosstheglobe(O’GormanandSchneider2009);however,thetimingandspatialdistributionofextremeeventsareamongthemostuncertainaspectsoffutureclimatescenarios(AllenandIngram2002,KarlandKnight1998).Despitethisuncertainty,recentexperiencewithdroughtsandlowflowsinmanyareasoftheUnitedStatesindicatethatevensmallchangesindroughtseverityandfrequencywillhaveamajorimpactonsociety,amongthemareductionindrinkingwatersupplies(Easterlingandothers2000,LuceandHolden2009).

Theindirectimpactsofclimatechangearerelatedtochangesintemperatureandatmosphericcarbondioxide.Intheshortterm,highertemperatureshavethepotentialtoincreaseevaporationandplantwateruseviatranspiration(astemperaturesincrease,theenergyavailableforevapotranspirationincreases),andthereforedecreaseexcessprecipitationavailableforstreamfloworgroundwaterrecharge.Warmertemperatureswillalsoinfluencethedurationandtimingofsnowmelt,acriticalfactorinecosystemswheresnowmeltdominateshydrologicprocesses.Theimpactsoftemperaturemaybeoffset(orexacerbated)bychangesinotherfactorsthatinfluenceevapotranspirationsuchasvaporpressure(warmairholdsmorewater),windpatterns(whichimpactboundarylayerresistance),increasesincarbondioxide(whichdecreasestomatalconductance),andchangesinnetradiation(influencedbychangesincloudcoverandaerosols).Inthelongerterm,awarmerclimateincombinationwithchangesinprecipitationwilllikelyshiftdistributionsoftreespecies,whichdifferconsiderablyintheamountofannualandseasonalwatertheyuseviatranspirationandinterception(Fordandothers2011,Sunandothers2011).Forexample,insomegeographicareas,ashiftfromhardwoodtopineforestsmayresultinyear-roundtranspirationandinterceptionandgreaterwateruse.Controlledstudieshavedemonstratedthatincreasedatmosphericcarbondioxidereducestranspirationinmanytreespecies,whichmaytranslateintoincreasedstreamflow

327chAPTeR 13. Forests and Water

Onaverage,waterstressduetothecombinedeffectsofpopulationandlandusechangewillincreaseintheSouthby10percent.

6.AllclimatechangescenariospredictedthattheSouthwouldlikelyseeincreasesinairtemperatureinthenext50yearsbutdifferedinpredictionsofprecipitationchangeacrosstheregion.ThecombinedeffectsofchangingtemperatureandprecipitationwillgenerallydecreasestreamflowacrosstheSouth(fig.13.13).Inaddition,streamflowwilllikelybecomemorevariablewithlowerflowsduringdroughtperiodsandhigherflowsduringwetperiodsthanexperiencedinthepast.

7.Watersupplystresswouldlikelyincreasesignificantlyunderallfourclimatechangescenarios(fig.13.14),largelycausedbyincreasesinwaterlossbyevapotranspirationresultingfromhigherairtemperatures,andalsobecauseofdecreasingprecipitationinsomeareas.Theeffectsofchangingclimateonwaterstresswillvarysignificantlyacrosstheregion(fig.13.15).Forexample,theWaSSImodelprojectsthatFrankfort,KYwillhavenegligiblechangeinwaterstressacrossthefourfutureclimatescenarios,whileOklahomaCity,OK,LittleRock,AR,andAustin,TXareprojectedtohavesignificantincreasesinwaterstress.

implications of Sea level change on coastal Areas

Sea-levelmayrisefrom0.4to2.0mbytheendofthe21stcentury(table13.5)(McMullenandJabbour2009,Rahmsorf2007,Solomonandothers2009).AlongtheAtlanticCoastinthestudyregionthereisapproximately7,297squaremiles(~4.6millionacres)ofcoastallandbelowanelevationof1.5meters(NorthCarolinaandFloridahavethemostcoastalareabelow1.5m),withanadditional5,573squaremiles(~3.5millionacres)ofcoastallandbetween1.5and3.5m.AlongtheGulfCoastthereisapproximately13,605squaremiles(~8.7millionacres)oflandbelowanelevationof1.5m(LouisianaandTexashavethemostcoastalareabelow1.5m),withanadditional6,430squaremiles(~4.1millionacres)ofcoastallandbetween1.5and3.5m(fig.13.16).Ifsealevelrose1.5mweestimatethat2,633squaremiles(~1.6millionacres)offorestscouldbeaffectedalongtheAtlanticCoast,and3,352squaremiles(~2.1millionacres)offorestscouldbeimpactedalongtheGulfCoast.Whenphysicalprocessesareconsideredbythecoastalvulnerabilityindex,alongtheAtlanticCoastNorthCarolinaandVirginiahavethemostcoastlineintheveryhigh-riskclass,andalongtheGulfCoast,LouisianaandTexashavethemostcoastlineintheveryhigh-riskclass(fig.13.17).

Projectionsofsealevelchangescanhelpmanagersidentifyportionsofthecoastlinethatcouldbemonitoredmoreclosely.Forexample,figure13.18showsthattheentireLouisianacoastlineisinthehighriskcategorywithcoastalareabelow1.5m,buttheGulfcoastportionofsouthernFloridaisranked

inthemoderateriskcategoryeventhoughitscoastalareaisalsobelow1.5m,suggestingthatitsresponsetoarisingseamaybeslowerthanifpredictedfromelevationalone(ThielerandHammar-Klose2000).Figure13.19showsthatportionsoftheNorthCarolinacoastlineandtheAtlanticcoastofFloridaareinthehighriskcategory,butbecausethosecoastalareasarebetween1.5and3.5m,asea-levelriseof1mmaynotaffectthosehigherelevationareas.

DiScuSSioN AND coNcluSioNS

Forestconversiontoagricultureorurbanlandusesconsistentlycausesincreasesindischarge,peakflow,andvelocityofstreams.Differencesinthenatureofhydrologicresponsestourbanizationamongsubregionsaresubstantial.Asexamples,thepronouncedeffectofurbandevelopmentonpeakflowsandstreamhydrographsfoundintheAppalachiansandPiedmontmaybeobscuredbynaturalprecipitationregimesinaridregions,suchaswesternTexaswherehydrographsfromlessdisturbedstreamsresemblethoseofurbanstreams(Grimmandothers2004).Similarly,thereductionsinbaseflowthatareoftenobservedfollowingincreasesinimperviousareainthePiedmontmaynotoccurintheflatterterrainoftheCoastalPlain.

Forestconversionsalsoresultinincreasesinsediment,waterchemistryindices,fecalcoliformandE.coli,andothersubstances.Becausedischargeandconcentrationsincreaseafterurbanization,loadsaregenerallyhigher.Physiographiccharacteristicssuchasslopeandsoiltexturestronglyinfluencehydrologyandsedimentexport,buttheirimpactonwaterchemistryislessthantheimpactofurbanization.ConversionofforestlandtourbanusesmayresultinhealthrisksforhumansasevidencedbylargeincreasesinfecalcoliformandE.coli,heavymetals,pharmaceuticals,andothersubstancesinstreamwater.Whileeffectivewatertreatmentmayovercomethisrisktodrinkingwater,thereremainssignificantpotentialfordirectcontactwithpollutedwaterasstreamsflowthroughresidentialareaspriortotreatment.

Eachriverbasinhasauniquelandusehistorythatmayhavelong-lastingeffects.Cuffneyandothers(2010)reportedthattheconversionofforesttourbanlandhadmorepronouncedeffectsonbenthicmacroinvertebratesinAtlanta;Birmingham,AL;andRaleigh,NCthantheconversionofagriculturetourbanlandhadinDallas,wherenaturalgrasslandhadalreadybeendegradedbyagricultureintherecentpast(anexampleantecedentlanduseimpactstakingprecedentoverhistoricallanduse).Infact,theirstudyfoundthatantecedentagriculturallandusemaskedtheeffectsofurbanizationinareasofhistoricforestuseaswell.

Physiographiccharacteristicscoulddeterminethethreshold,ortheresiliencetochange,thateachsubregiondisplaysin

333chAPTeR 13. Forests and Water

responsetochangesinlanduse.Forthisreason,McMahonandHarned(1998)recommendedincorporatingmeasuresofbothnaturalphysiographicvariationandeffectsofhumanactivityinwatershedstudiesandmanagementplans.Additionally,therehavebeensomeindicationsthatimpervioussurfaceincreasesmayhavehigherthresholdsforsignificantwaterdegradationintheCoastalPlainthanthePiedmontorSouthernAppalachians(Helmsandothers2009,MorganandCushman2005,Royandothers2003,Stednick1996,Utzandothers2009);butthisdoesnotappeartobetrueforallmeasures(Nagyandothers2012).

Theconceptofthresholdsofimperviousnessbeyondwhichsignificantdegradationofwaterqualityoccursisvagueandhadpreviouslybeenreportedat10-20percent(ArnoldandGibbons1996,BledsoeandWatson2001).However,somereportshavenotedsignificantchangesinwaterqualityatevenlowerlevelsofdevelopment,suchas<5percentimpervioussurface(Crim2007,Cuffneyandothers2010,Nagyandothers2012).Forinstance,at5percentimpervioussurface,Cuffneyandothers(2010)estimateda13to23percentdegradationofmacroinvertebrateassemblagescomparedtobackgroundconditions.Thissuggeststhatcaremustbetakenfromthefirststagesofdevelopmenttolimitimpactsonwaterresources.BoggsandSun(2011)suggestthatmaintaininghighETofvegetationinthegrowingseasoniskeytoreducingstormflowinurbanwatersheds.Furthermore,onceimpervioussurfacecoverexceeds30percent,deteriorationofwaterqualitybecomessevere(Calhounandothers2003,PaulandMeyer2001).Inareaswheredevelopmentisplannedorabouttobegin,itwouldbeveryusefultoidentifykeybioindicatorsfordetectingtheonsetofsignificantdegradation.

Amongthemostdramaticimpactsassociatedwithforestconversiontourbanoragriculturearechangesinaquaticpopulations.Thehighervelocityandchannelscouringassociatedwithurbanhydrologycreatesunstablehabitat,andthisiscompoundedbytheeffectsofdegradedwaterquality.Speciesrichnessandabundancegenerallydecline;andsomegroups,suchasmussels,maybeeliminatedfromparticularlocations.TheseimpactstendtobemostsevereintheAppalachians.Also,speciesthataretolerantofthealteredconditionsmayreplacethosethatareintolerant.AnexamplewasobservedintheGeorgiaPiedmontasreptilespeciesrichnessincreasedafterurbanization,whileamphibianrichnessdecreased(BarrettandGuyer2008).

Increasedintensificationofforestmanagementonasmallerlandbasecouldhaveimpactsonquantityandqualityofwater,especiallyatlocalscales.Ingeneral,anincreaseinpineplantationsorfastgrowinghardwoodspeciesmayresultingreaterwateruseviatranspiration(Fordandothers2011);however,themagnitudeandsignificanceofgreatertranspirationonwaterresourceswilldependonthe

communitytypethatisbeingreplacedandonsitespecifichydrologicprocesses.Inaddition,theimpactofgreaterwaterusemaybeoffsetbyanetreductionofforestcover.Increasedintensificationofforestmanagementactivitiesthatcreatemoresevereorfrequentsoildisturbance—suchassitepreparation,increasedharvestfrequencies,alargerroadnetwork,ormoretraffic—mayresultinincreasedsedimentandreducedwaterqualityifBestManagementPracticesarebypassed.

BasedonWaSSImodelresultsunderthefourfutureclimatescenariosconsideredinthischapter,streamflowsandwatersupplywillgenerallydecreaseandbecomemorevariableoverthenext50to100years.However,magnitudesandeventhesignsofchangesinstreamflowsresultingfromclimatechangewillvaryconsiderablyacrosstheregion,withsomesmallareas,suchaswesternTexas,experiencingincreasesinwatersupply.Otherareaswilllikelyexperiencedecreasesinsupply,particularlyinFlorida,Oklahoma,andnorthernTexas.Overall,climate-induceddecreasesinwatersupplyandincreaseddemandfromagrowinghumanpopulationwilllikelyresultinanincreaseinwatersupplystressintothenextcentury.

Considerablevariabilityofwaterresourcepredictionsamongthefutureclimatescenariosandtheabsenceofoverlappingpredictionsforanyparticularsubregionconfoundthecertaintyoffutureprojections.Despitetheseuncertainties,theimportanceofwaterresourcesforhumanandaquaticlifearguesforfurtherresearchandactivemanagement.

Ourprojectionsindicateagreaterriskofsea-levelriseformanycoastalareasinthiscentury.Thermalinertiadictatesthatoncethewatersrise,curbsinfuturegreenhousegasemissionswillnotproduceaquickreversal.Therefore,unlikeprecipitationdrivenfloodevents,floodingduetosea-levelrisewillhavelong-termconsequences.Coastalinundationisoneofthemostvisibleimpactsofrisingsealevels.Areasthatwereoncedryfurtherinlandwillgraduallyshifttoepisodicallyinundated(duringhightidesandstorms)andthentopermanentlyinundated.Theimpactofsealevelrisetothepointofinundationisobvious,butotherimpactsmaybelessvisible,suchasthesaltwatermarshesthatexemplifyanecosysteminbalancebetweenfreshwaterandsalineenvironments.Theseuniqueplacesprovideimportantbreedinghabitatformanyterrestrialandaquaticanimalspecies.However,rapidlyrisingsealevelswillpermeatenon-salineforestsandgrasslands,causinglossesofexistingvegetationwithoutthepossibilityofreplacementbymoresalttolerantspecies.Oncetheexistingvegetationisdead,therootstructurethatbindsthesoilsystemtogetherandprovidesabufferfromincomingtideswillalsobelost,andcoastalerosionislikelytoaccelerate.Althoughcoastalerosionisanaturallyoccurringprocessinbarrierislandsandmanyotherareas,theincreaseinrateandseverity

334The Southern Forest Futures Project

thatislikelywithrisingsealevelscouldresultinagreatlyacceleratedlossofvaluablecoastalproperty.

Finally,acombinationofpressureonwaterresourcesfromincreasinghumanpopulationsandrisingsealevelscouldseverelyreducefreshwatersuppliesalongcoastalareas.Asfreshwaterisdrawnoutofshallowgroundwatersystems,adjacentbrackishwaterwouldlikelyfillthevoid,thusraisingtheriskofsaltwatercontaminationtodrinkingwatersupplies.Risesinsealevelwillfurtherincreasetheriskofcontaminationassalinewaterlevelsrise.ThelossofgroundwatersuppliesinplaceslikeFloridawouldhaveenormoussocialandeconomicimplicationsandmaybemoresignificantthancoastalinundationintheneartomediumfuture.

kNoWleDGe AND iNFoRmATioN GAPS

Paststudiesonforest-waterrelationsthathavebeenconductedprimarilyinforestedwatershedsarenotsufficienttoaddressissuesinmorecomplex,humandominatedlandscapes.AkeyissueistherelationshipbetweenincreasingurbanizationanddiminishingavailablewatersupplyforhumansintheSouth.Weneedtounderstandmoreaboutthenatureofthisrelationshipandhowitmaychangeacrossthearrayofsouthernphysiographicfeatures.Complexityincreaseswiththeinteractiveeffectsofmultipledriversincludinglandusechange,climatechange,populationgrowth,andthenaturalvariabilityinthehydrologiccycle.Abetterunderstandingiscriticallyneededinadvanceofthenextmajordrought,whoseimpactsmaybeexacerbatedbyexpectedincreasesinhumanpopulationsandimpervioussurfacesinmanyareasoftheSouth(chapter4).

Theramificationsofurbanizationonsurfacewaterandsubsequently,humanhealthisanothertopicthatdeservesgreaterattention.VeryhighcountsoffecalcoliformandE.colihavebeendocumentedinurbanstreams,butthepotentialriskstohumanhealthhavenotyetbeenassessed.Researchisneededoncoastalareas,whichhavenotbeenadequatelystudiedandareexpectedtoundergohighpopulationgrowthanddevelopmentratesincomingyears.Also,thesensitivitiesofaquaticorganismstourbanizationhavebeendemonstratedbutnotquantified,andshouldbemorefullyunderstoodsothattheycanserveasbioindicatorsofimpendingdegradationtosurfacewaterresources.Thefocusofthischapterwasonsurfacewaterimpactsassociatedwithlanduseconversion,butliteraturesearchesproducedlittleinformationontherelationshipsofgroundwatertolanduseandlandcover.Becausemanysoutherncommunitiesareconsideringexpandeduseofaquifers,believingthemtobe“droughtproof,”theywillneedtounderstandtheextenttowhichchangesinlandusemightaffectgroundwaterresources.

TheWaSSImodelprovidesageneralsummaryofwatersupplyanddemanddynamicsacrosslargeregionsoverextendedperiodsoftime,requiringassimilationandintegrationoflargedatasetsandtheuseofextensiveGISandcomputingresources.Theselargedatarequirementsnecessitateddevelopmentofsimplifyingassumptionstosimulatewaterresourcechangesinresponsetoclimatechange.Forexample,theWaSSImodelusedforthischapterdoesnotincludeprovisionsforwatersupplyreservoirstorageorinterbasinwatertransfers;italsoassumesthatallin-streamsurfacewaterisavailableforhumanuse(noecologicalflowisreserved)andthatriverflowsareroutedthroughtherivernetworkinstantaneouslyduringagivenmonth.Itisimportanttokeepinmindthattheseassumptionsmayimpactwatersupplystresspredictionsforsomeareasacrosstheregion.Futurelandusechangesarelikelytoaffectwaterqualityandextremehydrologysuchaspeakflowrate,issuesthatarenotaddressedyetbytheWaSSImodel.Thetradeoffsbetweenwaterresourcesandcarbonsequestrationarenotwellunderstoodandneedtobequantifiedbeforeembarkingonbioenergydevelopmentandforestmanagementtomitigateclimatewarming.

Comparedtothephysicsofoceanicthermalexpansion,relativelylittleisknownabouttherateofglobalwarming,thechangesinoceansurfacealbedo,ortheinputofwaterfromsnowandicemeltsonland.Manyoftheseunknownsarenotafunctionofsciencegaps,butratheruncertaintyaboutfutureincreasesingreenhousegasemissions.Conversely,thephysicsofthermalexpansionarewellunderstood.Aspredictionsofglobalwarmingratesimprove,theaccuracyofsea-levelrisewillalsoimprovesignificantly.Finally,demographicchangesandassociatedpressuresongroundwaterresourcesarealsounknown.Theseknowledgegapsneedtobeaddressedbeforeamorecompleteassessmentofclimatechangeonsea-levelriseispossible.

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