Freshwater Non- forested Wetlands: Floodplain Marsh By: Mattie J. E. Rush.
Chapter 13. Forests and Water - US Forest ServiceFunctions of Forested Wetlands and Riparian Forests...
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309 chAPTeR 13. Forests and Water Graeme Lockaby, Chelsea Nagy, James M. Vose, Chelcy R. Ford, Ge Sun, Steve McNulty, Pete Caldwell, Erika Cohen, and Jennifer Moore Myers 1 key FiNDiNGS • Forest conversion to agriculture or urban use consistently causes increased discharge, peak flow, and velocity of streams. Subregional differences in hydrologic responses to urbanization are substantial. • Sediment, water chemistry indices, pathogens, and other substances often become more concentrated after forest conversion. If the conversion is to an urban use, the resulting additional increases in discharge and concentrations will produce even higher loads. • Although physiographic characteristics such as slope and soil texture play key roles in hydrologic and sediment responses to land use conversion, land use (rather than physiography) is the primary driver of water chemistry responses. • Conversion of forest land to urban uses may decrease the supply of water available for human consumption and increase potential threats to human health. • Increases in urbanization by 2060 in the Appalachians, Piedmont, and Coastal Plain will increase imperviousness and further reduce hydrologic stability and water quality indices in the headwaters of several major river basins and in small watersheds along the Atlantic Ocean and Gulf of Mexico. 1 Graeme Lockaby is a co-lead author and Associate Dean and Professor, Auburn University School of Forestry and Wildlife Sciences, Auburn, AL 36849. Chelsea Nagy is a Graduate Research Assistant, Brown University, Department of Ecology and Evolutionary Biology, Providence, RI 02912. James M. Vose is a co-lead author and Project Leader, U.S. Department of Agriculture Forest Service, Southern Research Station, Center for Integrated Forest Science, College of Natural Resources, North Carolina State University, Raleigh, NC 27695. Chelcy R. Ford is an Ecologist, U.S. Department of Agriculture Forest Service, Southern Research Station, Coweeta Hydrologic Laboratory, Otto, NC 28763. Ge Sun is a Research Hydrologist, U.S. Department of Agriculture Forest Service, Southern Research Station, Eastern Forest Environmental Threats Assessment Center, Raleigh, NC 27606. Steve McNulty is a Supervisory Ecologist, U.S. Department of Agriculture Forest Service, Southern Research Station, Eastern Forest Environmental Threats Assessment Center, Raleigh, NC 27606. Pete Caldwell is a Research Hydrologist, U.S. Department of Agriculture Forest Service, Southern Research Station, Eastern Forest Environmental Threats Assessment Center, Raleigh, NC 27606. Erika Cohen is a Resource Information Specialist, U.S. Department of Agriculture Forest Service, Southern Research Station, Eastern Forest Environmental Threats Assessment Center, Raleigh, NC 27606. Jennifer Moore Meyers is a Resource Information Specialist, U.S. Department of Agriculture Forest Service, Southern Research Station, Eastern Forest Environmental Threats Assessment Center, Raleigh, NC 27606. • On average, water supply model projections indicate that water stress due to the combined effects of population and land use change will increase in the South by 10 percent by 2050. • Water stress will likely increase significantly by 2050 under all four climate change scenarios, largely because higher temperatures will result in more water loss by evapotranspiration and because of decreased precipitation in some areas. • Approximately 5,000 miles of southern coastline are highly vulnerable to sea level rise. iNTRoDucTioN Compared to all other land uses, southern forests provide the cleanest and most stable water supplies for drinking water, recreation, power generation, aquatic habitat, and groundwater recharge (Brown and others 2008, Jackson and others 2004, Sun and others 2004). Forests are unique among land covers because they are long-lived and relatively stable. However, they are subject to substantial structural and functional alterations by management practices and/ or natural disturbances, the intensity of which determines whether alterations are short or long-term. Water resources in the South are at risk of degradation from a growing population, continued conversion of forests to other land uses, and climate change. Urban and agricultural lands can impair water resources by introducing nutrients, sediment, bacteria, and other pollutants to streams. Additionally, altered hydrology—including higher peak flows, and lower baseflows and hydroperiods (Amatya and others 2006)—is common with forest conversion to other land uses. Together, these changes can modify the habitat and consequently the composition of aquatic (and riparian) communities. Historical land use practices have dramatically changed the landscape of the South. Soil erosion and sedimentation were prevalent throughout the region during the period of agricultural expansion in the 18 th and 19 th centuries. Evidence can still be seen today in the sediment deposits of floodplains. This massive topsoil erosion and depleted soil productivity was followed by a period of agricultural abandonment and reforestation throughout much of the South. Today major land uses include forestry, agriculture, Chapter 13. Forests and Water
Transcript of Chapter 13. Forests and Water - US Forest ServiceFunctions of Forested Wetlands and Riparian Forests...
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
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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( )
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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
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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.
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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;[email protected].
Figure13.5—Streamflow-durationcurvesincubicfeetpersecondbeforeandafterurbanization.(Source:L.Kalin,unpublisheddata;AuburnUniversity,SchoolofForestryandWildlifeSciences,Auburn,AL,36849;[email protected])
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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.
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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).
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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
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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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