VALIDATIONOFTHESWATMODELONALARGERWERBASINWITHPOINT

JOURNALOFTHEAMERICANWATERRESOURCESASSOCIATIONVOL.37,NO.5AMERICANWATERRESOURCESASSOCIATIONOCTOBER2001VALIDATIONOFTHESWATMODELONALARGERWERBASINWITHPOINTANDNONPOINTSOURCES1C.Santhi,J.G.Arnold,J.R.Williams,W.A.Dugas,R.Srinivasan,andL.M.Hauck2ABSTRACT:TheStateofTexashasinitiatedthedevelopmentofaTotalMaximumDailyLoadprogramintheBosqueRiverWater-shed,wherepointandnonpointsourcesofpollutionareaconcern.SoilWaterAssessmentTool(SWAT)wasvalidatedforflow,sedi-ment,andnutrientsinthewatershedtoevaluatealternativeman-agementscenariosandestimatetheireffectsincontrollingpollution.Thispaperdiscussesthecalibrationandvalidationattwolocations,HicoandValleyMills,alongtheNorthBosqueRiver.Calibrationforflowwasperformedfrom1960through1998.Sedi-mentandnutrientcalibrationwasdonefrom1993through1997atHicoandfrom1996through1997atValleyMills.Modelvalidationwasperformedfor1998.Timeseriesplotsandstatisticalmeasureswereusedtoverifymodelpredictions.Predictedvaluesgenerallymatchedwellwiththeobservedvaluesduringcalibrationandvali-dation(R20.6andNash-SuttcliffeEfficiency0.5,inmostinstances)exceptforsomeunderpredictionofnitrogenduringcali-brationatbothlocationsandsedimentandorganicnutrientsdur-ingvalidationatValleyMills.ThisstudyshowedthatSWATwasabletopredictflow,sediment,andnutrientssuccessfullyandcanbeusedtostudytheeffectsofalternativemanagementscenarios.(KEYTERMS:watershedmanagement;totalmaximumdailyload;erosion;sedimentation;phosphorusloading;dairymanuremanage-ment.)INTRODUCTIONTheUnitedStatesEnvironmentalProtectionAgen-cy(USEPA)reportednutrientenrichmentasthemajorcauseforimpairmentoflakesandotherwaterbodiesintheUnitedStates(USEPA,1994).USEPA'swaterqualityinventoryreportof1996indicatedthat40percentofthesurveyedrivers,lakes,andestuarieswerepollutedrelativetotheirdesignateduses(USEPA,1998).Torestorethequalityofthesewaterbodies,theTotalMaximumDailyLoad(TMDL)pro-cesswasestablishedbysection303(d)oftheCleanWaterAct(CWA).ATMDLquantifiespollutantsourcesandmaximumallowableloadstothecon-tributingpointandnonpointsourcessothatthewaterqualitystandardsareattainedtoprotectdrink-ingwater,aquaticlife,andotherwateruses(USEPA,1998).OncethenecessarypollutantreductionlevelsareidentifiedthroughestablishmentofTMDLs,con-trolmeasuressuchasbestmanagementpractices(BMPs)aretobeimplemented.TheTexasNaturalResourceConservationCom-mission(TNRCC)isimplementingTMDLprojectsinTexasincooperationwiththeTexasStateSoilandWaterandConservationBoard(TSSWCB).Ofthe386classifiedwaterbodies,147havebeenlistedonthe1998303(d)list.OneoftheseprojectsisintheBosqueRiverWatershedinNorthCentralTexas,wherephosphorusfromconfinedanimal-feedingoperationsisaconcern(TNRCC,1999).Runofffromcroplandandurbanareas(nonpointsources)andeffluentfromwastewatertreatmentplants(WWTPs)(pointsources)areotherpollutionsourcesinthiswatershed.WaterqualitysimulationmodelscanassistwithTMDLdevelopmentthroughsimulatingloadstoreceivingwaterbodiesundervariousBMPs.Modelsincombinationwithobservationaldatafromhistori-calandcurrentmonitoringprogramswillprovidetheinformationforTMDLwaste/loadallocationsandimplementationstrategies.Inthisstudy,thehydro-logic/waterqualitymodel,SoilWaterAssessmentTool1PaperNo.00044oftheJournaloftheAmericanWaterResourcesAssociation.DiscussionsareopenuntilJune1,2002.2Respectively,PostDoctoralResearchScientist,BlacklandResearchCenter,720EastBlacklandRoad,Temple,Texas76502-9622;HydraulicEngineer,USDA/ARS,808EastBlacklandRoad,Temple,Texas76502;ResearchScientist,BlacklandResearchCenter,720EastBlacklandRoad,Temple,Texas76502-9622;ProfessorandResidentDirector,BlacklandResearchCenter,720EastBlacklandRoad,Temple,Texas76502-9622;AssociateProfessorandDirectorofSpatialSciencesLab,BlacklandResearchCenter,700EastUniversityDrive,Suite104,CollegeStation,Texas77840;andAssistantDirector,TexasInstituteforAppliedEnvironmentalResearch,P.O.BoxT-0410,Stephenville,Texas76402(E-MaillSanthi:[e-mail]).JOURNALOFTHEAMERICANWATERRESOURCESASSOCIATION1169JAWRASanthi,Arnold,Williams,Dugas,Srinivasan,andHauck(SWAT)wasusedtostudytheeffectofdifferentBMPsinreducingthepollution.TheobjectiveofthispaperistodescribethecalibrationandvalidationoftheSWATmodelforflow,sediment,organic,andmineralnutrientsintheBosqueRiverWatershed.ThemodelwasthenusedtostudytheimpactofdifferentBMPsontheBosqueRiverWatershedandidentifysuitableBMPsforachievingwaterqualitytargets(Santhietal.,2000).SWATModelDescriptionSWATisahydrologic/waterqualitymodeldevel-opedbyUnitedStatesDepartmentofAgriculture-AgriculturalResearchService(USDA-ARS)(Arnoldetal.,1998).Itisacontinuoustimemodelthatoper-atesonadailytimestep.Theobjectiveinmodeldevelopmentwastopredicttheimpactofmanage-mentonwater,sediment,andagriculturalchemicalyieldsinlargeungagedbasins.Tosatisfytheobjec-tive,themodel(a)isphysicallybased(calibrationisnotpossibleonungagedbasins);(b)usesreadilyavailableinputs;(c)iscomputationallyefficienttooperateonlargebasinsinareasonabletime;and(d)iscontinuousintimeandcapableofsimulatinglongperiodsforcomputingtheeffectsofmanagementchanges.SWATusesacommandstructureforroutingruno