THE POTENTIAL OF SURFACE RUNOFF MANIFESTATION...
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STUDIA UBB GEOGRAPHIA, LVIII, 2, 2013, pp. 39 ‐ 47 (RECOMMENDED CITATION) THE POTENTIAL OF SURFACE RUNOFF MANIFESTATION OBTAINED ON THE BASIS OF THE DIGITAL ELEVATION MODEL. CASE STUDY: THE SUBCARPATHIAN SECTOR OF BUZĂU CATCHMENT AREA R. COSTACHE 1 , R. PRĂVĂLIE 2 ABSTRACT. The Potential of Surface Runoff Manifestation Obtained on the Basis of the Digital Elevation Model. Case study: the Subcarpathian Sector of Buzău Catchment Area. The analyzed sector of Buzău catchment area is situated in the Curvature Subcarpathians, recognized as one of the areas in Romania with the highest values of potential surface runoff. The digital elevation model is one of the most important environmental variables used in the analysis of surface runoff. Thus, the present study tries to quantify the manifestation potential of surface runoff exclusively on the basis of the digital elevation model, determining seven morphometric and hydrographic indicators derived from it. Under these circumstances, the present work represents a preliminary analysis of the potential of surface runoff of the analyzed area. The final susceptibility of surface runoff depends on other important factors such as lithology, soil texture and land cover. According to the spatialisation of the Surface Runoff Index, obtained by superimposing the seven factors derived from the digital elevation model, areas within the basin with high susceptibility of surface runoff were delineated. They represent the genetic premises of risk hydrological phenomena such as flash‐floods. Keywords: digital elevation model, surface runoff, Buzău catchment area. 1 INTRODUCTION Knowing the pattern of surface runoff is essential because it represents the genetic premises of risk hydrological phenomena such as flash‐floods, events ranked in the category of natural hazards with the highest degree of danger. Globally, it is estimated that flash‐floods, the final product of surface runoff, represent one of the most important natural hazards with extremely adverse consequences on the socio‐ economic elements (Lumbrosso & Gaume, 2012). In the global context, the extreme events of surface runoff are caused mainly by the global climate change context with negative implications on rainfall conditions (IPCC, 2007), and on changes in the land use (Marchi et al., 2010). Regional and local surface runoff depends on both climate peculiarities, but especially on the characteristics of the geographical area (geological, morphometric and hydrographic peculiarities of the landforms, land cover, etc). 1 Faculty of Geography, University of Bucharest, Master Degree in Hydrology and Climatology, email: [email protected] 2 Faculty of Geography, University of Bucharest, Master Degree in Hydrology and Climatology, email: [email protected]
Transcript of THE POTENTIAL OF SURFACE RUNOFF MANIFESTATION...
STUDIAUBBGEOGRAPHIA,LVIII,2,2013,pp.39‐47(RECOMMENDEDCITATION)
THEPOTENTIALOFSURFACERUNOFFMANIFESTATIONOBTAINEDONTHEBASISOFTHEDIGITALELEVATIONMODEL.CASESTUDY:THE
SUBCARPATHIANSECTOROFBUZĂUCATCHMENTAREA
R.COSTACHE1,R.PRĂVĂLIE2ABSTRACT.ThePotentialofSurfaceRunoffManifestationObtainedontheBasisof theDigitalElevationModel. Case study: the Subcarpathian Sector ofBuzăuCatchment Area. The analyzed sector of Buzău catchment area is situated in theCurvature Subcarpathians, recognized as one of the areas in Romaniawith the highestvaluesofpotentialsurfacerunoff.Thedigitalelevationmodelisoneofthemostimportantenvironmentalvariablesusedintheanalysisofsurfacerunoff.Thus,thepresentstudytriesto quantify themanifestation potential of surface runoff exclusively on the basis of thedigital elevation model, determining seven morphometric and hydrographic indicatorsderived from it. Under these circumstances, the presentwork represents a preliminaryanalysisof thepotentialofsurfacerunoffof theanalyzedarea.The finalsusceptibilityofsurfacerunoffdependsonother important factorssuchas lithology,soil textureandland cover. According to the spatialisation of the Surface Runoff Index, obtained bysuperimposingthesevenfactorsderivedfromthedigitalelevationmodel,areaswithinthebasinwithhighsusceptibilityofsurfacerunoffweredelineated.Theyrepresentthegeneticpremisesofriskhydrologicalphenomenasuchasflash‐floods.Keywords:digitalelevationmodel,surfacerunoff,Buzăucatchmentarea.1INTRODUCTIONKnowing thepatternof surface runoff isessentialbecause it represents the
geneticpremisesofriskhydrologicalphenomenasuchasflash‐floods,eventsrankedin the category of natural hazardswith the highest degree of danger. Globally, it isestimated that flash‐floods, the final product of surface runoff, represent one of themost importantnaturalhazardswithextremelyadverseconsequenceson thesocio‐economicelements(Lumbrosso&Gaume,2012).
Intheglobalcontext,theextremeeventsofsurfacerunoffarecausedmainlybytheglobalclimatechangecontextwithnegativeimplicationsonrainfallconditions(IPCC,2007),andonchangesinthelanduse(Marchietal.,2010).Regionalandlocalsurfacerunoffdependsonbothclimatepeculiarities,butespeciallyonthecharacteristicsofthegeographicalarea(geological,morphometricandhydrographicpeculiaritiesofthelandforms,landcover,etc).
1Faculty of Geography, University of Bucharest,Master Degree in Hydrology and Climatology,email:[email protected]
2Faculty of Geography, University of Bucharest,Master Degree in Hydrology and Climatology,email:[email protected]
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Althoughthereareseveralscientificmethodologiesforestimatingthepatternofsurfacerunoffandthustheassessmentofflash‐floodrisk,theirrigorousforecastingisoneofthemostdifficulttasksinoperationalhydrology(Javelleetal.,2010).Thisisduetothelargenumberofenvironmentalvariablesneededtobetakenintoaccountbothintermsofstaticanddynamicpointofview.Mostmethodologiesarebasedonthe analysis of two fundamental components responsible for the surface runoff:precipitationandgeographicalbedrockwithitspeculiaritiesmentionedabove.
One of the fundamental components of geographical bedrockwith amajorrole in shaping the surface runoff on slopes is the digital elevation model (Matei,2012).ThispaperaimstoanalyzeintermsofstaticthepotentialofsurfacerunoffintheanalyzedSubcarpathianarea,takingintoaccountthemostimportantgeographicalfactors,derivedsolelyfromthedigitalelevationmodel.
2GEOGRAPHICALFEATURESINTERMSOFDIGITALELEVATIONMODEL
ANDTHEIMPACTONTHEREGIMEOFSURFACERUNOFFThe Subcarpathian sector ofBuzău catchment area is located in the central
South‐EasternRomania,andisspreadoutentirelyovertheCurvatureSubcarpathianunit(Fig.1).Ithasanareaofabout1600km2,representingathirdoftheentiredrainagebasin of Buzău River whose area amounts to approximately 5000 km2. The entirecatchmentareaofBuzăuformsanintegralpartofSiretcatchmentarea,asitisafirstorder tributary. Altitudes of the study area range from 114 m, recorded at theentrancetotheplainofBuzăuRiver,and950mintheNorth‐East,inBisocaHills.
Terrain slope, a very important indicator in defining the potential of waterdrainage on slopes, falls between 0.06° and 29.7°, while its average value is 8.9°. Thehighestvaluesarerecordedgenerallyatthecontactwiththemountainswhilelowvaluesare registered in Cislău, Pătârlagele, and Pârscov basins. Slopes over 15° are mostfavorableforsurfacerunoff(Bilașco,2008)andtheycoverabout11%ofthestudyarea.
In addition to slope gradient, an essential role in surface runoff analysis isplayedbyLSfactorratio,anindicatorthatquantifiestherelationshipbetweenslopeand slope length. If the two sides have the same slope but different lengths, theirbehaviorwillbedifferentinhydrologicalmodelingprocesses(Constantinescu,2006).ThesurfacerunoffhashighervaluesastheLSfactorratiovalueishigher.Studyareaindexvaluesrangefrom0inmeadowareasanddepressionsandreachvaluesofover6onslopesofthemainhillyunits.
Also,areascharacterizedbysurfacerunoffareoutstandingandwithprofilecurvaturevalues.Convexareaswithsurfacerunoffrepresent42%ofthetotal,whilethe55%ofsurfaceleakageoccurslessaccelerated.Theremaining3%oftheareasofstudyarethestableareas.
Also,highvaluesofdrainagedensity(over4km/km2)recordedalongthemainvalleys of the study area, Buzău, Slănic, Sărățel, near their confluencewith tributaries,represent an important factor in delineating areas with high susceptibility in theoccurrenceofhydrologicalrisks.Adensehydrographicalnetworkmakeshighwaterandflash‐floodsregimetobecomeatorrentialtype(Bilașco,2008).Besidesdrainagedensity,animportantroleinthemanifestationofsurfaceflowhastheconvergenceofthedrainagenetwork,whichhashighervaluesinthenorthernhalfofthecatchmentarea.
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Fig.1.LocationoftheSubcarpathiansectorofBuzăucatchmentareainRomania
Slope aspect is an important factorwith direct influences on the flow regime.
Generally, slopeswith southern exposure are favorable for increasing upward thermalconvection and therefore to the formation of torrential rains especially in summerafternoons(Roșu,1980).IntheSubcarpathiansectorofBuzăucatchmentarea,over40%oftheslopeshaveageneralsouthernaspect.Theshapeofsecondarybasinshasalsovitalsignificance in defining the time ofwater concentration in themainstem. Thereby, themorecirculartheshapeofthebasins,thelowerthetimeofwaterconcentration(Pișotaetal.,2010).ThehighestvaluesofthecircularityindexarefoundinthesecondarydrainagebasinsofValeaBoului,Olari,Comisoaia,PecineagaMurătoare.
3METHODOLOGYTohighlightthesurfacerunoffexposedareasintheSubcarpathiansectorof
Buzăucatchmentareaintermsofthedigitalelevationmodel,theSurfaceRunoffIndexwasdefinedandspatialized. Itwasobtained inGISbysummingupseven indicatorsderived from the digital elevation model, playing an essential role in defining themanifestationpotentialofsurfacerunoff:slopeoftheland,LSfactor,profilecurvature,slopeaspect,drainagedensity,convergence indexandtheshapeofsecondarydrainagebasins(fig.2).
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Fig.2.Spatialrepresentationofslopevalues(a),LSFactorindex(b),profilecurvature
(c),slopeaspect(d),drainagedensityvalues(e),convergenceindex(f)andcatchmentareashapeindex(g)
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Because their influence on the flowprocess is different, each indicatorwasgiven a differentweight in the final index delineation. Thus, factors such as terrainslopeorprofile curvaturehavebeenassignedgreater importance than theshapeofsecondarydrainagebasinsorslopeaspect,influencinglessthesurfacerunoff.
Intheearlystage,toobtainthedigitalelevationmodel,the1:25000topographicalmapwasusedascartographicsupportfromwhichcontourlineswereextracted.ThegeoreferencingofthetopographicalmapscorrespondingtothestudyareawasmadeusingtheGlobalMapper13softwarewiththeRomanianspecificprojectionsystem,Stereo70.
ThedigitalelevationmodelwasobtainedusingArcGIS9.3softwareandcontourlineswereinterpolatedtoacellsizeof20m,recommendedforhydrologicalstudies(Bilașco,2008).Indicatorssuchasslope,profilecurvature,slopeaspectandsecondarybasinswerederivedusingspecificextensiontoolsfromArcGIS9.3.
Thecatchmentareashape factorwasbasedonF/L2 formula,whereF‐basinarea isgiven inkm2,while theL‐basin length inkm.Themore theshape factorhasvaluescloseto1,themorecircularisthebasin,sothatsurfacerunoffwilloccurmoreintensely(Pișotaetal.,2010).
The assessment of drainage densitywas based on the determination of thehydrographicnetworkintheformofrasterbasedondigitalelevationmodelusingFlowAccumulation tool fromArcGIS 9.3. Nextwe calculated the sumof the lengths of riversectorsonaunitofonekm2,thisoperationbeingperformedbymeansofStatisticsBlocktool fromArcGIS9.3.Theother two indicatorsderived fromdigitalelevationmodel,LSFactorandConvergence IndexofhydrographicnetworkwereobtainedusingSAGAGIS2.0.8 software. The determinant of relationship between slope gradient and length isbasedontheformula:LS=(m+1)*(As/22,13)m(sinβ/0.0896)n,wherem=0.4andn=1.3 (Moore etal., 1993). Values close to 0 indicate a low slope combinedwith a smalllengthoftheslope,whilehighvaluesexpressahighslopecombinedwithalargelengthoftheslope,whichincreaseswaterflowrate(Constantinescu,2006).
Afterobtainingthesevenindicators,theywerereclassifiedintofiveclassesofvaluesdependingonhowtheircharacteristicsaffectwaterdrainageonslopes(table1).
Table1Classificationandindexingofenvironmentalfactorstoobtain
thefinalSurfaceRunoffIndex
Environmentalparameters/Percentoftotalfinalindex Type/values
Slope(˚)‐25% 0‐3 3‐7 7‐15 15‐25 >25L‐SFactor‐15% 0‐1.5 1.5‐3 3‐4.5 4.5–6 >6Profilcurvature
(radiani/m)‐25% 0.99‐1.51 0‐0.99 (‐1.8)‐0
Slopeaspect‐5% N.NE NW.E FlatZones SE.W S.SWDrainagedensity(km/km2)‐10%
0‐0.86 0.86‐1.69 1.69‐2.51 2.51‐3.34 3.34‐4.17
Convergenceindex‐10% >0 0‐(‐1) (‐1)‐(‐2) (‐2)–(‐3) (‐3)‐(‐100)Catchmentshapeindex‐10% 0.09 0.18–0.19 0.21–0.3 0.31–0.38 0.42–0.67
Scoregiven 1 2 3 4 5SurfaceRunoffIndex(class)
Verylow17.5‐20.8
Low20.8‐28.4
Medium28.4‐32.2
High32.2‐36
Veryhigh36‐45
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The granting of differentweights to analyzed factors (table 1) according totheir importance on surface runoff and their overlapping inGIS environment is thelaststepinthedeterminationoftheSurfaceRunoffIndexproposedinthispaper.
4RESULTSANDDISCUSSIONSThespatialisationoftheSurfaceRunoffIndexintheSubcarpathiansectorof
Buzăucatchment areawasobtainedbyoverlapping the analyzed factors. Its values,ranging between17.5 and 45, have been grouped into five classes according to thestandarddeviation:verylow17.5‐20.8,low20.8‐28.4,medium28.4‐32.2,high32.2–36andveryhigh36‐45(fig.3).
Fig.3.SpatialrepresentationofSurfaceRunoffIndexvaluesinBuzăucatchmentarea
The Subcarpathian sector of Buzău catchment area has high and very high
valuesofSurfaceRunoffIndex,over32.2onapproximately32%(approximately500km2)ofitstotalarea.Theseoccurespeciallyinareasofsteepslope,over15°,whereconvergenceof torrential organisms ishigher, in convex areas of the landscape, generally slopeswithsouthernexposure.Mostsuchareasoccurinhillyunits(BisocaHills,CornețuHills,PriporHills,DâlmahillsandBocuHills)nearthemountains,especiallyatthecontact
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withthem.Theseareas,wherethereisnoforestvegetationcover,areexposedtotheoccurrenceofhydrologicalriskphenomenasuchasflash‐floodsandotherscausedbysurfacerunofflikeintenseerosionofsoilandlandslides.
Thelargestshareinthestudyareacorrespondstoaveragevalues(28.4‐32.2)ofthe Surface Runoff Index, which is present in 39% of the area. These values arecharacteristic for the slopeswith an average gradient between 7° and 15° situatedgenerallyinhillyouterunits.
AreaswithlowandverylowvaluesofSurfaceRunoffIndexappearon29%oftheSubcarpathiansectorofBuzăucatchmentarea.Generally,lowmanifestationpotentialofsurfacerunoffoccursonlandswithaslopebelow3°,depressionslocatedinbasinswithanelongatedshape.ThesecorrespondtoCislău,Pătârlagele,NișcovandPârscovbasins.
AnanalysisofSurfaceRunoffIndexvaluesonthedrainagebasinsofthetributariesofBuzăuRiverintheSubcarpathiansector,clearlyhighlightsthatbasinswithashapeindexcloseto1,placedneartheCurvatureCarpathians,arethemostexposedtohighmanifestationpotentialof surface runoff.Weights close to60%ofhighandveryhighvaluesofSurfaceRunoffIndexoccurinthebasinsofSibiciu,Plăvăț,Pănătau,RușăvățandJghiabrivers(fig.4).Generally,thesebasinscoverasmallarea.Highvaluesoftheweightsofthetwocriticalclassesofover40%occurinbasinslikeBălăneasa,BâscaChiojduluiandSărățel(fig.4).
Fig.4.SpatialrepresentationofSurfaceRunoffIndexvaluesinthesecondarydrainagebasinsofBuzăucatchmentarea
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TheSlănicdrainagebasinpresentsoverallweightsof36%ofhighandvery
highvaluesofthepotentialofsurfacerunoff.Thislowerweightthantheonesabovementioned isdue to themore extensive areaof Slănicbasin, butmostof theuppersectorshowsveryhighvaluesof theSurfaceRunoff Index,particularly inPecineagatributarybasin.Forthisreason,flash‐floodsmayformespeciallyinthenorthernpartofthebasin.Fromthere,theyspreadtothelowersectorgeneratingfloods.
Thelessexposedbasinstosurfacerunoff,intermsofindicatorsderivedfromthedigitalelevationmodel,arethosewithanelongatedshape,generallyatthecontactwiththeplain,suchasNișcov,Blăjani,ValeaLargă,ValeaAdâncăandValeaBuzăului.
However, itshouldbenotedthat thisassessmentofsurfacerunoffvalues isrelative to the Buzău catchment area as a whole and to the delineated secondarybasins.Generally,besides thedigitalelevationmodel, thepotentialof surface runoffdependsonother important factorssuchas lithology,soil textureand landcoversothatthefinalresultsdependtoalargeextentonthesefactors.
5CONCLUSIONSTheanalysisofthedigitalelevationmodel,usingindicatorsderivedfromit,isone
of the major components necessary in evaluating the potential of the surface runoff.Becauseitisanenvironmentalcomponentthatisconstantovertime,unliketheclimaticfactorwhichischaracterizedbyhighdynamics,thedigitalelevationmodelisveryusefulinanalyzingthepotentialofthesurfacerunofffromastaticpointofview.
Intheseconditions,thedigitalelevationmodelcanprovideimportantinformationin the case ofmodificationof relatively unchanging environmental components likelandcoverdata.Inotherwords,ifSurfaceRunoffIndexshowshighvaluesespeciallyinsecondarybasinsofthenorthernhalfofBuzăucatchmentarea, it is imperativetomaintain thereahighweightof forests. In theoppositeevent, continued deforestation(specifictothisCarpathianareainthelastcentury)willbeinthefutureanimportantcause insurface flowaccelerationand in increased frequencyof flash‐floods, theworstaffectedareasbeingmostlythosedefinedonthebasisofthedigitalelevationmodel.
ACKNOWLEDGMENTSThispaperhasbeenfinancedthroughthecontractPOSDRU/86/1.2/S/57462,
strategic project “Education and professional training in support of the economicgrowthandthedevelopmentoftheknowledge‐basedsociety”,“Qualityinthehighereducation”,co‐financedbytheEuropeanSocialFund,throughtheSectoralOperationalProgrammefortheHumanResourcesDevelopment2007‐2013.
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