basin characteristics · 2017-11-10 · basin characteristics from hydrological processes at the...
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Basincharacteristics
Fromhydrologicalprocessesatthepointscaletohydrologicalprocessesthroughoutthespacecontinuum:
⤷ pointscaleà riverbasin
Thewatershedcharacteristics(shape,length,topography,…)
– controltheresponseoftheriverbasin⤷ shapeofthehydrograph⤷ soilwaterdynamics
– canbeusedtoidentifysimilarityofresponseamongdifferentbasins⤷ extrapolationandregionalisation
ofmodels
NB
Riverbasin,drainagebasin,watershed,catchmentareusedassynonimous
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Lecturecontent
Quantitativecharacterizationof
– topographicfeatures
– rivernetworkcharacteristics
– soil
– landcover
– interrelationshipsbetweenwatershedfeatures
Skript:Ch.IV.1,IV.1.1,IV.1.3,IV.1.4
Basincharacteristics
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Watersheddefinition
watershed:area oflanddrainingintoarivernetwork convergingtoanoutlet
theareaofthewatershedisdelimitedbythetopographic(=drainage)divideoutlinedonatopographicmap
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Topographicvsphreaticdivide
• theareacontributingtowaterbalancecorrespondstothatdelimitedbythetopographic(=drainage)divide
• theareacontributingtowaterbalanceDOESNOTcorrespondtothatdelimitedbythetopographic(=drainage)divideêClosingthewaterbalancerequirestheknowledgeofthephreaticdivide
TOPOGRAPHIC DIVIDE ⋍ PHREATIC DIVIDE
TOPOGRAPHIC DIVIDE
PHREATIC DIVIDE
GROUNDWATER FLOW IMPERVIOUSLAYER
IMPERVIOUS LAYER4Hydrology– Basincharacteristics– AutumnSemester2017
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Hypsometriccurveelevationrange[m]
ΔA[km2]
Atot[km2]
1685– 1500 3.4 3.4
1500– 1250 4.9 8.3
1250– 1000 7.6 15.9
1000– 750 4.1 20.0
750– 500 2.8 22.8
500– 443 1.3 24.1
2200
1900
1600
1300
1000
700
4002 40 6 8 10 12 14 16 18 20 22 24
H [masl]
A [km2]
1685masl
1500masl
1250masl
1000masl750masl
500masl
• cumulativefrequencydistributionofareaswithelevation⤷ watershedsteepness
• use⤷ watershedsimilarity⤷ hydropowerpotential (identifies
largecontributingareaswithhighelevationdifference)
H =hiAii∑Atot
= 1108 m
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Watershedaverageslope(Alvard-Horton’s)
di = distancebetweentwosubsequentcontourlines
li = lengthoftheaverageelevationlinebetweentwosubsequentcontourlines
• localslope si = Δhi /di vsbasinslope,
• localslope forapproximatelyrectangularareasbetweentwosubsequentcontourlinesofdifferenceh:
⤷ whereai =areabetweentwocontourlines
• basinslope forregularlyspacedelevationranges(contourlinesdifference=h):
⤷
whereL =totallengthofaverageelevationlinesA =totalarea
s
si =hdi
= hlidili
= hliai
si =hl1a1a1A+ hl2a2
a2A+ ...+ hln
ananA
= hAl1 + l2 + ...+ ln( ) = hL
A
NB
thedegreeofaccuracydependsonthecontourlinespacingandontheaccuracyoftherectangularareasapproximation
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Watershedaverageslope(gridmethod)
NB
theaccuracyofthemethoddependsonthenumberN oflocalslopeestimates
• localslope
si = Δhi /di
with Δh =contourlinesintervaldi =minimumdistancebetweentwo
contourlinesindifferentgridboxes
• basinslope
where N =numberofgridnodeswherelocalslopessi arecomputed
si =sii∑
N
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• modernsurveyingtechniques(airbornelaseraltimetry,satelliteradaraltimetry,…)makeavailabledigitalterrainmodels(DTMs)
• mostcommonlyDTMsareavailableinrasterformatà gridofsquareswithelevationsassociatetoeachnode
Watershedcharacteristics(automaticmethods)
!
ShadedreliefrepresentationoftheDTMoftheMaggia
Riverbasin(gridsize25×25m)
DTMscanbemanipulatedbymeansofGeographicalInformationSystems(GIS) toextractbasincharacteristics:
⤷ watersheddelineation
⤷ localandbasinslope
⤷ digitalrivernetwork– flowdirection– flowaccumulation(drainagearea)
⤷ riverprofile
⤷ aspect
⤷ …à
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ExampleofDRNextractedbyGISfromaDTM
DigitalRiverNetwork(DRN)
!
DigitalRiverNetwork
actualdrainagenetwork(“bluelines”)
}
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ExampleofDigitalSoilMap(Maggiariverbasin,Tessin,CH)
Otherwatershedcharacteristics– Soil
soiltype soildepth
!!
• use⤷ delineationof
infiltrationandsoilstoragecharacteristics
⤷ parameterisationofinfiltrationmodels
• problems⤷ accuracyofinformation
⤷ differentclassification/resolutionusedbydifferentcountries
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ExampleofDigitalLandUseMap(Maggiariverbasin,Tessin,CH)
Otherwatershedcharacteristics– Landuse
• use⤷ parameterisationof
models
– infiltration(e.g.SCS-CN)
– evapotranspiration
• problems⤷ representativenessof
mapdependsonrastersize
⤷ differentclassification/resolutionusedbydifferentcountries
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Otherwatershedcharacteristics– DrainageDensity
• itmeasurestheefficiencyofthebasindrainage(i.e.ofhowwellorhowpoorlyawatershedisdrainedbyrivers)
• itdependsuponbothclimate(e.g.rainfallregime)andphysicalcharacteristics(e.g.geology,slope,soil,landcover)ofthedrainagebasin
• forequalclimaticcharacteristicscanbeusedasproxyinformationforpermeability⤷ highDà lowpermeability
⤷ lowDà highpermeability
D = total length of streamsarea
=Li∑ i
A
VERYHIGHà D>100km/km2HIGHà D18÷28km/km2
LOWà D<3km/km2 AVERAGEà D7÷10km/km2
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Interrelationshipsofwatershedcharacteristics
!
!
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