Basincharacteristics

Fromhydrologicalprocessesatthepointscaletohydrologicalprocessesthroughoutthespacecontinuum:

⤷ pointscaleà riverbasin

Thewatershedcharacteristics(shape,length,topography,…)

– controltheresponseoftheriverbasin⤷ shapeofthehydrograph⤷ soilwaterdynamics

– canbeusedtoidentifysimilarityofresponseamongdifferentbasins⤷ extrapolationandregionalisation

ofmodels

NB

Riverbasin,drainagebasin,watershed,catchmentareusedassynonimous

Hydrology– Basincharacteristics– AutumnSemester2017 1

Lecturecontent

Quantitativecharacterizationof

– topographicfeatures

– rivernetworkcharacteristics

– soil

– landcover

– interrelationshipsbetweenwatershedfeatures

Skript:Ch.IV.1,IV.1.1,IV.1.3,IV.1.4

Basincharacteristics

2Hydrology– Basincharacteristics– AutumnSemester2017

Watersheddefinition

watershed:area oflanddrainingintoarivernetwork convergingtoanoutlet

theareaofthewatershedisdelimitedbythetopographic(=drainage)divideoutlinedonatopographicmap

3Hydrology– Basincharacteristics– AutumnSemester2017

Topographicvsphreaticdivide

• theareacontributingtowaterbalancecorrespondstothatdelimitedbythetopographic(=drainage)divide

• theareacontributingtowaterbalanceDOESNOTcorrespondtothatdelimitedbythetopographic(=drainage)divideêClosingthewaterbalancerequirestheknowledgeofthephreaticdivide

TOPOGRAPHIC DIVIDE ⋍ PHREATIC DIVIDE

TOPOGRAPHIC DIVIDE

PHREATIC DIVIDE

GROUNDWATER FLOW IMPERVIOUSLAYER

IMPERVIOUS LAYER4Hydrology– Basincharacteristics– AutumnSemester2017

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

5Hydrology– Basincharacteristics– AutumnSemester2017

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

6Hydrology– Basincharacteristics– AutumnSemester2017

Watershedaverageslope(gridmethod)

NB

theaccuracyofthemethoddependsonthenumberN oflocalslopeestimates

• localslope

si = Δhi /di

with Δh =contourlinesintervaldi =minimumdistancebetweentwo

contourlinesindifferentgridboxes

• basinslope

where N =numberofgridnodeswherelocalslopessi arecomputed

si =sii∑

N

7Hydrology– Basincharacteristics– AutumnSemester2017

• 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

8Hydrology– Basincharacteristics– AutumnSemester2017

ExampleofDRNextractedbyGISfromaDTM

DigitalRiverNetwork(DRN)

!

DigitalRiverNetwork

actualdrainagenetwork(“bluelines”)

}

9Hydrology– Basincharacteristics– AutumnSemester2017

ExampleofDigitalSoilMap(Maggiariverbasin,Tessin,CH)

Otherwatershedcharacteristics– Soil

soiltype soildepth

!!

• use⤷ delineationof

infiltrationandsoilstoragecharacteristics

⤷ parameterisationofinfiltrationmodels

• problems⤷ accuracyofinformation

⤷ differentclassification/resolutionusedbydifferentcountries

10Hydrology– Basincharacteristics– AutumnSemester2017

ExampleofDigitalLandUseMap(Maggiariverbasin,Tessin,CH)

Otherwatershedcharacteristics– Landuse

• use⤷ parameterisationof

models

– infiltration(e.g.SCS-CN)

– evapotranspiration

• problems⤷ representativenessof

mapdependsonrastersize

⤷ differentclassification/resolutionusedbydifferentcountries

! 11Hydrology– Basincharacteristics– AutumnSemester2017

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

12Hydrology– Basincharacteristics– AutumnSemester2017

Interrelationshipsofwatershedcharacteristics

!

!

13Hydrology– Basincharacteristics– AutumnSemester2017