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Modelling Macropores. Philipp Kraft. Schwingbach. ICON Project. Approach I. Use a 5-10x higher conductivity Examples : Everywhere , cmf applications until today. Approach II. Van Genuchten retention curve model is based on pore size distribution , assuming a normal distribution

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Modelling macropores

ModellingMacropores

Philipp Kraft




Approach i
Approach I

  • Use a 5-10x higherconductivity

  • Examples: Everywhere, cmfapplicationsuntiltoday


Approach ii
Approach II

  • Van Genuchtenretentioncurvemodelisbased on poresizedistribution, assuming a normal distribution

  • Byoverlayoftwo normal distributionswithdifferingmeanandstdev a closed form retentioncurvefor a bimodaldistributionispossible

  • Example: Durner 1994, implemented in HYDRUS


Approach i ii
Approach I+II

  • Onlyonestorage per numericallayer

  • Water in a numericallayerdoes mix perfectly

  • Macroporeandmicroporewaterhasthe same waterquality

  • Relation ofmacropores do not changewithwatercontent (noswellingeffects)


Approach iii
Approach III

  • Waterinfiltratesthroughmacroporesintodeeperlayers

  • No additional waterstorage, infiltrationhas a by pass aroundthe top soil

  • Example: BROOK 90, cmf.LayerByPass


Approach iii1
Approach III

Surface water

  • l=cell.surfacewater

  • r=cell.layers[0 ..1]

  • cmf.LayerByPass(l,r,Kmax,w0,beta)

Soillayer 1

Soillayer 2

Soillayer 3


Approach iv
Approach IV

  • Distinctmodelsofmacroporespaceandmicroporespace.

  • Resultsgetaveraged

  • Example: someHydrus 1D/2D applications


Approach v
Approach V

  • Twodistinctwaterstorages per layer

  • transportequationsformacropores (nocapillaryeffects)

  • transportequationsformicropores (Richards equation)

  • masstransferequationbetweenmacro- andmicropores

  • Example: MACRO


B a real macropore storage
b) A real Macroporestorage

Surface water

Richards eq.

Macrotransporteq.

Macropore 1

Soillayer 1

Macrotransporteq.

Richards eq.

Macropore 2

Soillayer 2

Richards eq.

Macrotransporteq.

Macropore 3

Soillayer 3

Masstransferequations


Macro pore transport
Macroporetransport

  • Withoutcapillaryrise, kinematicwaveisusable

  • cmf:

  • V – actualstored Volume

  • C – Capacityoflayer


Mass transfer
Masstransfer

  • saturationbased

  • headbased


Saturation based mass transfer
Saturation basedmasstransfer

  • Philip 1968

  • Jarvis 1994


Head based mass transfer
Head basedmasstransfer

  • Gerke & Van Genuchten


Cmf gradientmacromicroexchange for macro micropore exchange
cmf.GradientMacroMicroExchangeforMacro/Microporeexchange

Δx

Ψ(Macro)

Aggregate

Macropore

z

Ψ(Micro)


Examplary model setup
Examplarymodelsetup

  • 10 daysruntime

  • 1 daywith 50mm precipitation

  • 1 m soilcolumn, nogroundwaterpercolation

  • At thebeginning: hydrostaticequilibrium, 1m groundwaterlevel

  • siltysandsoil, 5% macropores, meanmacroporedistance 5cm

  • Non swellingsoil


But for swelling soils
But forswellingsoils?


Modelling macropores

Matrix watercontent


Why not use always dual porosity
Why not usealways dual porosity

  • Big jobforthesolver (anothertimescale, twicethestate variables)

  • Additional parameters (Conductivityofmacropores, macroporefraction, macroporedensity)


Questions to macropores

Questionstomacropores?