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# Modelling Macropores - PowerPoint PPT Presentation

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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### ModellingMacropores

Philipp Kraft

• Use a 5-10x higherconductivity

• Examples: Everywhere, cmfapplicationsuntiltoday

• Van Genuchtenretentioncurvemodelisbased on poresizedistribution, assuming a normal distribution

• Byoverlayoftwo normal distributionswithdifferingmeanandstdev a closed form retentioncurvefor a bimodaldistributionispossible

• Example: Durner 1994, implemented in HYDRUS

• Onlyonestorage per numericallayer

• Water in a numericallayerdoes mix perfectly

• Macroporeandmicroporewaterhasthe same waterquality

• Relation ofmacropores do not changewithwatercontent (noswellingeffects)

• Waterinfiltratesthroughmacroporesintodeeperlayers

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

• Example: BROOK 90, cmf.LayerByPass

Surface water

• l=cell.surfacewater

• r=cell.layers[0 ..1]

• cmf.LayerByPass(l,r,Kmax,w0,beta)

Soillayer 1

Soillayer 2

Soillayer 3

• Distinctmodelsofmacroporespaceandmicroporespace.

• Resultsgetaveraged

• Example: someHydrus 1D/2D applications

• Twodistinctwaterstorages per layer

• transportequationsformacropores (nocapillaryeffects)

• transportequationsformicropores (Richards equation)

• masstransferequationbetweenmacro- andmicropores

• Example: MACRO

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

Macroporetransport

• Withoutcapillaryrise, kinematicwaveisusable

• cmf:

• V – actualstored Volume

• C – Capacityoflayer

Masstransfer

• saturationbased

Saturation basedmasstransfer

• Philip 1968

• Jarvis 1994

• Gerke & Van Genuchten

Δx

Ψ(Macro)

Aggregate

Macropore

z

Ψ(Micro)

Examplarymodelsetup

• 10 daysruntime

• 1 daywith 50mm precipitation

• 1 m soilcolumn, nogroundwaterpercolation

• At thebeginning: hydrostaticequilibrium, 1m groundwaterlevel

• siltysandsoil, 5% macropores, meanmacroporedistance 5cm

• Non swellingsoil

But forswellingsoils?

Matrix watercontent

Why not usealways dual porosity

• Big jobforthesolver (anothertimescale, twicethestate variables)

• Additional parameters (Conductivityofmacropores, macroporefraction, macroporedensity)