Modelling Macropores

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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

Approach I
• Use a 5-10x higherconductivity
• Examples: Everywhere, cmfapplicationsuntiltoday
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
• Onlyonestorage per numericallayer
• Water in a numericallayerdoes mix perfectly
• Macroporeandmicroporewaterhasthe same waterquality
• Relation ofmacropores do not changewithwatercontent (noswellingeffects)
Approach III
• Waterinfiltratesthroughmacroporesintodeeperlayers
• No additional waterstorage, infiltrationhas a by pass aroundthe top soil
• Example: BROOK 90, cmf.LayerByPass
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
• Distinctmodelsofmacroporespaceandmicroporespace.
• Resultsgetaveraged
• Example: someHydrus 1D/2D applications
Approach V
• 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
Why not usealways dual porosity
• Big jobforthesolver (anothertimescale, twicethestate variables)
• Additional parameters (Conductivityofmacropores, macroporefraction, macroporedensity)