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Modelling Macropores

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

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  1. ModellingMacropores Philipp Kraft

  2. Schwingbach

  3. ICON Project

  4. Approach I • Use a 5-10x higherconductivity • Examples: Everywhere, cmfapplicationsuntiltoday

  5. 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

  6. Approach I+II • Onlyonestorage per numericallayer • Water in a numericallayerdoes mix perfectly • Macroporeandmicroporewaterhasthe same waterquality • Relation ofmacropores do not changewithwatercontent (noswellingeffects)

  7. Approach III • Waterinfiltratesthroughmacroporesintodeeperlayers • No additional waterstorage, infiltrationhas a by pass aroundthe top soil • Example: BROOK 90, cmf.LayerByPass

  8. 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

  9. Approach IV • Distinctmodelsofmacroporespaceandmicroporespace. • Resultsgetaveraged • Example: someHydrus 1D/2D applications

  10. Approach V • Twodistinctwaterstorages per layer • transportequationsformacropores (nocapillaryeffects) • transportequationsformicropores (Richards equation) • masstransferequationbetweenmacro- andmicropores • Example: MACRO

  11. 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

  12. Macroporetransport • Withoutcapillaryrise, kinematicwaveisusable • cmf: • V – actualstored Volume • C – Capacityoflayer

  13. Masstransfer • saturationbased • headbased

  14. Saturation basedmasstransfer • Philip 1968 • Jarvis 1994

  15. Head basedmasstransfer • Gerke & Van Genuchten

  16. cmf.GradientMacroMicroExchangeforMacro/Microporeexchange Δx Ψ(Macro) Aggregate Macropore z Ψ(Micro)

  17. Examplarymodelsetup • 10 daysruntime • 1 daywith 50mm precipitation • 1 m soilcolumn, nogroundwaterpercolation • At thebeginning: hydrostaticequilibrium, 1m groundwaterlevel • siltysandsoil, 5% macropores, meanmacroporedistance 5cm • Non swellingsoil

  18. But forswellingsoils?

  19. Matrix watercontent

  20. Why not usealways dual porosity • Big jobforthesolver (anothertimescale, twicethestate variables) • Additional parameters (Conductivityofmacropores, macroporefraction, macroporedensity)

  21. Questionstomacropores?

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