FLOW OF WATER IN UNSATURATED SOIL

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FLOW OF WATER IN UNSATURATED SOIL

• Krisztina Eleki
• March 07

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INTRODUCTION

• Most field processes occur in unsaturated
  conditions
• Supply moisture, nutrients

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UNSATURATED AND SATURATED FLOW

• PRINCIPLES
• Flow is - driven by hydraulic potential
• - occurs towards the lower potential
• Flow rate is - proportional to the potential
  gradient
• - affected by the geometry of the pores

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FLOW IN UNSATURATED VS SATURATED FLOW

• MOVING FORCE
• SATURATED SOIL gradient of a positive pressure
  potential
• UNSATURATED SOIL gradient of negative pressure
  potential matric suction matric potential

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MATRIC SUCTION/NEGATIVE PRESSURE POTENTIAL

• Due to physical affinity between water and soil
  matrix (particle surfaces, capillary pores)
• Pores getting air-filled suction develops
• If uniform along the column no moving force
• Water flows from where suction is lower to where
  it is higher

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FLOW IN UNSATURATED VS SATURATED FLOW

• HYDRAULIC CONDUCTIVITY (K)
• SATURATED SOIL all pores water filled and
  conducting
• Most conductive sandy or well aggregated soils -
  large continuous pores
• UNSATURATED SOIL some pores air filled
• Most conductive clayey soils small water
  retaining particles

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DESATURATION

• First big pores drain
• Separate and discontinuous
• pockets of water
• increased tortuosity
• Water moves
• As film creep alone the pore walls
• As tube flow through narrow water-filled pores
• Contact points! Texture clayey, Structure
  compacted

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RELATION OF CONDUCTIVITY TO SUCTION AND WETNESS

• Fig 8.2.
• Flow is maintained by suction differences
• Steady flow if flux (or product of gradient and
  K) constant
• Darcys Law q K ?H/ ?x
• Conductivity drops with the increase in suction
  along the length of the sample
• Fig. 8.3 Variation of
• wetness, matric potential
• and conductivity along a
• hypothetical column of
• unsaturated soil conducting a steady flow of
  water

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RELATION OF CONDUCTIVITY TO SUCTION AND WETNESS

• Steep decline of K with rising matric suction
• Soil water dynamics
• Fast wet soil processes
• Slow dry soil processes

• Slope of the flux vs. gradient K - varies with
  the average suction

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EQUATIONS OF UNSATURATED FLOW

• CONTINUITY EQUATION
• Based on the law of mass conservation
• inflow rate out flow rate rate of gain
• Difference in discharge flowing through two faces

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CONTINUITY EQUATION

• Continuity principle a V of soil gaining or
  losing water in accordance with the divergence of
  the flux
• ??/?t -?q ??/?t -div q
• Del (?) vector differential operator 3 D
  gradient in space spatial gradient of flux
• Divergence (Div) - scalar product of del operator

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COMBINED FLOW EQUATION

• from Darcys Law q -K(?)?H
• Matric suction head instead of K
• ?H hydraulic head gradient
• K(?) function is highly hysteretic
• Richards equation continuity equation combined
  with Darcys
• ??/?t ?K(?)?H