Infiltration, Runoff and Stream flow

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Infiltration, Runoff and Stream flow

• Ali Fares, PhD
• Watershed Hydrology, NREM 691
• UHM-CTAHR-NREM

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Presentation Outline

• Infiltration
• Definition theory
• Green-Ampt, Horton Philip equations
• Infiltration measurement
• Infiltration as affected by
• Soil type

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Infiltration

• Infiltration is the actual rate at which water is
  entering the soil at any given time(SCSA, 1976).
• Infiltration capacity Maximum rate (LT-1).

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Measuring infiltration
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Infiltration Characteristics

• Infiltration is influence by
• Rain intensity, - Soil type -Disturbance. - Crust
• Soil cover - etc.

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Soil Type Effects on Infiltration

• Sand soils have the highest infiltration rates
• Clay soils have the lowest infiltration rates.
• High organic matter improves infiltration rates.

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Water Flow in Soil

• The flux density, q, is the rate of water
  movement through a medium.
• q is a function of ?H and the hydraulic
  conductivity of the medium.
• ?H H1 H2 is the difference between total
  water potential inlet and outlet.
• H Pressure head Gravitational head

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DERIVATION OF RICHARDS EQUATION IN RECTANGULAR
COORDINATES
The general continuity equation is Q a
q where Q is the flow rate, volume/time
(L3/T) a is the cross-section area
perpendicular to the flow, (L2) q is the flow
velocity, length/time (L/T)
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FLOW THROUGH POROUS MEDIA
In unsaturated soil the total potential can be
estimated as the sum of the matric potential and
the gravity potential
Since the gravity potential only acts in the
vertical, or z-direction, the total potential, H,
can be replaced by the matric potential, h, in
all terms except the one involving z
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FLOW THROUGH POROUS MEDIA
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From Continuity of mass
Where Q is the volumetric water content and t is
time.
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Applying the Darcy Law to each velocity term
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Infiltration Equations

• Green-Ampt (1911)
• i ic b/I
• Where I is cumulative infiltration,
• ic and b are constants.
• There are several other equations
• Horton (1940)
• Philip (1957)

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Hortons Equation

• Assumes K and D are constants (no dependence on
  q)
• Richards equation reduces to the diffusion
  equation dq/dt D d2q/dz2
• Solve the equation for the rate of moisture
  diffusion at soil surface f(t) fc (f0 fc)
  ekt
• k Decay constant T-1

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