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Infiltration, Runoff and Stream flow

Infiltration, Runoff and Stream flow. Ali Fares, PhD Watershed Hydrology, NREM 691 UHM-CTAHR-NREM. Presentation Outline. Infiltration Definition & theory Green-Ampt, Horton & Philip equations Infiltration measurement Infiltration as affected by: Soil type. Infiltration.

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Infiltration, Runoff and Stream flow

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  1. Infiltration, Runoff and Stream flow Ali Fares, PhD Watershed Hydrology, NREM 691 UHM-CTAHR-NREM

  2. Presentation Outline • Infiltration • Definition & theory • Green-Ampt, Horton & Philip equations • Infiltration measurement • Infiltration as affected by: • Soil type

  3. 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).

  4. Double ring infiltrometer Tension infiltrometer Guelph Permeameter Xiaodong, 2002 Measuring infiltration

  5. Effect of Rain Intensity Effect of Initial Water Content Soil Cover Effects Infiltration Characteristics • Infiltration is influence by: • Rain intensity, - Soil type -Disturbance. - Crust • Soil cover - etc.

  6. Taylor & Ashcroft, 1972 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.

  7. H1 A Ksat L H2 Water Flow in Soil • The flux density, q, is the rate of water movement through a medium. • q is a function of ΔH and the hydraulic conductivity of the medium. • ΔH = H1 – H2 is the difference between total water potential inlet and outlet. • H = Pressure head + Gravitational head

  8. DERIVATION OF RICHARD’S 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)

  9. 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:

  10. z Dz x Dy Dx y FLOW THROUGH POROUS MEDIA

  11. From Continuity of mass Where Q is the volumetric water content and t is time.

  12. Applying the Darcy Law to each velocity term:

  13. 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) Infiltration Equations

  14. Horton’s Equation • Assumes K and D are constants (no dependence on q) • Richard’s 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) e–kt • k = Decay constant ~ T-1

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