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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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Tension infiltrometerDouble ring infiltrometer Guelph Permeameter 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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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 = H 1 – H 2 is the difference between total water potential inlet and outlet. H = Pressure head + Gravitational head L A H1H1 H2H2 K sat

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DERIVATION OF RICHARD ’ S EQUATION IN RECTANGULAR COORDINATES The general continuity equation is: Q = a q where Q is the flow rate, volume/time (L 3 /T) a is the cross-section area perpendicular to the flow, (L 2 ) q is the flow velocity, length/time (L/T)

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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: FLOW THROUGH POROUS MEDIA

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x y z xx yy zz

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

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Applying the Darcy Law to each velocity term: Applying the Darcy Law to each velocity term:

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Infiltration Equations Green-Ampt (1911): i = i c + b/I Where I is cumulative infiltration, i c and b are constants. There are several other equations: Horton (1940) Philip (1957)

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Horton ’ s Equation – Assumes K and D are constants (no dependence on q) – Richard’s equation reduces to the diffusion equation dq/dt = D [d 2 q/dz 2 ] – Solve the equation for the rate of moisture diffusion at soil surface f(t) = f c + (f 0 – f c ) e –kt – k = Decay constant ~ T -1

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