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

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

Tension infiltrometerDouble ring infiltrometer Guelph Permeameter Measuring infiltration

Infiltration Characteristics Infiltration is influence by: – Rain intensity,- Soil type -Disturbance. - Crust – Soil cover- etc.

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.

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

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)

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

x y z xx yy zz

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

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

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)

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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