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InfiltrationIntroductionGreen Ampt methodPonding time

Reading: Applied Hydrology Sections 5.1 to 5.6

depthWetting ZoneTransmissionZoneTransition ZoneSaturation ZoneWetting FrontqInfiltrationGeneralProcess of water penetrating from ground into soilFactors affectingCondition of soil surface, vegetative cover, soil properties, hydraulic conductivity, antecedent soil moistureFour zonesSaturated, transmission, wetting, and wetting front

InfiltrationInfiltration rate, f(t)Rate at which water enters the soil at the surface (in/hr or cm/hr)Cumulative infiltration, F(t)Accumulated depth of water infiltrating during given time period

tf, FFfInfiltrometers

Single RingDouble Ringhttp://en.wikipedia.org/wiki/Infiltrometer Infiltration MethodsHorton and Phillips Infiltration models developed as approximate solutions of an exact theory (Richards Equation)Green AmptInfiltration model developed from an approximate theory to an exact solution Green-Ampt AssumptionsWetted ZoneWetting FrontGround SurfaceDry Soil

= increase in moisture content as wetting front passes

= Suction head at sharp wetting frontConductivity, K

= Wetted depth

= Conductivity in wetted zonePonded Water

= Depth of water ponding on surface (small)Green-Ampt Soil Water VariablesWetted ZoneWetting FrontGround SurfaceDry Soil

= initial moisture content of dry soil before infiltration happens

= increase in moisture content as wetting front passes

= moisture content (volume of water/total volume of soil)

= residual water content of very dry soil

= effective porosity

= porosityGreen-Ampt Parameters(Data from Table 4.3.1)TexturePorosity nResidual Porosity rEffective Porosity eSuction Head (cm)Conductivity K (cm/hr)Sand0.4370.0200.4174.9511.78Loamy Sand0.4370.0360.4016.132.99Sandy Loam0.4530.0410.41211.011.09Loam 0.4630.0290.4348.890.34Silt Loam0.5010.0150.48616.680.65Sandy Clay Loam0.3980.0680.33021.850.15Clay Loam0.4640.1550.30920.880.10Silty Clay Loam0.4710.0390.43227.300.10Sandy Clay 0.4300.1090.32123.900.06Silty Clay 0.4700.0470.42329.220.05Clay 0.4750.0900.38531.630.03Green-Ampt Porosity (Data from Table 4.3.1)0.090.450.03Total porosity ~ 0.45Clay soils retain water in ~ 20% of voids when dryOther soils retain water in ~ 6% of voids when dryerConductivity and Suction Head(Data from Table 4.3.1)Suction Head, (cm)Conductivity, K (cm/hr)SandClaySilt LoamSilty Clay LoamLoamy Sand

Sandy ClaySandy LoamLoamSandy Clay LoamClay LoamSilty ClayGreen Ampt InfiltrationWetted ZoneWetting FrontPonded WaterGround SurfaceDry Soil

Green Ampt Infiltration (Cont.)Apply finite difference to the derivative, between Ground surfaceWetting front

Wetted ZoneWetting FrontGround SurfaceDry Soil

Green Ampt Infiltration (Cont.)

Nonlinear equation, requiring iterative solutionand integrate over time, givesUsetf, FFf13Initial Effective Saturation

Initial effective saturation 0 Se 1 Effective porosity

ExampleDetermine the infiltration rate and the cumulative infiltration after 1 hour on a clay loam soil with initial effective saturation of 30%. Assume water is ponded instantaneously on the surfaceParameters:TexturePorosity nResidual Porosity rEffective Porosity eSuction Head (cm)Conductivity K (cm/hr)Clay Loam0.4640.1550.30920.880.10

SolutionUse Excel Solver16Ponding timeElapsed time between the time rainfall begins and the time water begins to pond on the soil surface (tp)

Ponding TimeUp to the time of ponding, all rainfall has infiltrated (i = rainfall rate)

Potential InfiltrationActual InfiltrationRainfallAccumulated RainfallInfiltrationTimeTimeInfiltration rate, fCumulative Infiltration, F

ExampleClay Loam soil, 30% effective saturation, rainfall 1 cm/hr intensity What is the ponding time, and cumulative infiltration at ponding?How long does it take to infiltrate 2 cm of water?What is the infiltration rate at that time?

Infiltration after ponding has occuredSolution