INFILTRATIONPRESENTED BYKARUNAKAR KEMA

Infiltration General

Process of water penetrating from ground into soil

Factors affecting Condition of soil surface,

vegetative cover, soil properties, hydraulic conductivity, antecedent soil moisture

Four zones Saturated, transmission,

wetting, and wetting front

depth

Wetting Zone

TransmissionZone

Transition ZoneSaturation Zone

Wetting Front

Infiltration Infiltration rate

Rate at which water enters the soil at the surface (in/hr or cm/hr)

Cumulative infiltration Accumulated depth of water infiltrating

during given time periodt

dftF0

)()(

)(tf

dttdFtf )()(

InfiltrometersSingle Ring Double Ring

Infiltration Methods

Horton and Phillips Infiltration models developed as approximate solutions of an exact

theory (Richard’s Equation) Green – Ampt

Infiltration model developed from an approximate theory to an exact solution

Hortonian Infiltration Recall Richard’s

Equation Assume K and D are

constants, not a function of or z

Solve for moisture diffusion at surface

K

zD

zt

zK

zD

t

2

2

02

2

zD

t

ktcc effftf )()( 0

f0 initial infiltration rate, fc is constant rate and k is decay constant

Hortonian Infiltration

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2

Time

Infil

trat

ion

rate

, fk1

k3

k2

k1 < k2 < k3

fc

f0

Philips Equation Recall Richard’s

Equation Assume K and D are

functions of , not z Solution

Two terms represent effects of

Suction head Gravity head

S – Sorptivity Function of soil suction

potential Found from experiment

K

zD

zt

KtSttF 2/1)(

KSttf 2/1

21)(

Green – Ampt Infiltration

Wetted Zone

Wetting Front

Ponded WaterGround Surface

Dry Soil

0h

L

n

i

z

LLtF i )()(

dtdL

dtdFf

zh

Kz

Kf

fzhKqz

MoistureSoilInitialFront WettingtoDepth

i

L

Green – Ampt Infiltration (Cont.)

Apply finite difference to the derivative, between Ground surface Wetting front

Kz

Kf

Wetted Zone

Wetting Front

Ground Surface

Dry Soil

L

i

z0,0 z

fLz ,

KL

KKz

KKz

Kf f

00

FL

LtF )(

1

FKf f

Kz

Kf

1

LK

dtdL f

1

FKf f

dtdLf

Green – Ampt Infiltration (Cont.)

LtF )(

Wetted Zone

Wetting Front

Ground Surface

Dry Soil

L

i

z

LdL

dLdtK

f

f

CLLtKff

)ln(

Integrate

Evaluate the constant of integration

)ln( ffC

0@0 tL

)ln(L

LKtf

ff

Green – Ampt Infiltration (Cont.)

)ln(L

LKtf

ff

)1ln(f

fFKtF

1

FKf f

Wetted Zone

Wetting Front

Ground Surface

Dry Soil

L

i

z

Nonlinear equation, requiring iterative solution.

Soil Parameters Green-Ampt model requires

Hydraulic conductivity, Porosity, Wetting Front Suction Head

Soil Class Porosity Effective Porosity

Wetting Front

Suction Head

Hydraulic Conductivity

n e K (cm) (cm/h) Sand 0.437 0.417 4.95 11.78 Loam 0.463 0.434 9.89 0.34 Clay 0.475 0.385 31.63 0.03

re n

ees )1(

e

res

Effective saturation

Effective porosity

Ponding time

Elapsed time between the time rainfall begins and the time water begins to pond on the soil surface (tp)

Ponding Time

Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate)if ptiF *

1

FKf f

1

* p

f

tiKi

)( KiiKt f

p

Potential Infiltration

Actual Infiltration

Rainfall

Accumulated Rainfall

Infiltration

Time

Time

Infil

trat

ion

rate

, fC

umul

ativ

e In

filtr

atio

n, F

i

pt

pp tiF *

Example Silty-Loam soil, 30%

effective saturation, rainfall 5 cm/hr intensity

30.0/65.0

7.16486.0

e

e

shrcmK

cm

340.0)486.0)(3.01()1( ees

340.0*7.16

hr17.0))(65.00.5(0.5

68.565.0)(

KiKiiKt f

p