infiltration reading ah sec 4.3 to 4.4
DESCRIPTION
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 Transmission Zone Transition Zone Saturation Zone Wetting Front qTRANSCRIPT
Infiltration
Reading AH Sec 4.3 to 4.4
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 period
t
dftF0
)()(
)(tf
dttdFtf )()(
InfiltrometersSingle Ring Double Ring
http://en.wikipedia.org/wiki/Infiltrometer
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 2Time
Infil
trat
ion
rate
, f
k1
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
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
/65.07.16486.0
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