INFILTRATION

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INFILTRATION

• AE 520

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Infiltration Lesson Objectives

• Understand the factors affecting soil water
  infiltration
• Be able to estimate infiltration rate using the
  Horton Equation and the Green-Ampt Equation
• Understand the advantages of the various
  infiltration functions.

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INFILTRATE

• To cause (as a liquid) to permeate something by
  penetrating its pores or interstices.
• Websters Ninth New Collegiate Dictionary

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INFILTRATION

• The movement of water through the soil surface
  into the soil profile.
• After Precipitation, it is the most important
  process controlling the water balance on a field.
• Expressed as total depth or as a velocity called
  the infiltration rate

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Infiltration may be controlled by 3 types of
factors

• Supply Factors
• Surface Factors
• Profile Factors

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Infiltration Control Supply Factors

• The water supply for infiltration may be
• Rainfall
• Surface Pond Storage
• Irrigation
• Infiltration Rate will be the minimum of supply
  and Capacity

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Infiltration Control Surface Factors

• Degree of Surface Crusting
• Clay content of the surface
• Effect of drop impact
• Degree of Surface Porosity
• Soil texture
• Soil Structure
• Vegetation Cover
• Inwash of fine particles
• Cracks and other macropores

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EFFECT OF PROTECTIVE COVER ON INFILTRATION
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Infiltration Control Profile Factors

• Hydraulic Conductivity of the soil layers
• Moisture Content of the soil layers near the
  surface.
• Rate of internal drainage

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Measurement of InfiltrationSprinkler
Infiltrometer
Rainfall
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Infiltration with supply control
Supply rate
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Measurement of InfiltrationRing Infiltrometer
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Infiltration with Soil Control
Slope fc
DEPTH (F)
TIME
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Infiltration with Soil Control
fo
INFILTRATION RATE (f) DEPTH / TIME
fc
TIME
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Field Surface
Yp Ym
During Infiltration
YT
Static
Z
Soil Profile
Water Table
- 0 POTENTIAL
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Kostiakovs Equation
F CTn f CnTn-1
log F
n
1
C
log T
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Hortons Equation
Log(f0 - fc )
1
Log(f-fc)
K
t
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Darcys Equation
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SOIL MOISTURE CONTENT
0
ZONE OF SATURATION
TRANSITION ZONE
TRANSMISSION ZONE
DEPTH FROM INFILTRATION SURFACE
WETTING ZONE
WETTING FRONT
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HOLTANS CONCEPT OF INFILTRATION
F Fmin S
S TOTAL POROSITY - WILTING POINT MOISTURE ABOVE
THE CONFINING LAYER
F Fmin S - ASM Sr
ACCUMULATED INFILTRATION DEPTH
Fmin slope fc
TIME
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Holtans Equation
log (f-fc)
P
1
A
1
log ((S-F)/S)
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Green-Ampt Equation
KDQDY
f
1
K
1/F
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Green-Ampt Infiltration Equation
1
Soil Surface
Yp 0
K Constant
Yz 0
Q Q f constant
Wet
Lf
Ym Yf
Wetting Front
2
Yp 0
Q Qi constant
Yz -Lf
Dry
Ym Yi
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Green-Ampt Infiltration Equation
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Green-Ampt Infiltration Equation

• Both Yi and Yf are negative and Yi lt Yf .
  Therefore DY lt 0 and the negative signs cancel.

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Green-Ampt Infiltration Equation

• Total Depth of Infiltration F
• F Lf(Qf - Qi) LfDQ
• Lf F/DQ
• assuming the velocity V the infiltration rate f

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Green-Ampt Computations
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Infiltration Rate at the Beginning of a Time
Increment
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Infiltration Rate at the Beginning of a Time
Increment
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Average Infiltration Rate over a Time Period
Rearranging
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Upper Limit on the Infiltration During the Period
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Multiplying by F2 and Rearranging
F1 will always be known or assumed. Therefore F2
is the only unknown, and the function is a
quadratic.
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Quadratic Equation Solution for F2
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Quadratic Equation Solution for F2

• Only the sign in the quadratic equation applies.
• The actual value of F2 can not exceed F1 Supply
  during the period. At the end of each time step,
  this limit must be checked, and if the GA
  equation gives a larger value, then the supply
  will be limiting.

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