CE 394K.2 Hydrology

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CE 394K.2 Hydrology

• Infiltration
• Reading AH Sec 5.1 to 5.5
• Some slides were prepared by Venkatesh Merwade
• Slides 2-6 come from http//biosystems.okstate.edu
  /Home/mkizer/C20Soil20Water20Relationships.ppt

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Soil Water Measurement

• Neutron scattering (attenuation)
• Measures volumetric water content (?v)
• Attenuation of high-energy neutrons by hydrogen
  nucleus
• Advantages
• samples a relatively large soil sphere
• repeatedly sample same site and several depths
• accurate
• Disadvantages
• high cost instrument
• radioactive licensing and safety
• not reliable for shallow measurements near the
  soil surface
• Dielectric constant
• A soils dielectric constant is dependent on soil
  moisture
• Time domain reflectometry (TDR)
• Frequency domain reflectometry (FDR)
• Primarily used for research purposes at this time

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Soil Water MeasurementNeutron Attenuation
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Soil Water Measurement

• Tensiometers
• Measure soil water potential (tension)
• Practical operating range is about 0 to 0.75 bar
  of tension (this can be a limitation on medium-
  and fine-textured soils)
• Electrical resistance blocks
• Measure soil water potential (tension)
• Tend to work better at higher tensions (lower
  water contents)
• Thermal dissipation blocks
• Measure soil water potential (tension)
• Require individual calibration

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Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root
Zone Depth
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)
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Electrical Resistance Blocks Meters
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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

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Richards Equation

• Recall
• Darcys Law
• Total head
• So Darcy becomes
• Continuity becomes

Soil water diffusivity
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Philips Equation

• Recall Richards Equation
• Assume K and D are functions of q, not z
• Solution
• Two terms represent effects of
• Suction head
• Gravity head
• S Sorptivity
• Function of soil suction potential
• Found from experiment

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Infiltration into a horizontal soil column
Boundary conditions
T To for x 0, t gt 0
T Tn for t 0, x gt 0
x
0
Equation
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Measurement of Diffusivity by Evaporation from
Soil Cores
Air stream
q
x
q soil water flux evaporation rate
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Measurement of Diffusivity by Evaporation from
Soil Cores
http//www.regional.org.au/au/asa/2006/poster/wate
r/4521_deeryd.htm
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Numerical Solution of Richards Equation
(Ernest To)
http//www.ce.utexas.edu/prof/maidment/GradHydro20
07/Ex4/Ex4Soln.doc
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Implicit Numerical Solution of Richards Equation
t (j)
j j -1
x (i)
i-1 i i1
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Implicit Numerical Solution of Richards Equation
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Matrix solution of the equations
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T
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f
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F
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Definitions
Element of soil, V (Saturated)
Pore with water
solid
Pore with air
Element of soil, V (Unsaturated)
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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

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Infiltrometers
Single Ring
Double Ring
http//en.wikipedia.org/wiki/Infiltrometer
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Infiltration Methods

• Horton and Phillips
• Infiltration models developed as approximate
  solutions of an exact theory (Richards Equation)
• Green Ampt
• Infiltration model developed from an approximate
  theory to an exact solution

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Hortonian Infiltration

• Recall Richards Equation
• Assume K and D are constants, not a function of q
  or z
• Solve for moisture diffusion at surface

f0 initial infiltration rate, fc is constant rate
and k is decay constant
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Hortonian Infiltration
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Philips Equation

• Recall Richards Equation
• Assume K and D are functions of q, not z
• Solution
• Two terms represent effects of
• Suction head
• Gravity head
• S Sorptivity
• Function of soil suction potential
• Found from experiment

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Green Ampt Infiltration
Ponded Water
Ground Surface
Wetted Zone
Wetting Front
Dry Soil
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Green Ampt Infiltration (Cont.)
Ground Surface
Wetted Zone
Wetting Front

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

Dry Soil
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Green Ampt Infiltration (Cont.)
Ground Surface
Wetted Zone
Wetting Front
Dry Soil
Evaluate the constant of integration
Integrate
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Green Ampt Infiltration (Cont.)
Ground Surface
Wetted Zone
Wetting Front
Dry Soil
Nonlinear equation, requiring iterative solution.
See http//www.ce.utexas.edu/prof/mckinney/ce311
k/Lab/Lab8/Lab8.html
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Soil Parameters

• Green-Ampt model requires
• Hydraulic conductivity, Porosity, Wetting Front
  Suction Head
• Brooks and Corey

Effective saturation
Effective porosity
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Ponding time

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

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Ponding Time

• Up to the time of ponding, all rainfall has
  infiltrated (i rainfall rate)

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Example

• Silty-Loam soil, 30 effective saturation,
  rainfall 5 cm/hr intensity