CE 394K.2 Hydrology

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CE 394K.2 Hydrology

• Infiltration
• Reading for Today AH Sec 4.3 and 4.4
• Reading for Thurs AH Sec 5.1 to 5.5
• Subsequent slides prepared by Venkatesh Merwade

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Darcys Law

• K hydraulic conductivity
• q specific discharge
• V q/n average velocity through the area

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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

• 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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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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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