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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. Darcy’s Law. K = hydraulic conductivity q = specific discharge V = q/n = average velocity through the area. Definitions.

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ce 394k 2 hydrology

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

darcy s law
Darcy’s Law
  • K = hydraulic conductivity
  • q = specific discharge
  • V = q/n = average velocity through the area
definitions
Definitions

Element of soil, V

(Saturated)

Pore with

water

solid

Pore with

air

Element of soil, V

(Unsaturated)

infiltration

q

Saturation Zone

Transition Zone

Transmission

Zone

Wetting Zone

Wetting Front

depth

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
infiltration1
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
infiltration methods
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
Hortonian Infiltration
  • Recall Richard’s 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

philips equation
Philips Equation
  • Recall Richard’s 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
green ampt infiltration
Green – Ampt Infiltration

Ponded Water

Ground Surface

Wetted Zone

Wetting Front

Dry Soil

green ampt infiltration cont
Green – Ampt Infiltration (Cont.)

Ground Surface

Wetted Zone

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

Wetting Front

Dry Soil

slide12

Green – Ampt Infiltration (Cont.)

Ground Surface

Wetted Zone

Wetting Front

Dry Soil

Evaluate the constant of integration

Integrate

green ampt infiltration cont1
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/ce311k/Lab/Lab8/Lab8.html

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

Effective saturation

Effective porosity

ponding time
Ponding time
  • Elapsed time between the time rainfall begins and the time water begins to pond on the soil surface (tp)
ponding time1

Potential

Infiltration

Infiltration rate, f

Rainfall

Actual Infiltration

Time

Accumulated

Rainfall

Infiltration

Cumulative

Infiltration, F

Time

Ponding Time
  • Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate)
example
Example
  • Silty-Loam soil, 30% effective saturation, rainfall 5 cm/hr intensity