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CE 394K.2 Hydrology - PowerPoint PPT Presentation

CE 394K.2 Hydrology. Infiltration Reading AH Sec 5.1 to 5.5 Some of the subsequent slides were prepared by Venkatesh Merwade. Darcy’s Law. K = hydraulic conductivity q = specific discharge V = q/n = average velocity through the area. Richard’s Equation. Recall Darcy’s Law

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CE 394K.2 Hydrology

Infiltration

Reading AH Sec 5.1 to 5.5

Some of the subsequent slides were prepared by Venkatesh Merwade

• K = hydraulic conductivity

• q = specific discharge

• V = q/n = average velocity through the area

• Recall

• Darcy’s Law

• So Darcy becomes

• Continuity becomes

Soil water diffusivity

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

• Recall Richard’s Equation

• Assume K and D are functions of q, not z

• Solution

• Two terms represent effects of

• S – Sorptivity

• Function of soil suction potential

• Found from experiment

Boundary conditions

Θ = Θo for x = 0, t > 0

Θ = Θn for t = 0, x > 0

x

0

Equation:

Air stream

q

x

q = soil water flux = evaporation rate

http://www.regional.org.au/au/asa/2006/poster/water/4521_deeryd.htm

(Ernest To)

t (j)

j

j -1

x (i)

i-1 i i+1

Element of soil, V

(Saturated)

Pore with

water

solid

Pore with

air

Element of soil, V

(Unsaturated)

• 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

• 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

• 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

• Recall Richard’s Equation

• Assume K and D are functions of q, not z

• Solution

• Two terms represent effects of

• S – Sorptivity

• Function of soil suction potential

• Found from experiment

Ponded Water

Ground Surface

Wetted Zone

Wetting Front

Dry Soil

Ground Surface

Wetted Zone

• Apply finite difference to the derivative, between

• Ground surface

• Wetting front

Wetting Front

Dry Soil

Ground Surface

Wetted Zone

Wetting Front

Dry Soil

Evaluate the constant of integration

Integrate

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

• Green-Ampt model requires

• Hydraulic conductivity, Porosity, Wetting Front Suction Head

• Brooks and Corey

Effective saturation

Effective porosity

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

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)

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