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

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CE 394K.2 Hydrology

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  1. 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

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

  3. Definitions Element of soil, V (Saturated) Pore with water solid Pore with air Element of soil, V (Unsaturated)

  4. 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

  5. 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

  6. 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

  7. 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

  8. Hortonian Infiltration

  9. 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

  10. Green – Ampt Infiltration Ponded Water Ground Surface Wetted Zone Wetting Front Dry Soil

  11. Green – Ampt Infiltration (Cont.) Ground Surface Wetted Zone • Apply finite difference to the derivative, between • Ground surface • Wetting front Wetting Front Dry Soil

  12. Green – Ampt Infiltration (Cont.) Ground Surface Wetted Zone Wetting Front Dry Soil Evaluate the constant of integration Integrate

  13. 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

  14. Soil Parameters • Green-Ampt model requires • Hydraulic conductivity, Porosity, Wetting Front Suction Head • Brooks and Corey Effective saturation Effective porosity

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

  16. 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)

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

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