CE 394K.2 Hydrology

1. 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/C%20Soil%20Water%20Relationships.ppt

2. 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 soil’s dielectric constant is dependent on soil moisture • Time domain reflectometry (TDR) • Frequency domain reflectometry (FDR) • Primarily used for research purposes at this time

3. Soil Water MeasurementNeutron Attenuation

4. 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

5. 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)

6. Electrical Resistance Blocks & Meters

7. 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

8. Richard’s Equation • Recall • Darcy’s Law • Total head • So Darcy becomes • Continuity becomes Soil water diffusivity

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. Infiltration into a horizontal soil column Boundary conditions Θ = Θo for x = 0, t > 0 Θ = Θn for t = 0, x > 0 x 0 Equation:

11. Measurement of Diffusivity by Evaporation from Soil Cores Air stream q x q = soil water flux = evaporation rate

12. Measurement of Diffusivity by Evaporation from Soil Cores http://www.regional.org.au/au/asa/2006/poster/water/4521_deeryd.htm

13. Numerical Solution of Richard’s Equation (Ernest To) http://www.ce.utexas.edu/prof/maidment/GradHydro2007/Ex4/Ex4Soln.doc

14. Implicit Numerical Solution of Richard’s Equation t (j) j j -1 x (i) i-1 i i+1

15. Implicit Numerical Solution of Richard’s Equation

16. Matrix solution of the equations

17. Θ

18. f

19. F

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

21. 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

22. Infiltrometers Single Ring Double Ring http://en.wikipedia.org/wiki/Infiltrometer

23. 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

24. 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

25. Hortonian Infiltration

26. 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

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

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

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

30. 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

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

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

33. 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)

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