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Infiltration

Infiltration. ERS 482/682 Small Watershed Hydrology. Definitions. infiltration: process by which water enters the soil surface infiltration rate, f ( t ): rate at which water enters the soil surface water-input rate, w ( t ): rate at which water arrives at the soil surface

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Infiltration

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  1. Infiltration ERS 482/682 Small Watershed Hydrology

  2. Definitions • infiltration: • process by which water enters the soil surface • infiltration rate, f(t): • rate at which water enters the soil surface • water-input rate, w(t): • rate at which water arrives at the soil surface • infiltration capacity, f*(t): • maximum rate at which infiltration can occur • depth of ponding, H(t): • depth of water standing on the surface [L T-1] [L T-1] [L T-1] [L]

  3. Definitions • percolation • downward movement of water through the soil • hydraulic conductivity, Kh: • rate at which water moves through a porous medium under a unit potential-energy gradient • sorptivity, Sp: • rate at which water will be drawn into an unsaturated soil in the absence of gravity forces • soil-water pressure or matric potential, : • water pressure (tension) head in a soil • air-entry tension, ae: • pressure head when significant volumes of air begin to appear in soil pores; occurs at the capillary fringe (i.e., height of the tension-saturated zone) [L T-1] [L T-1/2] [L] [L]

  4. Why is infiltration important?

  5. Why is infiltration important? • Determines availability of water for overland flow • Flood prediction

  6. Why is infiltration important? • Determines availability of water for overland flow • Flood prediction • Irrigation plans

  7. Why is infiltration important? • Determines availability of water for overland flow • Flood prediction • Irrigation plans • Runoff pollution • Determines how much water goes into the soil • Groundwater estimates • Water availability for plants

  8. Soil column Infiltration conditions • No ponding:

  9. Infiltration conditions • No ponding: • Saturation from above: Soil column

  10. Infiltration conditions • No ponding: • Saturation from above: • Saturation from below: Soil column

  11. Capillarity: • Sorptivity • Matric potential • Gravity: • Percolation • Hydraulic conductivity Figure 5.2: Manning (1987)

  12. What are models? Models are representations of the real world A model is a conceptualization of a system that retains the essential characteristics of that system for a specific purpose.

  13. Assumptions for most infiltration models • Water moves vertically • Homogeneous soil • Soil volume > pore size • Moving water is liquid only • Water movement not affected by • Airflow in soil pores • Temperature • Osmotic gradients

  14. Infiltration models • Horton • Kostiakov • Green-Ampt • Philip • Others

  15. What we want to quantify… Infiltration rate, f(t) w tsat Time, t

  16. What we want to quantify… Infiltration rate, f(t) w K*h tsat Time, t

  17. f(t)=f*(t) f(t)<f*(t) Infiltration rate, f(t) Runoff Time, t What we want to quantify… K*h tsat,1 tsat,3 tsat,2

  18. What we want to quantify… Figure 4.2: Brooks et al. (1991)

  19. Exponential function: Horton model • Infiltration rate resembles a decreasing exponential function: where e = 2.71828…

  20. f(t) = fc + (f0 – fc)e-kt x x Infiltration rate, f(t) x x x x x x Time, t Horton model f0 fc

  21. f(t) = Kkt- Infiltration rate, f(t) Time , t Kostiakov model

  22. Water H(t) Wet soil  =  zf(t) wetting front Dry soil  = 0 Capillary suctionat wetting front Soil column Green-Ampt model • Based on • Darcy’s law (Eq. 6-8b)

  23. zf(t) Green-Ampt model Figure 8.10: Hornberger et al.(1998)

  24. Dry soil  = 0 Soil column Green-Ampt model • Initially (before rain)  = 0, H(t) = 0, f= 0

  25. Storage  Dry soil  = 0 time when rain stops Soil column Green-Ampt model w = rainfall rate • If w < K*h: H(t) = 0   > 0 Kh() until t = tw

  26. Storage  Dry soil  = 0 time when ponding starts Soil column Green-Ampt model • If w > K*h: H(t) = 0   = Kh() up to K*h until t=tp

  27. Dry soil  = 0 Soil column Green-Ampt model • If w > K*h: for t>tp = Equation 6-40 (error in book)

  28. Change inwater content zf(t) Dry soil  = 0 Soil column Green-Ampt model • If w > K*h: for t>tp = • Volume infiltrated • H(t) ~ 0 • rate Equation 6-42

  29. Table 6-1 measure Table 6-1 Equation 6-46 with Table 6-1 Green-Ampt model • Difficulties with model • Need to know • Porosity,  • Initial water content, 0 • K*h • f • See Examples 6-6 and 6-7

  30. where t = time since ponding began Sp= sorptivity Kp = hydraulic conductivity Dry soil  = 0 Soil column Philip model • For t>tp • Volume infiltrated =

  31. Dry soil  = 0 Soil column Philip model • Works after ponding only • Used for characterizingspatial variability of infiltrometer measurements =

  32. Other models • Richard’s equation • Physically-based • Numerically intensive • Morel-Seytoux and Khanji model • Includes viscous resistance • Smith-Parlange model • Account for different rates of changing hydraulic conductivity with water content

  33. Measuring infiltration • Flooding (ring) infiltrometers • Single ring • Double ring • Rainfall-runoff plot infiltrometers

  34. Cylinder infiltration True infiltration Water-entry pressure head  0.5ae Ring infiltrometers Bouwer (1986)

  35. ponding begins; determined in Example 6-7 Estimating infiltration parameters Box 6-2 and Example 6-9 Data from Example 6-8

  36. Estimating infiltration parameters Box 6-2 and Example 6-9

  37. Estimating infiltration parameters Box 6-2 and Example 6-9 Least squares approach: Find the parameters that provide the ‘best fit’ of the model to the observed data ‘best fit’ occurs when sum of the squared differences between measured and modeled values is minimized

  38. note errorin book! Estimating infiltration parameters Box 6-2 and Example 6-9 Equations 6B2-8 and 6B2-9

  39. sum sum sum sum Estimating infiltration parameters Box 6-2 and Example 6-9

  40. Variability of infiltration • Factors that affect infiltration rate • Water-input rate or depth of ponding • Hydraulic conductivity at the surface • Organic surface layers • Frost • Swelling-drying • Inwashing of fine sediment • Anthropogenic modification

  41. Variability of infiltration • Factors that affect infiltration rate • Water-input rate or depth of ponding • Hydraulic conductivity at the surface • Organic surface layers • Frost • Swelling-drying • Inwashing of fine sediment • Anthropogenic modification

  42. Variability of infiltration • Factors that affect infiltration rate • Water content of surface pores • Surface slope and roughness • Chemical characteristics of soil • hydrophobicity • Physical/chemical properties of water Figure 4.5: Brooks et al. (1991)

  43. Infiltration capacity Infiltration 0.5 1.0 0 Fraction of watershed area Point  watershed??? • Manley (1977) approach K*+h Rainfall rate w runoff

  44. Philip equation: • Measure at several locations • Calculate area-weighted average of Sp and Kp Areal-weighted average of infiltration Point  watershed??? • Areal-weighted averages

  45. Point  watershed??? • Divide watershed into subareas • Soil properties • Initial conditions • Etc. • Calculate areally-weighted infiltration

  46. Example: Incline Creek Watershed Sullivan et al. 1996 • Objective: determine which data collection techniques are best for quantifying spatial variations in surface infiltration • Used Philip equation

  47. Watershed size: 7.2 km2

  48. Performed 50 tests with disk permeameter

  49. Performed 50 tests with disk permeameter • Sites were selected based on: • Accessibility • Minimal surface disturbance • Macropores were absent • Tried to pick sites that represented different soil types and vegetative cover

  50. Created GIS coverages • Soil types • Vegetative groupings • Used field method to determine average areal % of vegetation classification per disk- permeameter test • Calculated weighted values for Ks based on average areal % vegetation cover

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