1. Infiltration : Objectives
Understand the three processes of infiltration
Understand the difference between rate (flux) and volume of infiltration
Identify physical and biological factors that affect infiltration rates and volumes
Learn about methods for measuring infiltration
Understand why infiltration varies in space and time
Identify management effects on infiltration rates and volumes

2. What is infiltration?
The movement of water through the air-soil interface
It is one of the things that can happen to precipitation that reaches the soil surface
Infiltration is the actual rate at which water is entering the soil
at any given time (SCSA, 1976)

4. Processes of infiltration
Entry through the soil surface (infiltration)
Storage in the soil profile (soil moisture)
Transmission through the soil profile- (percolation)

5. Infiltration
Precipitation reaching the ground may infiltrate.
This is the process of moving from the atmosphere into the soil.
Infiltration may be regarded as either a rate or a total. For example: the soil can infiltrate 1.2 inches/hour. Alternatively, we could say the soil has a total infiltration capacity of 3 inches.
Note that in both cases the units are Length or length per time!

6. Infiltration, cont
Infiltration is nearly impossible to measure directly - as we would disturb the sample in doing so.
We can infer infiltration in a variety of ways (to be discussed at a later point).
The exact point at which the atmosphere ends and the soil beings is very difficult to define and generally we are not concerned with this fine detail!
In other words, we mostly want to know how much of the precipitation actually enters the soil.

7. Percolation.....
Once the water infiltrates into the ground, the downward movement of water through the soil profile may begin.

8. Percolation.....
The percolating water may move as a saturated front - under the influence of gravity…
Or, it may move as unsaturated flow mostly due to capillary forces.

9. Percolation….the point
The vertical percolation of the water into various levels or zones allows for storage in the subsurface – .
This stored subsurface water is held and released as either evaporation, transpiration, or as streamflow eventually reaching the watershed outlet.

10. Infiltration nomenclature
i = intensity of rainfall (rate) (length/time)
f = infiltration rate- measure of hydraulic conductivity (length/time)
F = infiltrated volume (Length3) or depth (L)
If i < f what happens?
If i > f what happens?

11. Infiltration
Infiltration is the actual rate at which water is entering the soil at any given time(SCSA, 1976).

12. What factors affect infiltration?
Flow influences
Head (ponding)
Viscosity (function of temperature)
Water quality
Soil chemistry
Soil and water temperature
Air entrapment

13. What factors affect infiltration?
Soil surface conditions
Land use
Vegetation cover
Roughness and slope
Cracking and crusting
Surface sealing, swelling

14. What factors affect infiltration?
Hydrophobicity
Dryness
Heat
Plant chemicals
Aromatic oils
Other chemicals
Fire

15. What factors affect infiltration?
Subsurface conditions
Soil
Hydrologic group (A B C D)
Texture
Porosity
Depth
Shrink and swell
Layering
Spatial variability
Structure

16. What factors affect infiltration?
Subsurface conditions
Root system
Water table depth
Subsurface drainage
Water release relationship
Hydraulic conductivity

17. Factors that affect surface and subsurface conditions that affect infiltration
Mechanical processes, plowing,
Frost- freeze-thaw cycles
Litter layer, organic matter
Compaction
Antecedent soil water condition
Chemical activity
Biological activity
Microbial activity

18. Soil Type Effects on Infiltration
Sand soils have the highest infiltration rates
Clay soils have the lowest infiltration rates.
High organic matter improves infiltration rates.
Taylor & Ashcroft, 1972

19. Effect of Initial Water Content
Soil Cover Effects

20. How do we measure infiltration?
Single ring infiltrometer
Constant head (ponded depth)
Results tend to be higher than that due to rainfall
Point scale

21. Infiltrometer ring
Fall-2004/9227A886-A1FE-408F-BFED-1665F5E5B6B8/0/1_72_lecture_13.pdf

22. How do we measure infiltration?
Rainfall simulators
Needle drip systems
Stand pipes
Sprinkler nozzles
Rotating boom
All measure input of water and output of water (runoff)- difference is the amount infiltrated
Plot scale
Need lots of water, vehicles, plot boundaries

23. How do we measure infiltration?
Average infiltration method
Small basins or plots
Use storms with bursts of rain
Compute the amount of rain in the burst
Separate the runoff volume due to the burst
Difference is infiltrated volume

24. How do we measure infiltration?
Soil surveys
Usually report infiltration ranges for various soil types
Example rates
Sand mm/hr
Sandy loam 50 mm/hr
Loam mm/hr
Silt loam mm/hr
Light clay mm/hr

25. Management effects on infiltration
Compaction or alteration of soil surface and vegetative cover
Grazing, skidding logs, recreational use, vehicles, plowing
Even low ground pressure skidders can increase bulk density by up to 45% at a depth of 15 cm – frequent travel over wet soils

26. Management effects on infiltration
Compaction or alteration of soil surface and vegetative cover
Grazing, cropping and logging
Changes interception, organic matter layers, rooting depths, ground cover

27. Infiltration rate over time
Infiltration rate i(t), cm/hr
Infinite at time t = 0?
i(t) can’t exceed precipitation rate
Zero at time t = ∞?
Soil Physics 2010

28. Infiltration rate over time
Infiltration rate i(t), cm/hr
Infiltration rate can be either soil-limited or rain-limited
i(t) can’t exceed precipitation rate
Not really. Soil behind (above) the wetting front isn’t 100% saturated. Some people write ic instead.
As t → ∞, i(t) → Ks
Soil Physics 2010

29. Single-ring Double-ring
Measuring infiltration: ring infiltrometer
Falling head method: Pour in water, wait for steady flow, then measure water depth over time.
Constant head method: Maintain a constant water level, and measure how much water that requires over time.
Single-ring Double-ring
Soil Physics 2010

30. Measuring infiltration
There is less effect of the ring size on the results when using the double-ring:
Maintain equal depths, but only measure flow into inner ring.
Outer ring will supply most of the horizontal flow, so inner ring gives mainly vertical
Water is applied to the soil surface at a positive pressure
Soil Physics 2010

31. Double Ring Infiltrometer
Measure rate of fall in inner ring
Infilration

32. Estimating infiltration at the scale of a catchment (watershed):
Measure baseflow before rainfall
Measure rainfall
Measure streamflow
Estimate runoff by baseflow separation
Estimate: Infiltration = rainfall - runoff
Soil Physics 2010

33. These models have 2 main purposes:
Infiltration models
Green & Ampt (1911)
Horton (1930)
Kostiakov (1932)
Philip (1957)
There are many others, but we won’t study them.
These models have 2 main purposes:
Explain the observed infiltration patterns
Predict future infiltration
Soil Physics 2010

34. Infiltration by Horton’s method

42. No theory: this is purely empirical
Kostiakov’s model t
i(t)
with i : infiltration rate, L/T
t : time, T
B, n : fitting parameters
usually n ≈ 1/2
No theory: this is purely empirical
No physical interpretation of B and n.
Note that i(0) = ∞, and i(∞) = 0.
Frequently this model fits the data better than more physically-based models.
Soil Physics 2010

43. No physical interpretation of b. Note that i(0) = ∞, and i(∞) = ic.
Green & Ampt’s model t
i(t) ic
with i : infiltration rate, L/T
ic : final i : i(∞), L/T
t : time, T
b : fitting parameter
I : cumulative infiltration, L
No physical interpretation of b.
Note that i(0) = ∞, and i(∞) = ic.
Assumes all flow is saturated flow
Soil Physics 2010

44. Philip’s model t i(t) ic with i : infiltration rate, L/T
ic : final i : i(∞), L/T
t : time, T
s : sorptivity, L/T0.5
Exact solution of Richards’ equation, with additional assumptions
Infinite series, but only 1st 2 terms used
Doesn’t work well at short times
Sorptivity isn’t used much outside of Australia (J. R. Philip was Australian)
Soil Physics 2010

45. Infiltration into a soil water column1 2

46. Green-Ampt method Relative saturation Incremental change
in soil water content
as wetting front passes

47. Green-Ampt Parameter Definitions
Relative saturation
Incremental change
in soil water content
as wetting front passes

48. Math for Green Ampt

49. Green- Ampt Parameters