1. Toby’s & Jake’s notes combined
Infiltration 1-3
Jake’s lectures
Toby’s & Jake’s notes combined
Soil Physics 2010

2. Infiltration
Infiltration is a surface process: water (e.g., rain) moves from above to below the soil surface.
As water moves in, air must either (1) escape, or (2) be compressed below the infiltrating water. Generally air escapes through larger pores, which slows the infiltration. Sometimes you see bubbles in puddles while it’s raining.
(Note that Richards’ equation does not account for air flow: it is only about water flow.)
Soil Physics 2010

3. Infiltration is important for:
Soil water recharge (e.g., for plants)
Groundwater recharge
River baseflow
Erosion (water that doesn’t infiltrate runs off)
Flooding
Contaminant movement
Soil Physics 2010

4. Soil properties important to infiltration:
Texture, pore size distribution
Hydraulic conductivity
Structure (including macropores for air escape)
Antecedent (initial) water content
Wettability
Layering
Infiltration is driven by both gravity and matric potential
Drier soil has greater matric potential pulling water in, and more porosity available to hold that water.
Soil Physics 2010

5. More infiltration Less infiltration
Soil management affects infiltration
More infiltration
Good soil structure
Plants, root channels
Tillage (occasionally)
Organic matter
Drained
Less infiltration
Roads, roofs, etc.
Compaction
Tillage (frequently)
Bare soil surface
Soil Physics 2010

6. Infiltration rate over time
Infiltration rate i(t), cm/hr
Why this decrease?
At short times:
Air escapes more easily
Greater hydraulic gradient
Soil Physics 2010

7. 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

8. 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

9. (discussed further in next file)
The wetting profile q
saturated
Initial volume wetness qi
transmission zone
Note: not saturated!
wetting front
depth
Why is the
wetting front sharp?
(discussed further in next file)
Soil Physics 2010

10. 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

11. 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

12. Measuring infiltration: the tension infiltrometer
(Developed in part here at ISU. Patent holders are Ankeny, Horton, and Kaspar)
Water is applied to the soil surface at a negative pressure
Bubble tower
Reservoir
Steady infiltration at a given tension y gives estimate of K(y)
Soil Physics 2010

13. 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

14. These models have 2 main purposes:
Infiltration models
Green & Ampt (1911)
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

15. 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

16. 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

17. 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