Infiltration 1-3
Jake’s lectures
Toby’s & Jake’s notes combined
Soil Physics 2010
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
Infiltration
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
Soil properties
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
Soil management affects infiltration
More infiltration
Less infiltration
•
•
•
•
•
•
•
•
•
Good soil structure
Plants, root channels
Tillage (occasionally)
Organic matter
Drained
Soil Physics 2010
Roads, roofs, etc.
Compaction
Tillage (frequently)
Bare soil surface
Infiltration rate i(t), cm/hr
Infiltration rate over time
Why this decrease?
At short times:
Air escapes more easily
Greater hydraulic gradient
time
Soil Physics 2010
Infiltration rate i(t), cm/hr
Infiltration rate over time
Infinite at time t = 0?
i(t) can’t exceed precipitation rate
Zero at time t = ∞?
time
Soil Physics 2010
Infiltration rate i(t), cm/hr
Infiltration rate over time
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
time
Soil Physics 2010
The wetting profile
q
saturated
transmission zone
depth
wetting front
Why is the
wetting front sharp?
(discussed further in next file)
Soil Physics 2010
Note: not saturated!
Initial
volume
wetness qi
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
Soil Physics 2010
Double-ring
Measuring infiltration
Water is applied
to the soil
surface at a
positive pressure
Soil Physics 2010
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
Measuring infiltration: the tension infiltrometer
(Developed in part here at ISU. Patent holders are Ankeny, Horton, and Kaspar)
Steady infiltration at
a given tension y
gives estimate of K(y)
Soil Physics 2010
Reservoir
Water is applied to
the soil surface at a
negative pressure
Bubble
tower
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
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
Kostiakov’s model
i(t)
t
i t   Bt
n
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
Green & Ampt’s model
i(t)
ic
t
I t    i t  dt
Soil Physics 2010
b
i t   ic 
I t 
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
Philip’s model
i(t)
ic
t
Soil Physics 2010
i t   ic 
s
2 t
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)