Soil Physics 2010

Infiltration 1-3

Jake’s lecturesToby’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 • 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

Infiltration rate over time

time

Infi

ltra

tion

rat

e i(t)

, cm

/hr

Why this decrease?At short times:

Air escapes more easilyGreater hydraulic gradient

Soil Physics 2010

Infiltration rate over time

time

Infi

ltra

tion

rat

e i(t)

, cm

/hr Infinite at time t = 0?

i(t) can’t exceed precipitation rate

Zero at time t = ∞?

Soil Physics 2010

Infiltration rate over time

time

Infi

ltra

tion

rat

e i(t)

, cm

/hr

i(t) can’t exceed precipitation rate

As t → ∞, i(t) → Ks

Infiltration rate can be either soil-limited or rain-limited

Not really. Soil behind (above) the wetting front isn’t 100% saturated. Some people write ic instead.

Soil Physics 2010

The wetting profile

dep

th

Initial volume

wetness i

wetting front

saturated

transmission zone

Why is the wetting front sharp?

(discussed further in next file)

Note: n

ot saturated

!

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.

Soil Physics 2010

Single-ring Double-ring

Measuring infiltration

Water is applied to the soil surface at a positive pressure

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

Soil Physics 2010

Soil Physics 2010

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

Steady infiltration at a given tension gives estimate of K()

Res

ervo

ir

Bubble tower

Measure baseflow before rainfall

Measure rainfall Measure streamflow Estimate runoff by

baseflow separation Estimate: Infiltration

= rainfall - runoff

Soil Physics 2010

Estimating infiltration at the scale of a catchment (watershed):

Green & Ampt (1911) Kostiakov (1932) Philip (1957)

Soil Physics 2010

Infiltration models

There are many others, but we won’t study them.

These models have 2 main purposes:Explain the observed infiltration patternsPredict future infiltration

Soil Physics 2010

Kostiakov’s model

with i : infiltration rate, L/Tt : time, TB, n : fitting parameters

usually n ≈ 1/2

nBtti

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.

t

i(t)

Soil Physics 2010

Green & Ampt’s model

with i : infiltration rate, L/Tic : final i : i(∞), L/T t : time, Tb : fitting parameterI : cumulative infiltration, L

tIb

iti c

No physical interpretation of b.

Note that i(0) = ∞, and i(∞) = ic.

Assumes all flow is saturated flow

t

i(t)

ic

dttitI

Soil Physics 2010

Philip’s model

with i : infiltration rate, L/Tic : final i : i(∞), L/T t : time, Ts : sorptivity, L/T0.5

t

siti c

2

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)

t

i(t)

ic