Soil Physics 2010

Outline

• Announcements

• More tension infiltrometers

• More infiltration

Soil Physics 2010

• One more review session in G217 Agronomy, today, noon – 2:00 pm.

• Homework 5 due Wednesday after Spring Break

• Homework 5 is now posted. There is a pdf file, and an Excel file.

Announcements

Measuring infiltration

Tension infiltrometer(developed in part here at ISU;patent held by Ankeny, Horton, & 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

Soil Physics 2010

Soil Physics 2010

Mariotte bottle

How do you supply water under tension?

h

Air coming out of this hose has a suction h:That’s what is needed to pull air down, and bubbles out of the inlet tube.

Air inlet

Air outlet

Pulling air down is like

pulling water up

Soil Physics 2010

Mariotte bottle

Mariotte bottle, part 2Air inlet

This supplies water at a constant tension – as used in the hydraulic diffusivity experiment.

B

h

Soil Physics 2010

Mariotte bottle

Mariotte bottle, part 3

h

Air inlet

This is a useful way to supply water at a constant head…

or to maintain water at a constant height.

Soil Physics 2010

Mariotte bottle, part 4

h1

Water exits tube at

h = –h1 + h2

Control Mariotte

Supply Mariotte

h2

Change h by moving end of tube up or down.

Res

ervo

ir

Bubble tower

Soil Physics 2010

Tension infiltrometerMariotte control bottle (“bubble tower”)

and supply bottle (“reservoir”) in a single portable unit.

Big reservoir, small h controlFewer holes in big reservoir

Res

ervo

ir

Bubble tower

Soil Physics 2010

The point of the tension infiltrometer

The tension infiltrometer allows field measurement of a few points

on the K(h) curve, near saturation where it changes

fastest.

i

Soil Physics 2010

Why is the wetting front sharp?

z

L

K(

)

1) Need less gradient when is near s. Need big gradient at low .

2) If it weren’t, it would become sharp.

3) At the front, capillary forces dominate. Behind the front, gravity rules.

Soil Physics 2010

Why is the wetting front sharp?

i

z

3) At the front, capillary

forces dominate. Behind the front, gravity rules.

Behind the front, m near zero. Water in large pores can stay in large pores → high K.

At the front, m is strong. Water in large pores is pulled into smaller pores → K drops.

i

Soil Physics 2010

More about infiltrationSpecifically the Green & Ampt model

z

m initial condition

m = 0 at saturation

L

Potential difference from surface to wetting front:

Distancefrom surface to wetting front:

L + m

L

Gradient

Soil Physics 2010

Green & Ampt model

i

z

L

L + m

L

Gradient

As I (infiltration) increases, gradient decreases.

0

5

10

15

20

0 1 2 3 4 5

infi

ltra

tio

n r

ate,

cm

/hr

time, hrs.

soil-limited

soil and rain-limited

tIb

iti c

t

s ti

bKti

dttitI

Soil Physics 2010

Green & Ampt model

i

z

0

5

10

15

20

0 1 2 3 4 5

infi

ltra

tio

n r

ate,

cm

/hr

time, hrs.

soil-limited

soil and rain-limited

antecedent volume wetness: 0.23Suction at wetting front: 433 cmporosity 0.47Ksat 0.1 cm/hrtime step 0.025precipitation rate 5 cm/hravailable porosity 0.24

Instantaneous raterain-limited soil- rain and soil

mean time, cumulative distance to hydraulic limited limited cumulativehr precip wetting front gradient infilt infilt infiltration

0.025 0.125 0.104 4157.80 415.780 5.000 0.1250.050 0.250 0.208 2079.40 207.940 5.000 0.2500.075 0.375 0.313 1386.60 138.660 5.000 0.3750.100 0.500 0.417 1040.20 104.020 5.000 0.500

Soil Physics 2010

Philips model

Recall that horizontal infiltration can be modeled as a diffusion process, with x(t) t½

But for vertical infiltration, the gradient is always at least 1, so x(t) → Ks at large t.

So for vertical infiltration, the short-time infiltration rate is i(t) t½ , but the long-term behavior is more like i(t) t.

Soil Physics 2010

J. R. Philips on infiltration

Infinite series solution, with the first 2 terms dominating:

...22

3

24

3002 tA

tAtKA

t

sti

Kt

sti

2 KttsdttitI

Early time: diffusion term dominatesLate time: constant term dominates

– conceptually like Green & Ampt