Soil Physics 2010

Outline

• Announcements

• Where were we?

• Measuring water potential

• Unsaturated flow

Soil Physics 2010

Announcements

• Giant clam-eating shark found in Kansas (no kidding! See

http://news.bbc.co.uk/earth/hi/earth_news/newsid_8530000/8530995.stm)

• Homework 4 due March 3. File posted online: http://www.agron.iastate.edu/soilphysics/SP_HW4.pdf

Soil Physics 2010

Competition for pores

m

draining

wetting

When both water and air are present (middle range):

• Water is in the smaller pores

• Air is in the larger pores

Where were we?

Matric potential

Overview

Soil Physics 2010

Where’s the wilting point?

1.5 MPa(15 bars)

Soil Physics 2010

Measuring water potential

TensiometerPsychrometerEmbedded medium

Soil Physics 2010

Tensiometer

porous ceramic cup

water-filled shaft

cap, tightly sealed

sealed refiller (optional)

Water in shaft equilibrates with soil water through the ceramic cup.

Water in the tensiometer is at negative pressure

Water cavitates

around -0.085 MPa

Wanted:m here

Soil Physics 2010

TensiometerWater in shaft equilibrates with soil water through the ceramic cup.

Water in the tensiometer is at negative pressure

Water cavitates

around -0.085 MPa

• Use degassed water to delay cavitation

• Many caps have a manual vacuum pump

• Check and refill frequently when soil is dry

• Volume of water added to surrounding soil is small

Pressure transducer

Mercury& water

manometer

Vacuum gage

Soil Physics 2010

Reading the tensiometer

Like a certain exam question… equal force pulling down on

each side

Mercury& water

manometerSoil Physics 2010

Reading the mercury tensiometer

m

w

HgHgwm hh

Equal downward force on each side

Tube has water and mercury – no air

hw

hHg

w

HgHgh

m

w

wwh

Soil Physics 2010

New: polymer tensiometerMartine van der Ploeg

• Polymer swells on absorbing water

• Pressure transducer measures pressure due to swelling

• Good to -1.6 MPa

Soil Physics 2010

Psychrometer

• Small soil sample placed in holder

• Close door, start

• Sample equilibrates with chamber air (2-5 minutes)

• Measure water vapor pressure p of chamber air

• Calculate psy:

0

lnp

p

M

RTompsy

Soil Physics 2010

Embedded medium

Noborio

• Insert into the soil a material with a known (m) relationship

• The known material equilibrates with the soil

• Measure in the known material

• Read the soil’s m from the known (m) curve

Decagon

Soil Physics 2010

Unsaturated flow

You’re on, Joyce(and then Richards’ equation was derived)

Soil Physics 2010

m

draining

wetting

• Water is only in the smaller pores, which have lower flow (Poiseuille’s law)

• Air fills the larger pores, so:

• Less cross-sectional area for flow

• More tortuous flowpath, because water has to flow around big pores

Why does K decrease as decreases?

Rawls. Brakensiek, and Saxton, 1982

Ks versus K(S), 1982 predictions

Soil Physics 2010