Soil Properties andSoil Water Movement

Environmental Hydrology

Lecture 4

Objectives

• Understand soil properties affecting soil water retention and movement

• Understand physics governing soil water movement in unsaturated zone

• Learn methods for measuring soil physical properties and infiltration rates

• Explore effects of landuse and disturbance on infiltration into soils

Photo credit: Dept. of Land Resources & Environmental Science, Montana State University

Soil texture

Ward & Trimble, Fig 3.1

20% sand

70% silt

10% clay

Soil texture

Soil textural triangle from Christopherson, 2001. Elemental Geosystems. Prentice Hall

Soil matrix

Vt = Va + Vw + Vs

… or

Vt = Vv + Vs

Ward & Trimble, Fig 3.3

Porosity

Vt = Va + Vw + Vs

Vt = Vv + Vs

Measure of void spaces in a soil, expressed as fraction or percentage

… by substitution

porosity (n) = 1 – Vs / Vt

Soil water content

Volumetric water content (v) = Vw / Vt

Gravimetric water content (g) = mw / ms

Bulk density

Bulk density (b) = mt / Vt

Dry bulk density (dry) = ms / Vt

…we’ll derive some of these for soils sampled in lab!

Soil water movement

Image credit: Michigan State University Extension

What controls soil water movement?

Ward & Trimble, Figure 3.4.

Surface tensions for various interfaces(from CRC Handbook of Chemistry & Physics, 68th ed.)

Substance Formula In contact with Temp (oC) (dynes/cm)

Water H2O Air 20 72.75

Acetone C3H6O Air 20 23.70

Benzene C6H6 Air 20 28.85

Water 20 35.00

Mercury Hg Water 20 375

Soil water movement

gravitational forces = surface tension forces*

(r12) h w g = (2r1) cos

* Under equilibrium conditions. see Ward & Trimble eq 3.12 for definition of terms

To move water through the soil, gravitational forces must exceed surface tension forces

Soil Moisture Characteristic Curves

Ward & Trimble, Figure 3.5.

Pore geometry controls relationship between moisture content and pressure (or soil suction).

hysteresis

Shape of characteristic curve depends on soil texture

Ward & Trimble, Figure 3.6

Ward & Trimble, Figure 3.9.

Water available to plants depends upon soil texture (because of pore size effects on filling/drainage!)

Flow through porous media

Q = K dh/dL

L

where K = hydraulic conductivity.

But, unsaturated hydraulic conductivity is a function of moisture content and soil suction!

Measuring water movement in the unsaturated zone

Double-ring infiltrometer (infiltration rate)

Guelph permeameter (hydraulic conductivity)

Tensiometers (soil tension)

Time domain reflectometry (soil moisture)