Soil Properties and Soil 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, Elemental Geosystems. Prentice Hall

Soil matrix V t = V a + V w + V s … or V t = V v + V s Ward & Trimble, Fig 3.3

Porosity V t = V a + V w + V s V t = V v + V s Measure of void spaces in a soil, expressed as fraction or percentage … by substitution porosity (n) = 1 – V s / V t

Soil water content Volumetric water content (  v ) = V w / V t Gravimetric water content (  g ) = m w / m s

Bulk density Bulk density (  b ) = m t / V t Dry bulk density (  dry ) = m s / V t …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, 68 th ed.) SubstanceFormulaIn contact withTemp ( o C)  (dynes/cm) WaterH2OH2OAir AcetoneC3H6OC3H6OAir BenzeneC6H6C6H6 Air Water MercuryHgWater20 375

Soil water movement gravitational forces = surface tension forces* (  r 1 2 ) h  w g = (2  r 1 )  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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)