3e Soil Movement

total available water-holding capacity

  • accounts for all plant available water

  • soil properties differ between horizions

total awhc example

see slides for calculations

estimating total awhc using texture

  • texture is a primary factor influencing soil moisture retention

  • the plant available water in a horizon can be estimated as follows:

    • for example, if a soil horizon had a loamy, very fine sand (LVFS) texture and was 27cm thick, the awhc is

      • 27cm soil *.10cm water/cm soil=2.7 cm water

  • coarse fragments are excluded from this estimate because rocks don’t hold water

  • if a soil horizon was 35cm thick, had 20% coarse fragments and a sandy clay structure (SC) the awhc is:

    • AWHC = 35cm soil x 0.15 cm water/cm soil x (1-0.20) = 4.2 cm water

  • when estimating total awhc you MUST:

    • calculate to a depth of 150 cm or depth of root limiting layer

    • assume the bottom horizon extends to 150 cm if the hole was only dug to a shallower depth

  • root limiting layers include:

    • r horizons

    • cr horizons

  • “water retention class” is determined as follows:

soil moisture retention curve

  • aka:

    • water content-matric potential curve

    • soil moisture curve

    • soil moisture characteristic curve

    • soil moisture characteristic

  • a graph depicting the relationship between soil moisture content and soil tension

  • soil moisture potential range

    • 0 kpa(water table) to -106kpa (oven dry)

    • sometimes graphed on a log scale

    • sometimes graphed w a break in the axis

  • gravimetric method

    • “gold standard”

    • requires destructive sampling

    • water content = (wet weight - dry weight)/dry weight

soil moisture retention curve and texture

  • clays:

    • high surface area

    • high porosity

    • most water storage

  • sands:

    • lowest surface area

    • lowest porosity

    • least water storage

  • loams:

    • in the middle

tensiometer

  • consists of a water filled stiff plastic tube, a porous tip, and a vacuum gauge

  • water is pulled out of the tensiometer and into the soil by tension

  • a vacuum forms in the tube equal to the tension of the soil near the tip

neutron probe

  • one of the more accurate moisture measurement methods

  • detects density of the slow neutron cloud surrounding the probe/detector

  • the wetter the soil, the more dense the cloud

tdr probe (time domain reflectomatry)

  • developed originally for sensing breaks in buried power lines

  • electrical pulse sent out and reflected back to sensor

  • time it takes depends on soil moisture