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Soil Water Content
Soil Moisture Content
Water that may be evaporated from soil by heating at 1050C to a
constant weight
mass of water evaporated (g)
Gravimetric moisture content (w) =
mass of dry soil (g)
volume of water evaporated (cm3)
Volumetric moisture content (q) =
volume of soil (cm3)
q= w*
bulk density of soil
density of water
3
cm
g

3
cm
g
g
3
cm 3  g g cm
g
g cm 3 g
cm 3
mass of dry soil (g)
Bulk density of soil (r) =
volume of soil (cm3)
Example: A soil is sampled by a cylinder measuring 7.6 cm
in diameter and 7.6 cm length. Calculate gravimetric and
volumetric water contents, and wet and dry bulk densities
using the following data:
1. Weight of empty cylinder
= 300 g
2. Weight of cylinder + wet soil = 1000 g
3. Weight of cylinder + oven dry (1050C) soil = 860 g
Volume of cylinder = p*r2*h = 3.14*(7.6/2)2*7.6 = 345 cm3
Weight of wet soil = 1000 – 300 = 700 g
Weight of dry soil = 860 – 300 = 560 g
Dry bulk density = 560/345 = 1.62 g cm-3
Gravimetric moisture content = (700-560)/560 = 0.25 or 25%
Volumetric moisture content = r *w = 1.62*0.25 = 0.41 or 41%
Know how to do these calculations for
quiz on Friday
Calculating dry soil weight basis of
samples for analysis
Weigh drying pan, moist soil subsample + pan,
Oven dry the subsample at 105C for 24 hr,
Weigh the dried soil + pan.
Calculate the moisture content (w):
w = (g moist soil – g dry soil)/(g dry soil – pan)
Rearrange the eqn to solve for dry soil wt.
Dry soil wt = g moist soil / (1 + w)
Methods for measuring soil
water content
Direct method
Indirect methods
(Gravimetric)
Electrical
properties
(need to calibrate)
Radiation
technique
Acoustic
method
Thermal
properties
Chemical
methods
-Neutron scattering
-g- ray attenuation
Electrical
Conductance
Dielectric constant
- Gypsum blocks
- Nylon blocks
- Change in
conductance
TDR
Principles underlying different methods of
assessment of soil water content
Water Content
Direct
Gravimetric: evaporating water at 1050C (be able to do the calc’ns)
Indirect
Neutron scattering:
Thermalization
Time domain reflectrometry:
Dielectric constant
Soil Water (matric) Potential
In-direct:
Watermark (granular matrix
sensor), gypsum block
Direct: Tensiometer
Calibrating field instruments
http://www.bae.ncsu.edu/programs/extension/evans/ag452-3.html
Gently tap a tube into the soil to take an
undisturbed sample from the center of
the effective root zone.
Trim the soil at each end of the tube to the
tube length so that the soil occupies the
exact tube volume.
Calibration for moisture content
•
•
•
•
Measure and weigh the tube
Weigh the field moist soil + tube
Oven dry the soil from the tube
Calculate:
W = g water/g dry soil = (wet – dry) / dry soil
Db = g dry soil / cm3 volume soil
Θ = (W x Db) / Dw
• Compare lab moisture content to field
measurements
• For water potential, compare water retention
curves derived in lab using pressure plates.
Water retention
curves: Water
content vs pressure
or tension
Note: clay holds more
water at a specific
water potential than
sand or loam;
Water is held tighter
at a given water
content in clay than in
sand.
Structure is
predominant at low
potentials; as soil
dries out, texture is
more important
Effect of structure on water flow
www.soils.umn.edu/.../soil2125/doc/s7chp3.htm
The flow of water in soil
Saturated and unsaturated
flow
Saturated flow
Ksat = Q/A x L/(Ψ1 - Ψ2)
where Q is volume of
water in time (t)
A is area of cross section
Ksat is saturated hydraulic
conductivity of soil (how
fast water moves)
L is length of column
Ψ is the water potential at
points 1 and 2
Flux can be thought of as water flowing from a hose. The flux is the
rate of water discharged by the hose, divided by the cross-sectional
area of the hose.
http://soils.usda.gov/technical/technotes/note6fig1.jpg
Saturated flow in soils
• The pores are full of water and matric
potential is considered to be negligible
because at least some of the water is a
long distance from solid surfaces
• Under these conditions, flow is:
Rapid - moving through large pores
Driven by gravity and sometimes
Hydrostatic pressure if water is ponded
http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm
http://www.montcalm.org/montcalmold/media/planningeduc/tn_gwa5.jpg
Saturation
wet
unsaturated
dry
Unsaturated flow
Soil moisture
content changing
with depth
Unsaturated flow – most common in soils
• Occurs along soil surfaces, not through large pores.
• Driven by matric forces that are much stronger than gravity.
Gravity is not sufficiently strong to exert a significant influence on
unsaturated flow because much of the soil water adheres to solid
surfaces.
• Unsaturated flow is slow.
• Even though the driving force is usually greater than for
saturated flow, the resistance to flow is enormous.
• Water will flow toward a lower (more negative) potential
regardless of direction (up, down, laterally). In other words
it will flow towards:
drier medium
salty solution
finer texture (small pores)
http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm
http://wwwlb.aub.edu.lb/~webeco/SIM215soilwater_files/image004.gif