Lecture 17

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Vadose Zone


Fetter 6.1-6.5


Vadose Zone


Unsaturated/Vadose Zone or zone of Aeration


3 phase system
Vapor liquid solid

water, gases


water, contaminants






Void ratio
V = Vv + Vs
Gravimetric Water Content of Soil-
g = Gravametric water content (%)
Ww = Mass of water in soil (M)
Ws = Mass of solid particles (M)
Volumetric Water Content-
= Volumetric water content (dimensionless ratio)
Vw = Volume of water in rock
Saturation Ratio-
Tells what the ratio of water filled voids is.
Dry Bulk Density-
Particle Density-
n = 100
n = 100



How do you measure water content


Collect soil sample
Gravimetric - dry soil in oven at 105 degrees (do not dehydrate minerals)
Measure Wm (pre-drying) wet
Ws (post drying) solids
Mass of water = Wm - Ws
Volumetric water content -




(1.) Neutron moisture logger
-put on access tube in soil
-lower probe which measures H atoms(Neutrons)
-must calibrate
(2.) TDR Time Domain Reflectometery



n =
Remember, water table is surface where pore water pressure = atmospheric
-below water table, pore water pressure > atmospheric
-above water table, pore water pressure < atmospheric
Water under Tension in vadose zone
Near the water table- water subject to upward attraction due to surface tension and molecular attraction of liquid and solid phase capillary.


The attraction of solid for liquid draws water up the tube until weight of column of water

offsets upward force.


 Rise of fluids in capillary tube


h = height of capillary rise (L)
= the angle of meniscus with the capillary tube (degrees)
g = (L/T2)
R = radius of capillary tube (L)



water at 18oc


Assume soil is bundle of tubes(pore throats)
hc depends on the widest part of pore.
fine or coarse grained soils have greater hc

Capillary Rise (cm)

Fine silt


coarse silt


very fine


fine sand


medium sand


coarse sand


very coarse sand


fine gravel



Smaller pore openings have greater tension
Pore openings are not uniform
Capillary fringe is irregular


Capillary Fringe - Zone above water table where water is drawn upward by capillary



If water table is close to land surface, capillary forces can provide direct evaporation.



Fluid Pressure in Vadose Zone is negative.


Tensiometer - used to measure negative pressure head .


Field Capacity - maximum amount of water that soil can hold against gravity.


If more soil moisture than soil tension can hold then gravity drainage


Wilting Point - soil moisture content below which plants are unable to withdraw soil



i.e. sandy loam soil - soil moisture curve



Summary Information on Vadose Zone Water State Measurement and Monitoring Methods


 Method  Property Measured  Accuracy/Range
Vadose Zone Soil Water Potential Measurementa
Porous Cup Tensiometers Capillary pressure 0 to -85 kPab

0 to -80 kPac

Thermocouple Psychrometers Relative humidity -200 to -8,000 kPab

-100 to -5,000 kPac

Water Activity Meter Relative humidity 0 to -31,600 kPa
Resistance Sensors Resistance -50 to -1,500 kPac

    Gypsum Blocks

Resistance 0 to -30 kPab

    Fiberglass/Nylon Cells

Resistance No limitsb
Electrothermal Methods Heat transfer 0 to -200 kPa
Osmotic Tensiometers Osmotic + pressure potential 0 to -1,500 kPab
Filter-Paper Method Water content -10 to 100,000 kPa
Electro-Optical Sensors Optical properties 0 to -2,400 kPa
Vadose Zone Soil Water Content Measurementa


Gravimetric Weight d
Gamma-Gamma Radiation d
Neutron Moisture Probe Radiation d
Dielectric Sensors Dielectric d
Time Domain Reflectometry Dielectric d
Nuclear Magnetic Resonance Magnetic field d
Electro-Optical Sensors Optical properties d
CAT Scan Radiation d
Thermal Infrared Remote sensing d
Active Microwave Remote sensing d
Four-Electrode Method Resistivity d
Salinity Sensors Conductivity d
Electromagnetic Induction Conductivity d


Boldface = most commonly used methods


aMoisture content can be determined form measurement of soil water potential and vice versa

by the use of a moisture characteristic cure, which relates matric potential to water content.

The pascal is the Standard International unit for measuring pressure used by the Soil Science

Society of America. The bar is commonly used as a pressure unit in vadose zone

investigations: 1 kPa = 1 centibar.

bIndicated by Rehm et al. (1985).

cIndicated by Bruce and Luxmoore (1986).

dMost methods for measuring moisture content are accurate to around 1%. Gravimetric methods

and nuclear methods can be accurate to 0.1% or less.


Three types of porous-cup tensiometers.


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