Lecture 17
Vadose Zone
Fetter 6.1-6.5
Vadose Zone
Unsaturated/Vadose Zone or zone of Aeration
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| Vapor | liquid | solid | |||
| water, gases |
water, contaminants |
soil |
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| Definitions: | |||
Porosity
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Void ratio
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| V = Vv + Vs | |||
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| Gravimetric Water Content of Soil- | |||
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 g = Gravametric water content (%) |
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| Ww = Mass of water in soil (M) | |||
| Ws = Mass of solid particles (M) | |||
| Volumetric Water Content- | |||
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 = Volumetric water content (dimensionless ratio) |
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| Vw = Volume of water in rock | |||
| Saturation Ratio- | |||
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| Tells what the ratio of water filled voids is. | |||
| Dry Bulk Density- |  |
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| Particle Density- |  |
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n = 100 |
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n = 100 |
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How do you measure water content
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| 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 | ||||||
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| Volumetric water content - | ||||||
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Indirectly,
| (1.) Neutron moisture logger | ||
| -put on access tube in soil | ||
| -lower probe which measures H atoms(Neutrons) | ||
| -must calibrate | ||
| (2.) TDR Time Domain Reflectometery | ||
| -conductivity | ||
Example:
| Measure |  |
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n =  |
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| 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
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| h = height of capillary rise (L) | |
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 =
the angle of meniscus with the capillary tube (degrees) |
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| g = (L/T2) | |
| R = radius of capillary tube (L) |
Example-
| water at 18oc | ||
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| Assume soil is bundle of tubes(pore throats) | |
| hc depends on the widest part of pore. | |
| fine or coarse grained soils have greater hc | |
| Sediments | Capillary Rise (cm) |
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| Fine silt | 750 |
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| coarse silt | 300 |
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| very fine | 100 |
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| fine sand | 50 |
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| medium sand | 25 |
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| coarse sand | 15 |
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| very coarse sand | 4 |
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| fine gravel | 1.5 |
| Smaller pore openings have greater tension | |||
| Pore openings are not uniform | |||
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Capillary Fringe - Zone above water table where water is drawn upward by capillary
action.
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 
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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
moisture.
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
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| 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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ENV 302 - Lectures
