Vadose Zone Notes
Field Camp 2000
page: 1
Vadose Zone




Region between the water table and the ground surface
Pores filled with water and air; unsaturated or partially saturated
May be locally saturated; perched water table
How to characterize storage and transmission in vadose zone?
Flow process and related effects of interest in many disciplines
 Agronomy
Effects on crops: water/air flow, structure, chemistry, plant
biology, insects, worms, microbes.
Drainage: too much/not enough—crop dependent
Nutrients: Necessary to support crops, can supplement
Strength, adhesion: plowing
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


Soil physics: Movement of water, solutes
Soil chemistry: Reactions, transport
Soil development/classification
Civil engineering
Physical characteristics
Strength: Bearing capacity, excavation
Landslide:
Shallow slides following short, heavy
rain
Deep seated slides after long rain,
recharge and gw effects on slope
stability
Permeability: Drainage for building, dams
Flow thru landfill liners and caps
Leakage out of canals
Infiltration: Runoff
Flooding
Filtration of solids: Septic systems (transport of
viruses),
Shringe/swell (change in moisture) foundations
Frost heave (volume change from freezing):
Erosion and sedimentation: gulley formation, silt
mobilization
Chemical effects
Corrosion of buried structures, tanks
Vadose Zone Notes
Field Camp 2000
page: 2
Durability of building materials


Geology
Weathering of rocks
Soil development
Erosion, landscape development, geomorphology
Hydrogeology
Infiltration and recharge of aquifers
Evaporation and transpiration losses of water
Contaminant source zone/transport
BASIC PHYSICAL CHARACTERISTICS
Soil: Solids (minerals, organic material)
Pores
liquid (water, organic compounds, salts)
gases (nitrogen, oxygen, CO2, water vapor).
Moisture content, density, porosity
Moisture content, porosity, degree of saturation and related values are ratios of the
weight or volume of basic constituents of soil (solid, water, gas).
w: water content (mass wetness):
 volumetric moisture content:
b: bulk or wet unit weight:
w: unit weight of water
d: dry unit weight:
s: unit weight of soil solids
e: void ratio:
n: porosity:
S: degree of saturation:
weight water/weight solids
volume water/total volume
total weight/total volume
1 gm/cm3; 62.4 lbs/ft3
weight solids/total volume
weight solids/volume solids
volume voids/volume solids
volume voids/total volume
volume of water/volume pores
Characterizing basic soil properties with samples
You can obtain all the basic soil properties listed above by obtaining a
sample of a soil and measuring it in the laboratory.
1. Determine water content w, bulk unit weightb, dry unit weight d. To
do so requires knowing the volume of the sample in place, the initial
weight, and the weight after drying. Volume is obtained by measuring
Vadose Zone Notes
Field Camp 2000
page: 3
the volume of the hole from which the sample was obtained, or using a
thin-walled sampler and extruding, or by weighing the sample in air
and water (coat the sample with wax to prevent slaking).
2. Calculate volumetric water content:  = w (d/w)
3. Calculate porosity using n = (s - d)/s = 1- d/s
Usually you would measure s in the lab, but for reconnaisance studies you
can assume: s = 2.65 gm/cm3.
Calculate void ratio using: e = n/(1 - n).
4. Calculate degree of saturation: S 
w s
e
Moisture content in the field
Often it is infeasible to obtain samples for measuring moisture content.
There are a variety of methods for measuring moisture content in situ using
special sensors.
Electrical resistance
ER of soil depends on water content + texture, salts,
mineralogy, etc.
ER of uniform material (gypsum, nylon, fiberglass)
depends mostly on water content
So, make block of uniform material and install electrodes,
calibrate resistance versus moisture content.
Nylon, fiberglass: resistance function of
water and electrolytes in water.
Gypsum (plaster of paris): electrolyte
concentration constant=saturated calcium
sulfate. So independent of electrolyte
concentration.
Moisture blocks will deteriorate with time.
Resistance is temperature sensitive, must
compensate. But can be connected to a
recorder.
Neutron probe
Source of fast neutrons created by mixing an
alpha emitter (radon or americium)
with beryllium.
Vadose Zone Notes
Field Camp 2000
page: 4
Fast neutrons emitted and collide with atoms
in the soil, slow down from
collisions.
The loss of kinetic energy is greatest for
collisions with nuclei of similar
mass. Hydrogen mass closest to
neutrons.
Device counts the number of slow neutrons.
Slow neutrons proportional to the
amount of hydrogen (water) present.
Sphere of influence. Averages over volume,
size depends on water content
(football size)
Other sources of hydrogens will also scatter
neutrons. PVC casing, organic
material, hydrated minerals.
+ Measure moisture profile using access
tube
+ Relatively easy to do, fast
- Requires calibration for each hole or site
- Safety
- Effectiveness reduced with PVC, clay
- Average value/ sensitivity
Time Domain Reflectometry
Uses very high frequency electromagnetic
pulse
Coaxial cable and ss waveguide/ simple
Measures dielectric constant
Water = 78
Solids = 2 to 4
Air = 1
Correlate apparent dielectric constant with
water content.
+ Relatively independent of soil type.
+ Easy to do
+ Rapid
- Requires some interpretation of wave
- Expensive electronics
Strength
Compressive and shear strength
Vadose Zone Notes
Field Camp 2000
page: 5
Agriculture; plowing
Geotechnical; Construction, bearing capacity, landsliding
Character of soil
Strength of hand sample
Composition
Water content
Cementation
Measurement
Laboratory tests using load frames
Field tests
Shear vane
Penetration testing
Simple Chemistry
Some important ions
Ca2+, Mg2+, Na+, K+, Fe3+
HCO3-, SO42-, Cl-, NO3pH (activity of protons)
Measure pH of water or slurry
Use meter with electrode
Measure water directly
Make 1:1 (soil:deionized water by volume) for soil
Hydrogen ions (proton) produced by micoorganisms in soil
C (organic matter) + O2  CO2
CO2 + H2O  H2CO3
H2CO3  HCO3-+ H+
HCO3- CO3-+ H+
Hydrogen ions produced by weathering of aluminosilicates
Al3+ + H20  Al(OH)3 + 3H+
Expect to find low pH in A horizon, or in organic-rich sediments.
pH buffered by carbonate, expect increase in pH with depth where soils
contain carbonate.
Eh (activity of electrons)
Measure of oxidizing/reducing, aerobic/anaerobic
Express in mV
Measure electrical potential relative to potential defined by a halfreaction, usually Ag-AgCl.
Vadose Zone Notes
Field Camp 2000
page: 6

Solid and dissolved species present will depend on pH and Eh.
Microbial reactions will also depend on pH and Eh conditions.
Porbaix diagrams show stability fields of mineral and ionic species.
Distribution of carbonate species as a function of pH at 20cC. From Fetter, 1994.
Table 1
pH
2.00
3.00
4.00
5.00
6.00
6.38
7.00
8.00
9.00
10.00
10.38
11.00
12.00
I 3.00
Carbonic Acid
99.99%
99.9%
99.6%
96.0%
70.6%
50.0%
5.2%
2.3%
Bicarbonate Ion
0.01%
0.04%
0.4%
4.0%
29.4%
50.0%
94.8%
97.7%
96.0%
70.6%
50.0%
5.2%
2.3%
0.2%
Carbonate ion
4.0%
29.4%
50.0%
94.8%
97.7%
99.8%
Vadose Zone Notes
Field Camp 2000
page: 7
Vadose Zone Notes
Field Camp 2000
page: 8
FLOW IN PARTIALLY SATURATED SOIL
Flow in partially saturated soil obeys Darcy’s Law, just as flow in
saturated material. This means that flux is proportional to gradient in head, and
the ratio of the flux and the head gradient is the hydraulic conductivity. Flow in
partially saturated soil differs from flow in saturated soil, however, because the
hydraulic conductivity in partially saturate soil decreases with the moisture
content. In saturated soil, the hydraulic conductivity is constant
Hydraulic head in partially saturated soil: Soil water potential
Hydraulic head in partially saturated soil is termed soil water potential, or matric
potential. Recall that
Hydraulic head = Energy/(unit weight volume)
Units of pressure and pressure head
1 atm = 1.01325 bars = 101.325 kPa = 760 mm Hg = 10.333 m H20 =
14.69 psi = 33.9 ft H20 = 29.921 inches Hg
Total head = Pressure head + elevation head
 = h + p/
No downward flow will occur if the head is uniform with depth. This will occur if
the pressure head pressure head changes linearly with depth to offset change in
elevation head.
Total head
elevation head
Pressure head
Assume equilibrium with soils in vadose zone.
Measure pressure head relative to atmospheric pressure
Vadose Zone Notes
Field Camp 2000
page: 9
Pressure head = 0, atmospheric, at w.t.
Implies that pressure heads in vadose zone are negative, suction.
Negative potential in soil= matric potential, matric suction. Negative with respect
to atmospheric, but often presented as positive.
Similar to capillary tube, development of negative pressure. Curvature of water in
the pore indicates that there must be a pressure difference. Water also attracted to
soil surfaces by adsorption, so capillary bundle not complete
Can have matric suctions of many bars in dry soil.
Measuring Soil Moisture Potential
Piezometer measures potential in saturated system
Piezometer fills with air in partially saturated, no good in vadose zone
Use a fine-grained ceramic on tip of access tube = tensiometer
The ceramic is hydroscopic, design so air pressure of 1 atm required to enter pores
of ceramic. Tensiometers only work for suctions less than about 0.7 bars.
Measure pressure (suction) inside tube=pressure head in soil
Total head = pressure head (from gauge) + elevation head.
Note that suction head is usually given as a positive value, and vacuum gauges
will read positive values. However, you suction head is negative value if we stick
with the reference frame of hydrology, where p/ = 0 defines the water table,
p/0 below the water table, and p/0 above the water table.
Use to estimate water content at a point
Relative: irrigation
Absolute from soil moisture characteristic
Use to determine direction of water flow
Water infiltrating to recharge aquifer, or evaporating?
Vadose Zone Notes
Field Camp 2000
page: 10
Vacuum gauge reads positive value. Gauge = d - p/
 = h + p/definition
Gp = gauge x (-10) change units
d
H
p/ = d + Gp
Porous cup
definition
 = h+ d + Gp
p/
h
Datum (arbitrary elevation)
Determining the vertical hydraulic gradient
The vertical hydraulic gradient can be determined using two tensiometers at
different depths. Assume that z, the vertical axis, points upward. The hydraulic
gradient is the difference in head divided by the difference in elevation between
two measurement points. A negative hydraulic gradient means the head decreases
in the upward direction. A negative hydraulic gradient defined in this manner
means that water is moving upward through the soil.
Example Exercise
You install a cluster of tensiometers at different depths and measure the
pressure head on the gauge at each one. Determine the total head at each
tensiometer and determine the hydraulic gradient. What direction is the
water moving?
Vadose Zone Notes
Field Camp 2000
page: 11
Depth
(cm)
30
60
90
120
150
Gauge
(cbar)
35
20
15
12
10
G x -10
(cm)
-350
-200
-150
-120
-100
P/
(cm)
-320
-140
-60
0
50
h
(cm)
120
90
60
30
0
Total head
(cm)
-200
-50
0
30
50
Gradient
Depth
(cm)
30
60
90
120
150
Gauge
(cbar)
12
16
18
14
12
G x -10
(cm)
P/
(cm)
h
(cm)
Total head
(cm)
Gradient
Problems with tensiometers
1.
2.
3.
4.
5.
6.
Must be good contact between porous cup and soil. Same pore sizes
Fragile
Air coming out of solution will affect performance of gauge
Takes time to equilibrate
Suctions greater than 1 atm, water boils (0.8 practical upper limit).
Temperature differences cause pressure changes (shield from sun).
5
1.66
1
0.66
Vadose Zone Notes
Field Camp 2000
page: 12
Soil Moisture Characteristic Curves
Relation between moisture content and soil suction.
Depends on structure of soil pores.
How water drains from pores during suction/wetting
Shape of curve depends on distribution of pore sizes
Air-entry suction
saturated
clay
Water
content
sand
Irreducible moisture for sand
suction
Soil-moisture characteristic curves for sand and clay

Soil may remain saturated until critical value of suction is reached=air entry
suction (bubbling)

Characteristic curve decreases gradually for material with broad range of pore
sizes (silty clay).

Curve decreases abruptly for narrow range of pore sizes (well sorted sand)

Curve approaches irreducible moisture content at high suction

Field capacity: Moisture content when soils have drained by gravity alone

Wilting point: Moisture content of a soil when plants (sunflower used as
reference crop) are unable to grow.

Absolute water capacity: Difference between the Field Capacity and Wilting
Point. Water available for plants.
Vadose Zone Notes
Field Camp 2000
page: 13
Hydraulic conductivity as function of moisture content
K decreases with increasing suction/decreasing moisture
May decrease over several orders of magnitude as soils go from saturated to damp.
Hydraulic conductivity of sand decreases faster than clay as soils drain
Sands used as a capillary barrier. Impede flow at low water contents, act as a
drain if saturated—with proper design.
Ksat
clay
Log K
sand
suction
Hydraulic conductivity of clay and sand as a function
of suction in soil
It is often convenient to use a function to describe the hydraulic
conductivity as a function of suction. One of the simplest functions was proposed
by Gardner as
K = Ksat -p
The constant  depends on the type of soil, and it has units of 1/L. It is possible to
use this equation to estimate the hydraulic conductivity of partially saturated soils
using tensiometer data and information about the saturated hydraulic conductivity.
Vadose Zone Notes
Field Camp 2000
page: 14
Estimating hydraulic conductivity of partially saturated soils
The hydraulic conductivity of soils can be measured as a function of the
suction, or the water content, although such measurements are difficult to make.
What is often done is to estimate the parameters in a function that describes the
hydraulic conductivity as a function of suction. For example, a technique that
would allow us to estimate Ksat and  would provide a means to estimate K under
partially saturated conditions.
The Guelph pemeameter is designed to measure Ksat and in partially
saturated soils. It works by holding the head in an initially dry borehole at a
constant level and measuring the flow into the boring. This is done using a clever
mechanism called a Marriotte bubbler. The flowrate required to maintain constant
head is determined, then the head is changed slightly and the processes is repeated.
Those two flowrates, and the two heads are then used to calculate K and .
Water flux in partially saturated soil
Once K and  are known, it is then possible to calculate K using
tensiometer data. The head gradient in partially saturated soil was determined
earlier. The product of the head gradient and K is the flux. Thus, it is possible to
estimate the vertical flux in the vadose zone using these methods. A downward
flux that is beyond the root depth would be equal to, or at least related to, the
recharge flux. An upward flux would be the rate of evapotranspiration.