Laboratory Exercise for Teaching Principles of Water Movement in Soils
D. Eli McMillan
INTRODUCTION
Soil water movement principles are often difficult for students to grasp and
understand, in part because the vocabulary and measurements are difficult for students
to conceptualize. Thus, students benefit from a hands-on demonstration of the
measurements and calculations used to quantify soil water movement. With this
laboratory exercise, we are developing a tool for instructors to use in their teaching of
Darcy’s Law and the associated values of matric potential, flux, and hydraulic
conductivity. These water movement principles are demonstrated to the students by
means of a soil column that has ten manometers inserted into it and a Mariotte system
water source. With the manometers and the constant water level provided by the
Mariotte water source, the soil column can be used to show water and soil interacting in
the lab to simulate field situations.
MATERIALS & METHODS
Construction of a steel column and base to contain soil was the first portion of this
project. The column is a large piece of rectangular tubing steel 21.9 cm wide by 17.1
cm deep by 81 cm tall, the metal of the column is 0.468 cm in thickness. The column
was welded onto a steel plate with dimensions of 48 x 48 cm, and a thickness of 0.655
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cm. The bottom of the column is drained by a threaded access hole that is 1.75 cm in
diameter and fitted with a brass elbow hose barb, which was obtained from McMasterCarr wholesalers. The column assembly is held up by a foot at each of the four corners.
The feet are 9.0 cm tall by 3.1 cm diameter pipe welded to the bottom plate.
The column was constructed by first drilling a 1.71 cm hole in the middle of the
large steel base plate. Into the hole was welded a pipe threaded nipple for the drain.
The ten 1.25 inch holes for the manometers were then marked and cut into the face of
the rectangular steel tubing. The tubing was centered over the drain and plate, then
welded into place. The welds must be water tight. The feet of the column were then
welded on each corner of the bottom side of the plate and the drain fitted with the hose
barbed brass elbow. The entire column was then sanded smooth, primed, and painted
inside and out with quality rust resistant paint.
An important aspect of the column is to be able to drain it quickly and easily
without removing the soil. We have found the best method of doing this is to create an
assembly of screens in the bottom of the column that forms a barrier not allowing the
soil to pass through the brass elbow drain. In the bottom is placed a 0.25 inch thick
plastic sheet with 0.125 inch holes drilled in a 1.0 cm grid. The underside of this screen
has 0.25 inch risers attached to it that allow water to flow underneath it. Pea size gravel
is placed on top of this first screen to a thickness of 3 cm and an identical plastic
screen, minus the risers, is placed on top of the gravel. On the top of this assembly is
fitted a sheet of synthetic fiber mesh (door screening).
Water must enter the system through the bottom of the column, but we found that
using the drain port was not effective because it wetted unevenly and would air-lock.
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For access of water, we constructed a flow tube from 0.50 inch stainless steel tubing 30
cm in length, drilled with 0.125 inch holes every 1 cm on each of the four sides along its
entire middle section, with 5 cm on each end nonperforated. The stainless steel tube
was inserted into spun 8 micron fiberglass filter rope obtainable from Fisher Scientific as
Pyrex Brand Glass Wool, catalog number 11-388. Materials other than fiberglass were
tested but were found to create an air barrier. Holes 1.25 inch in diameter were drilled
into the column on the long sides in the middle 1.75 inch from the bottom. The water
flow tube within its fiberglass sleeve was then inserted into the holes in the column and
#7 stoppers were inserted on each side to form a seal between the tube and the hole
through which it was inserted in the column.
Into the ten holes that were drilled into the face of the column, manometers were
installed through #7 stoppers after the column was filled with soil and compacted.
Manometers for this system consist of 10 tensiometers that are constructed from 6 inch
lengths of 0.5 inch, Schedule 80, PVC pipe drilled with a 0.64 inch bit then beveled the
edge with a 0.875 inch bit to fit the ceramic cup. Each batch of ceramic cups are
somewhat different in size; the 0.64 inch bit that we used will not be right for all cups but
is approximately the size to use. Devcon 30 Minute Epoxy was used to glue the
ceramic cup into place and allowed to sit for 24 hours so the epoxy will harden. The
flexible tubing portion of the manometers was attached to a vertical reading board with
Gripper Clips. The board was painted black to make the water level in the tubing easier
to read. A 2-meter stick was attached to the middle of the board to allow for continuous
reading of the water level in the tubes from 1 cm at the bottom to 200 cm at the top. A
list of Materials used in construction of manometers and the reading board are:
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10 – 5 inch lengths of 0.5 inch, Schedule 80, PVC pipe
10 – Ceramic cups (Soil Moisture Equipment Corporation)
10 – 12 foot lengths of 0.25 inch clear flexible tubing
Devcon 30 Minute Epoxy
10 – #7 neoprene stoppers
0.75 inch X 15 inch X 78 inch wood board
30 Gripper Clips (Gibson Good Tools, Inc.)
The Mariotte system was responsible for getting water into the system in a
consistent manner so that the water level in the soil column does not change after
establishment of equilibrium. It does this by restricting the flow of water out of the water
source to the level of the submerged air vent in the sealed water source. The water
source is contained in a 10 gallon glass water carboy (bottle). A #6 ½ neoprene stopper
is used in the bottle opening. The stopper has three holes cut in it, sizes 11, 10, and 7
mm; the 11 mm hole allows the air vent tube to enter the bottle, the 10 mm hole allows
the water siphoning tube to exit the bottle, the 7 mm hole suspends the water level
monitoring ruler (a neon green cm rule glued to a rod). A 0.95 cm (0.375 inch) ID
flexible tubing 150 cm in length connecting the water source to the stainless steel flow
tube supplies water to the column. A 1.11 cm (0.44 inch) ID flexible tubing was used as
a coupler between the 0.95 cm flexible tubing and the 0.95 ID rigid tubing.
Dispersion of the soil aggregates can be a problem in an environment in which
the soil is saturated with a low salt concentration solution over an extended period of
time. A 0.005 M CaCl2 solution was prepared and used in the column for all water
needs (Altfelder et al., 2001).
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The soil used in the column must be able to readily conduct water to 0.5 m in
height. The soil mixture we used contained 5% vermiculite by mass, added to a silt
loam soil. The soil was also sifted through a 4.75 mm sieve to provide uniform size. A
moist soil, concrete mixer method was employed to mix the soil with the vermiculite to
prevent stratification in the column because of the difference in their densities. This was
done by loading the measured soil and vermiculite masses into the mixer. As the mixer
was turning, water was sprayed onto the soil and vermiculite with a garden pesticide
applicator.
The ability of a soil to conduct water is related to the nature of the porous
material. As such, an evenly prepared (compacted) soil material greatly assists the
illustration of water movement principles in a soil column. Wet settling was the best way
for us to achieve a uniform, compacted soil. An effective way to compact granular
materials is to vibrate them (Hillel, 2004). A siphon bottle was elevated even with the
top of the column and attached to the drain hole on the bottom. The water was allowed
to fill the column and pond on the top of the soil. While the soil was wet, the column
was vibrated to allow the soil to settle. The wet soil was allowed to set several hours. A
vacuum line was then connected to the drain at the bottom of the column and the water
was vacuumed off. This procedure was repeated four times.
After the soil had dried and settled completely so there was no more movement,
the tensiometers were inserted. Each hole in the soil was bored slightly smaller than
the diameter of the tensiometers and to slightly less than half the distance into the soil
column. This was to ensure that the ceramic cup was seated firmly into the soil on all
sides. The manometer tubes were primed and then attached to the tensiometers. This
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is most easily achieved before the stopper and tubing are inserted into the tensiometer.
By filling the tubing with water from the air open end (the end opposite of that which will
be connected to the tensiometer), and then the tensiometer with water, all air is forced
out. While the water is still running out the tube into the tensiometer, connect the tube
into the tensiometer and insert it into the bored hole in the soil column. Insertion should
be a smooth motion to avoid loosening the soil in a way that provides poor cup-to-soil
contact. A top-down view of the steel column with inserted manometers (sans soil) is
shown in Figure 1.
Figure 1. Top-down view of steel column with inserted manometers, sans soil.
When set-up of the system was complete, water was allowed to enter the column
and reach equilibrium in the zero flow condition in which there was no evaporation (due
to a cover placed on top of the soil column). It required approximately 8 days for all the
manometers to flatten out at the same level (reach equilibrium). After that level was
recorded, the cap was removed and an incandescent light was turned on above the soil
to increase evaporation. A 60 watt house light was used at 20 cm distance above the
soil surface. This induced sufficient drying without causing the surface to become so
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dry that it insulated the lower levels and prevented water from moving up to the surface.
When the column reached water equilibrium in this constant flow condition, the
manometers showed a curvilinear pattern with the driest, lowest reading, being with the
manometer closest to the soil surface. The readings across the manometer board were
done with a carpenter’s level so that measurements would be most accurate.
The completed system should have the soil column elevated to allow for the most
negative readings in the manometers. We placed the complete Steady State System
on a sturdy lab cart so it would be mobile for demonstrations (Figure 2).
Figure 2. Picture of the completed soil column system with the Mariotte water
source, steel column with inserted manometers, and board for reading of
manometers (left to right in photo).
DISCUSSION
Darcy’s Law of soil water flow can be considered to have three basic sections.
V(At)-1 is the water flow rate, often termed the water flux. K is the hydraulic conductivity,
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a soil’s ability to conduct water. [(TPH2-TPH1)(L)-1] is the change in total potential head
per unit of linear distance, often referred to as the driving force.
DARCY’S LAW
V (At)-1 = -K [(TPH2-TPH1)(L)-1]
V = Volume of H2O Flow
A = Cross-sectional area of soil perpendicular to water flow
t = Duration of time for V to pass from soil column
K = Hydraulic Conductivity
TPH = Total Potential Head at positions 1 and 2
L = Linear distance between the two manometers under consideration
To demonstrate the measurements and calculations that go into quantifying soil
water movement, we established two steady-state water flow situations. In each of the
two situations, we proceeded to take measurements from the soil column, perform the
needed calculations, and report the results.
NO-FLOW SITUATION
We first established a situation where a water table would exist within the soil
column and there would be no water movement. We established those conditions by
placing water in the column where the water table (surface) was at about 20 cm above
the base of the column. We then turned the water source off and covered the top of the
steel column to prevent evaporation of water from the soil. The soil column was then
allowed to equilibrate during an eight-day wait. After equilibration, the water level in all
10 manometers was at 121.5 cm on the reading board. The base of the soil column
was at 100.0 cm on the reading board.
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Table 1 presents the readings and the calculated potential heads for the no flow
situation. Col. D presents the calculated pressure potential head (PPH) values (Col. B –
Col. C), with PPH being the length of water column in equilibrium with soil water. Col. E
presents the gravitational potential head (GPH) values (Col. C – the gravitational
reference plane taken as the base of the soil column), with GPH being the elevation of
the manometer with respect to the reference elevation. Col. F presents the total
potential head (TPH) values (Col. D + Col. E), with TPH being the sum of pressure
potential and gravitational potential heads.
Table 1. Readings and calculated data with the no-flow situation.
A
B
C
D
E
F
Manometer
Manometer
Manometer
PPH
GPH
TPH
no.
rdg. (cm)
elev. (cm)
(cm)
(cm)
(cm)
1 (top)
121.5
166.5
-45.0
66.5
21.5
2
121.5
160.0
-38.5
60.0
21.5
3
121.5
153.5
-32.0
53.5
21.5
4
121.5
147.5
-26.0
47.5
21.5
5
121.5
141.0
-19.5
41.0
21.5
6
121.5
135.0
-13.5
35.0
21.5
7
121.5
128.0
-6.5
28.0
21.5
8
121.5
122.5
-1.0
22.5
21.5
9
121.5
116.5
5.0
16.5
21.5
121.5
110.5
11.0
10.5
21.5
10 (bottom)
The distributions of PPH, GPH, and TPH by depth within the soil column are
presented in Figure 3. Liquid water flow in soil moves from regions of higher to regions
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of lower TPH. If TPH at the two positions of interest is a single value, then no flow takes
place—there is no driving force (potential gradient). In this first soil column example,
TPH is a single value by depth within the column. Therefore, there is no water flow
within the soil columns, and that is clearly evident from Figure 3.
Figure 3. The distribution of PPH, GPH, and TPH by depth within the soil column
for the no-flow situation.
STEADY-STATE EVAPORATION SITUATION
We then established a situation where a water table would exist within the soil
column and there would be evaporation of water from the soil surface. We established
those conditions by using the Mariotte system to maintain the constant water table
depth and by using a light bulb to maintain a constant evaporation rate. The entire
system was then allowed to reach steady-state water flow conditions where water
volume entry from the water bottle equaled the volume of water evaporated from the soil
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column. This steady-state equilibrium flow was reached in about 10 days after the
system was started (water flow and light source). We then monitored water outflow
volume from the bottle for 5.3 days of steady-state flow. We measured water level in
the water bottle at five times during the flow to be sure we had a constant flow volume
per unit time. During 5.31 days (127.4 hours) the total volume of water flow from the
glass bottle was 785 cm3. The cross-sectional area of the soil column was measured at
375 cm2. Therefore, the steady-state water flow rate was 0.394 cm/day [(V/At)].
Table 2 presents the readings and calculated potential head values for the
steady-state evaporation situation. Col. D, E, and F contain the calculated values of
pressure potential head, gravitational potential head, and total potential head,
respectively.
Table 2. Readings and calculated data with the steady-state evaporation situation.
A
B
C
D
E
F
Manometer
Manometer
Manometer
PPH
GPH
TPH
no.
rdg. (cm)
elev. (cm)
(cm)
(cm)
(cm)
1 (top)
14.4
166.5
-152.1
66.5
-85.6
2
82.3
160.0
-77.7
60.0
-17.7
3
105.7
153.5
-47.8
53.5
5.7
4
115.0
147.5
-32.5
47.5
15.0
5
119.8
141.0
-21.2
41.0
19.8
6
121.0
135.0
-14.0
35.0
21.0
7
121.4
128.0
-6.6
28.0
21.4
8
121.5
122.5
-1.0
22.5
21.5
9
121.6
116.5
5.1
16.5
21.6
121.6
110.5
11.1
10.5
21.6
10 (bottom)
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l
The distributions of PPH, GPH, and TPH by depth within the soil column are
presented in Figure 4. Water always flows from regions of higher to regions of lower
TPH. Therefore, water is flowing upward throughout the soil column. The lower PPH
values at shallower soil depths indicate the soil is drier at the shallower depths. With
the drier soil conditions at shallower depths, the hydraulic conductivity would be less.
Also, at the shallower soil depths the driving force [(TPH2-TPH1)/L] is greater.
Figure 4. The distribution of PPH, GPH, and TPH by depth within the soil column
for the steady-state evaporation situation.
The greater driving force and the lesser hydraulic conductivity values at
shallower soil depths when multiplied together yield the same flow rate (flux) as in the
deeper soil depths (less driving force and greater hydraulic conductivity).
Earlier, we showed the steady-state flux value was 0.394 cm/day. The 10
manometer depths mark the boundaries of nine soil layers. By taking the TPH values of
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each soil depth from Table 2 (Col. F) and the distance between manometer elevations
(Col. C), the total potential head gradient [(TPH2-TPH1)/L] in each of the nine layers
was calculated. Dividing the flux value by the gradient value yields the hydraulic
conductivity of the soil layer. Hydraulic conductivity of a layer is expressed as a function
of the mean PPH within that layer. Hydraulic conductivity vs. pressure potential head of
eight layers is presented in Figure 5. With such slight differences in manometer
readings at deeper soil depths, we were not able to distinguish a difference between
manometers 9 and 10. Therefore, there is no difference in TPH and no flow in the
lowest layer bounded by manometers. The strong relationship that has hydraulic
conductivity decreasing sharply with soil drying is shown in Figure 5.
Figure 5. The relationship between hydraulic conductivity and pressure potential
head within the soil column.
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CONCLUDING REMARKS
We established a situation where there would be no water flow. The fact that
there was no flow was clearly evident from the measured data and calculated results.
We then established a situation with steady-state water flow through the soil column.
Students obtain experience at measuring and calculating gravitational potential head,
pressure potential head, total potential head, total potential head gradient (driving
force), water flow rate through the column (flux), and hydraulic conductivity. All are
components of the principles and use of soil water theory (Darcy’s Law). This hands-on
experience is especially beneficial to students who are relatively new to the concept of
water potential and the quantification of soil water flow.
LITERATURE CITED
Altfelder, S., T. Streck, M.A. Maraqa, and T.C. Voice. 2001. Nonequilibrium
sorption of dimethylphthalate—compatibility of batch and column techniques. Soil Sci.
Soc. Am. J. 65:102-111.
Hillel, D. 2004. Introduction to environmental soil physics. Elsevier Academic
Press.
ACKNOWLEDGEMENTS
Many thanks to Dr. Loyd R. Stone from the Department of Agronomy at Kansas
State University for the laboratory, making teaching funds available, his advice, editing,
and additions to this work.
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