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```					        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 McMaster-

Carr 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

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

10 (bottom)               121.5                  110.5               11.0   10.5        21.5

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.

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

respectively.

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

10 (bottom)             121.6                110.5                11.1           10.5    21.6    l

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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

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

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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,

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,

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