FINAL PROJECT REPORT

for

Subcontract No. 1995-27

from the

# University of Oklahoma

under

Cooperative Agreement NA37RJ0203

National Oceanic and Atmospheric Administration

Office of Global Programs

\"Meeting GEWEX/GCIP Measurement Needs by Adding

Automated Measurement of Soil Moisture and Temperature Profiles

to the DOE ARM/CART Southern Great Plains Site\"

Subcontractors:

Dr. Ronald L. Elliott and Dr. Glenn O. Brown

Biosystems and Agricultural Engineering Department

Oklahoma State University

Stillwater, Oklahoma

June 1, 1998

# INTRODUCTION

In 1995, the University of Oklahoma\'s Cooperative Institute for
Mesoscale Meteorological Studies was awarded a research grant by the
Climate and Global Change Program of the NOAA Office of Global Programs.
The project was entitled \"Meeting GEWEX/GCIP Measurement Needs by
Adding Automated Measurement of Soil Moisture and Temperature Profiles
to the DOE ARM/CART Southern Great Plains Site\" (Schneider and Fisher,
1997). The co-principal investigators, Dr. Jeanne M. Schneider and Dr.
Peter J. Lamb, subcontracted a portion of the work to Oklahoma State
University. Specifically, the subcontractor was responsible for
characterizing the soils and performing independent soil-moisture
measurements at each of 21 facilities (field research sites) in the
ARM/CART study area in Oklahoma and Kansas. This report presents the
results of those investigations, and discusses the field and laboratory
procedures that were used to acquire the data.

## **FIELD SAMPLING**

The locations of the ARM/CART Central Facility and Extended Facilities
were determined from maps provided by Site Operations. Site visits and
digging operations were coordinated through Site Operations. In order to
minimize site disturbance and safety risks, soil sampling was performed
using hand tools. All samples were collected by Mr. Brandon Claborn, OSU
Biosystems and Agricultural Engineering student, usually with the
assistance of Dr. Jeanne Schneider, project co-PI.

After arrival at each facility, the specific location of the 6\' x 6\'
SWATS plot was determined. \[SWATS is the acronym for \"Soil Water and
Temperature System\", and refers to the sensors installed as part of the
OGP-sponsored project.\] The surface features in the immediate vicinity
of the SWATS plot were visually assessed in order to determine the most
representative areas for soil sampling. Unless otherwise directed by
Site Operations, all digging occurred just outside the electric fence
enclosing the Energy Balance Bowen Ratio (EBBR) system (usually 5 m
directly west of the EBBR\'s exchange mechanisms). Power to the electric
fence was disconnected for the duration of the visit.

A preliminary look at the different soil horizons and their respective
depths was needed before any sampling could be done. This was
accomplished by digging a sample pit. The pit was adjacent to the SWATS
plot and just outside the fence of the EBBR. The soil inside the SWATS
plot remained undisturbed. The sample pit was dug using a sharpshooter
shovel. The pit had dimensions of approximately 70 cm x 15 cm x 60 cm (L
x W x D), with the extracted soil loaded into buckets to facilitate
refilling. This pit exposed the soil profile and soil horizons to visual
inspection; descriptions of the soil layers (2 to 4 in number), and
their respective depths were recorded. The soil color of each layer was
determined by using the Munsell Soil Color Chart. The texture-by-feel
method was used to determine the soil textures qualitatively. The soil
was replaced in the observation pit in the reverse order that it was
taken out, and original sod was replaced in all areas so the site was
returned as close to its original state as possible.

Undisturbed core samples were collected for laboratory determination of
soil-water-retention characteristics and bulk density. These samples
were taken using a model number 0200 soil core sampler from Soilmoisture
Equipment Corporation (see \"Piston Samplers\" in ASTM D 4700 -- 91).
Core samples were taken from each soil horizon at each of two different
locations near the sample pit. To obtain samples from depths other than
at the surface, a hole was dug using a post hole digger to reach the
desired depth for sampling without compacting the soil. The soil sampler
holds three numbered rings each with a height of 3 cm and an outside
diameter of 5.72 cm (2.25 in). The sampler has a wedge cutting tip to
help provide undisturbed samples. The sampler was driven into the ground
by using the driving hammer. The rings were removed and then the sample
was cut transversely to separate the rings. The rings were sealed with
plastic caps and placed in numbered containers for transport to the
laboratory at Oklahoma State University. The sampling depths, ring
numbers, and container numbers were recorded on the data sheet. Some bag
samples of soil were also collected for particle-size and organic-carbon
analyses. The bag labels were recorded on the data sheets.

The sampling holes were returned as close to the original state as
possible. The locations of the test pits, sample locations, and any
other disturbed areas were recorded on the data sheet. Other special
characteristics of the site, including ground cover, were also recorded.

Two additional visits were made to each site in order to obtain samples
for direct measurement of soil water. These were to provide independent
data for validation of the SWATS measurements of soil water content at
the top 4 depths (5, 15, 25, and 35 cm). Ideally, the two visits to a
given site were made at times of contrasting soil moisture conditions,
but this was not always possible. The same digging and coring procedures
as described above were used for the soil moisture sampling, except that
a ring with a height of 6 cm was used in place of two 3-cm rings. The
6-cm tall sample was centered on the depth of interest. Triplicate
samples were obtained (4 depths in each of 3 holes, for a total of 12
ring samples). The samples were placed in numbered cans and transported
to the laboratory in an insulated chest.

# LABORATORY PROCEDURES

Organic Carbon

Organic carbon analyses were carried out in the laboratory of Dr.
Nicholas T. Basta, Associate Professor of Soil Chemistry in the Plant
and Soil Sciences Department at Oklahoma State University. The
Walkley-Black Method (Nelson and Sommers, 1996) was used. Organic matter
percentage was estimated by doubling the organic carbon percentage
determined in the laboratory (Nelson and Sommers, 1996).

Particle Size (Texture)

Particle size distributions were analyzed in the laboratory of Dr. Glenn
O. Brown, Associate Professor in the Biosystems and Agricultural
Engineering Department at Oklahoma State University. Samples were
prepared according to ASTM D 421 -- 85 (1985). Hydrometer and wet
sieving procedures were used (ASTM D 422 -- 63, 1963; ASTM D 1140 -- 92,
1992). The laboratory data were entered into a spreadsheet, and the
resulting particle size distributions were plotted. Percentages of sand,
silt, and clay were calculated, and used to assign the soil textural
class according to the USDA classification system (i.e., the \"soil
triangle\").

Soil Water Retention

Water retention characteristics were measured in Dr. Brown\'s laboratory
using the undisturbed field samples and pressure-plate methodology (ASTM
D 2325 -- 68, 1968). Water contents were determined at 7-8 progressively
increasing pressures up to a maximum of 15 bars. \"Hanging column\"
tests were used to obtain additional data points in the wet (low
pressure) end of the range (Klute, 1986).

Generally, 4 field samples were available for a given soil layer. The 4
resulting \"release curves\" (i.e., pressure versus volumetric water
content) were plotted and visually compared. Samples that were obvious
outliers or that had physically inconsistent data points were eliminated
from further analysis. In some cases, visual inspection of the sample
ring indicated a likely problem with soil-to-plate contact during the
test. This type of data screening and interpretation is common when
analyzing the results of soil water retention tests. However, for over
half of the 50 different soil layers analyzed, all 4 sample results were
deemed to be \"good\".

Bulk Density

The \"good\" soil-water-retention samples discussed above were also used
to determine bulk densities. Bulk density was calculated as the oven-dry
(105 C for 24 hours) weight of the sample, less the tare weight of the
ring, divided by the ring volume.

Water Content

Soil samples collected during the two return visits to each site were
weighed immediately upon arrival at the laboratory. Within a few days,
they were placed in a baffled oven at 105 C for 24 hours. The dry
weights were recorded, along with the tare weight of the can.
Gravimetric water content (g g^-1^) was calculated as the weight of
water divided by the dry weight of soil. Since the samples were of a
known volume, values of volumetric water content (m^3^ m^-3^) and bulk
density (g cm^-3^) could also be calculated.

**RESULTS AND ANALYSIS**

The appendices of this report contain the collected data and the results
of follow-on analyses. The number of appendices varies depending upon
the version of the report. Version 1 is the shortest and contains
summary tables of organic carbon, texture, soil water retention fitting
parameters, and bulk density. Added in Version 2 are the results of the
field sampling of water content, the detailed particle size analyses,
the laboratory soil water retention data, and graphs showing the
retention data and the equations which were fit to the data. Finally,
the rather voluminous Version 3 also includes the detailed output from
the program (RETC) that was used to fit the equations to the soil water
retention data. The appendices are listed below, with accompanying
descriptions.

Appendix A Organic Carbon and Organic Matter

This is a summary table of the measured organic carbon percentage in the
near-surface soil at each site. Also included is the organic matter
percentage, which is estimated as twice the organic carbon percentage.

Appendix B Texture

This is a summary table of the percentages of sand, silt, and clay
fractions in each soil layer at each site. Also listed is the
corresponding USDA texture class as determined from the \"soil
triangle\".

Appendix C Parameters for Soil Water Retention Models

This consists of one table for each site, containing the fitted values
of the parameters in the van Genuchten and Brooks-Corey equations for
relating soil water pressure to volumetric water content:

$\theta = \theta_{r} + \frac{\left( \theta_{s} - \theta_{r} \right)}{\left\lbrack 1 + (\alpha\ h)^{n} \right\rbrack^{m}}$
van Genuchten

#### $\begin{array}{r}
\theta = \theta_{r} + \frac{\left( \theta_{s} - \theta_{r} \right)}{(\alpha\ h)^{\lambda}}\ (\alpha\ h) > 1 \\
 \\
\theta = \theta_{s}\ (\alpha\ h) \leq 1
\end{array}$ Brooks-Corey 

θ = volumetric water content (m^3^ m^-3^)

θ~r~ = residual water content (m^3^ m^-3^)

θ~s~ = saturated water content (m^3^ m^-3^)

h = soil water pressure (bar), positive for suction (unsaturated)
conditions

α = scaling parameter (bar^-1^)

n = empirical parameter (dimensionless)

m = 1 -- (1/n)

λ = pore-size distribution parameter (dimensionless)

Note that θ~r~ and θ~s~ are fitting parameters and should not be
interpreted as lower and upper limits of field water content. In the
Brooks-Corey equation, the break at h = 1/α is called the \"air entry
pressure\" (when the large pores first begin to empty). The Brooks-Corey
equation is a special case of the van Genuchten equation (n approaching
infinity with the product λ = mn remaining finite).

The fitting of the two equations to the soil water retention data was
done using the program \"RETC\" (van Genuchten et al., 1991). RETC uses
non-linear, least-squares optimization. As such, the fitting results can
be sensitive to the initial estimates of the parameters. In this
analysis, initial parameter estimates were obtained from a texture-based
table of typical values (see page 41 of the above-referenced report).

RETC was used to generate two sets of parameters. In the first case, all
four parameters (θ~r~, θ~s~, α, and either n or λ) were treated as
unknowns and allowed to \"float\" during the optimization process. In
the second case, θ~s~ was fixed at a value determined by:

$\theta_{s} = 1 - \frac{\rho_{b}}{2\text{.}\text{65}}$

where ρ~b~ is the soil\'s bulk density (g cm^-3^) and 2.65 is the
assumed particle density. See Appendix D for the bulk density values
that were used. Note that, in some instances, the RETC optimization
process set θ~r~ equal to zero.

Appendix D Bulk Density

This is a summary table of bulk densities representing the average of
the values obtained from the soil water retention samples.

Appendix E Water Content

This consists of one sheet for each site, containing the results of each
of the two follow-up visits for field sampling of water content.
Included are data on volumetric water content, gravimetric water
content, and sample bulk density, for each of the four depths (5, 15,
25, and 35 cm). The tables contain the data for each of the three
replicate samples, as well as the corresponding mean, standard
deviation, and coefficient of variation.

Appendix F Particle Size

This consists of two sheets for each soil layer at each site. The first
is the spreadsheet that was used to enter the laboratory data and
perform the necessary calculations. The second is a graph showing the
resulting particle size distribution.

Appendix G Soil Water Retention Data

This consists of one sheet for each site, containing the observed soil
water retention data as obtained from laboratory tests using pressure
plates and hanging columns.

Appendix H Graphs of Soil Water Retention Data and Fits (WCS Unknown)

These are graphs showing the observed soil water retention data along
with the van Genuchten and Brooks-Corey fits to the data. These RETC
fits are for the case where all equation parameters were treated as
unknowns. There is one graph for each soil layer at each site.

Appendix I Graphs of Soil Water Retention Data and Fits (WCS Fixed)

These are graphs showing the observed soil water retention data along
with the van Genuchten and Brooks-Corey fits to the data. These RETC
fits are for the case where the saturated water content was treated as a
known parameter. There is one graph for each soil layer at each site.

Appendix J Detailed Output from RETC Fits to Soil Water Retention Data

(WCS Unknown)

This appendix consists of the unabridged output from the RETC model for
fitting retention parameters, for the case where the saturated water
content was treated as an unknown. There are two pages of output for
each soil layer at each site. Also included for completeness are the
values of the fitted parameters (Appendix C) and the accompanying graphs
(Appendix H).

Appendix K Detailed Output from RETC Fits to Soil Water Retention Data

(WCS Fixed)

This appendix consists of the unabridged output from the RETC model for
fitting retention parameters, for the case where the saturated water
content was treated as a known parameter. There are two pages of output
for each soil layer at each site. Also included for completeness are the
values of the fitted parameters (Appendix C) and the accompanying graphs
(Appendix I).

## ACKNOWLEDGMENTS

Appreciation is extended to Dr. Nick Basta for his willingness to run
the organic carbon tests.

This project would not have been possible without the dedicated efforts
of many students and staff in the OSU Biosystems and Agricultural
Engineering Department:

Brandon Claborn Tom Underwood

John Roll Shellie Miller

Diana Loudenslager Jason Vogel

Hope Nickels Ken Fisher

REFERENCES

American Society for Testing and Materials. 1963. Standard Test Method
for Particle-Size Analysis of Soils. D 422 -- 63, Philadelphia,
Pennsylvania.

American Society for Testing and Materials. 1968. Standard Test Method
for Capillary-Moisture Relationships for Coarse- and Medium-Textured
Soils by Porous-Plate Apparatus. D 2325 -- 68, Philadelphia,
Pennsylvania.

American Society for Testing and Materials. 1985. Standard Practice for
Dry Preparation of Soil Samples for Particle-Size Analysis and
Determination of Soil Constants. D 421 -- 85, Philadelphia,
Pennsylvania.

American Society for Testing and Materials. 1991. Standard Guide for
Soil Sampling from the Vadose Zone. D 4700 -- 91, Philadelphia,
Pennsylvania.

American Society for Testing and Materials. 1992. Standard Test Method
for Amount of Material in Soils Finer Than the No. 200 (75 μm Sieve). D
1140 -- 92, Philadelphia, Pennsylvania.

Klute, A. 1986. Water Retention: Laboratory Methods. Chapter 26 in the
Second Edition of Methods of Soil Analysis, Part 1, Physical and
Mineralogical Methods, Agronomy Monograph No. 9, American Society of
Agronomy -- Soil Science Society of America, Madison, Wisconsin.

Nelson, D. W. and L. E. Sommers. 1996. Total Carbon, Organic Carbon, and
Organic Matter. Chapter 34 in Methods of Soil Analysis, Part 3, Chemical
Methods, SSSA Book Series No. 5, Soil Science Society of America --
American Society of Agronomy, Madison, Wisconsin.

Schneider, J. M. and D. K. Fisher. 1997. Meeting GEWEX/GCIP Measurement
Needs by Adding Automated Measurement of Soil Moisture and Temperature
Profiles to the DOE ARM/CART Southern Great Plains Site. Preprints,
13^th^ Conference on Hydrology, American Meteorological Society, Long
Beach, California, February.

van Genuchten, M. T., F. J. Leijand and S. R. Yates. 1991. The RETC Code
for Quantifying the Hydraulic Functions of Unsaturated Soils.
EPA/600/2-91/0655, U.S. Environmental Protection Agency, Robert S. Kerr
Environmental Research Laboratory, Ada, Oklahoma. 85 pp.

### APPENDIX A

ORGANIC CARBON AND ORGANIC MATTER

A summary table of organic carbon percentages (measured) and organic
matter percentages (estimated) in the near-surface soil.

### APPENDIX B

TEXTURE

A summary table of sand/silt/clay percentages and the corresponding USDA
texture class for each soil layer.

### APPENDIX C

PARAMETERS FOR SOIL WATER RETENTION MODELS

Values of the parameters in the van Genuchten and Brooks-Corey equations
for relating soil water pressure to volumetric water content. Curve
fitting was done using \"RETC\", first with all 4 parameters treated as
unknowns, and then with the saturated water content (WCS) fixed based on
the average value of bulk density for that layer. The parameters \"N\"
and \"LAMBDA\" are for the van Genuchten and Brooks-Corey equations,
respectively.

### APPENDIX D

BULK DENSITY

A summary table of bulk densities representing the average of the values
obtained from the soil water retention samples.

### APPENDIX E

WATER CONTENT

Volumetric and gravimetric water contents determined from field sampling
at the same depths as the SWATS sensors.

### APPENDIX F

PARTICLE SIZE

Detailed results of particle size analyses, including graphs of particle
size distributions.

### APPENDIX G

SOIL WATER RETENTION DATA

Observed soil water retention data, as obtained from laboratory tests
using pressure plates and hanging columns.

### APPENDIX H

GRAPHS OF SOIL WATER RETENTION DATA AND FITS

(WCS UNKNOWN)

Graphs showing the observed soil water retention data along with the van
Genuchten and Brooks-Corey fits to the data. These fits are for the case
where all equation parameters, including the saturated water content
(WCS), were treated as unknowns.

### APPENDIX I

GRAPHS OF SOIL WATER RETENTION DATA AND FITS

(WCS FIXED)

Graphs showing the observed soil water retention data along with the van
Genuchten and Brooks-Corey fits to the data. These fits are for the case
where the saturated water content (WCS), was treated as a known
parameter.

### APPENDIX J

DETAILED OUTPUT FROM RETC FITS TO

SOIL WATER RETENTION DATA

(WCS UNKNOWN)

Details on the RETC fits to the observed soil water retention data, for
the case where the saturated water content (WCS) was treated as an
unknown parameter. The graphs from Appendix H are included for
convenience. In the RETC output, \"MTYPE = 3\" indicates the van
Genuchten model, and \"MTYPE = 5\" indicates the Brooks-Corey model.

### APPENDIX K

DETAILED OUTPUT FROM RETC FITS TO

SOIL WATER RETENTION DATA

(WCS FIXED)

Details on the RETC fits to the observed soil water retention data, for
the case where the saturated water content (WCS) was treated as a known
parameter. The graphs from Appendix I are included for convenience. In
the RETC output, \"MTYPE = 3\" indicates the van Genuchten model,
\"MTYPE = 5\" indicates the Brooks-Corey model, and an \"INDEX\" value
of 0 for WCS indicates that it is a fixed parameter.