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SMEX03 Regional Ground Soil Moisture Data: Oklahoma

Summary

The parameters for this data set include gravimetric soil moisture, volumetric soil moisture, bulk density, and surface and soil temperature for the Oklahoma study region. Summary files containing field averages are also provided. This data set is part of the Soil Moisture Experiment 2003 (SMEX03). The United States portion of SMEX03 was conducted during June and July of 2003; the Oklahoma data were collected from 2 to 17 July 2003. Data are provided in tab-delimited ASCII text files, and are available via FTP.

These data were collected as part of a validation study for the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E). AMSR-E is a mission instrument launched aboard NASA's Aqua Satellite on 04 May 2002. AMSR-E validation studies linked to SMEX are designed to evaluate the accuracy of AMSR-E soil moisture data. Specific validation objectives include assessing and refining soil moisture algorithm performance; verifying soil moisture estimation accuracy; investigating the effects of vegetation, surface temperature, topography, and soil texture on soil moisture accuracy; and determining the regions that are useful for AMSR-E soil moisture measurements.

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Jackson, T. and M. Cosh. 2006. SMEX03 Regional Ground Soil Moisture Data: Oklahoma. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.

Overview Table

Category Description
Data format Tab-delimited ASCII text
Spatial coverage and resolution

Southernmost Latitude: 34.49º N
Northernmost Latitude: 36.80º N
Westernmost Longitude: 98.29º W
Easternmost Longitude: 97.97º W

Temporal coverage and resolution 2 to 17 July 2003
File naming convention Od_param_type.txt, where
d
=
either N for Northern or S for Southern
param
=
either GVSM for soil moisture data or Temp for temperature data
type
=
either Raw for raw data or Sum for summary data
File size Range from approximately 35 KB to 126 KB
Parameter(s) Gravimetric and volumetric soil moisture, bulk density, surface and soil temperature
Procedures for obtaining data Data are available via FTP.

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator(s)

Thomas J. Jackson
Hydrology and Remote Sensing Laboratory
US Department of Agriculture (USDA) - Agricultural Research Service (ARS)
Beltsville, MD 20705
USA

Mike Cosh
Hydrology and Remote Sensing Laboratory
US Department of Agriculture (USDA) - Agricultural Research Service (ARS)
Beltsville, MD 20705
USA

Ron Elliot
Oklahoma State University
Stillwater, OK 74078
USA

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Acknowledgements

Many graduate students and volunteers worked to collect the field data. The investigators thank the SMEX03 Science Team and the National Soil Tilth Lab for their assistance. The investigators also thank NASA for their generous contributions to the study. This work was supported by the NASA Aqua AMSR, Terrestrial Hydrology and Global Water Cycle Programs. We would specifically like to acknowledge the contributions of:

2. Detailed Data Description

Format

The data are in tab-delimited ASCII text files. The column headings and the information in each column are described in the following tables.

The raw soil moisture data files, ON_GVSM_Raw.txt and OS_GVSM_Raw.txt, contain the following information (ON_GVSM_Raw.txt does not contain a stop time column).

Column Heading Description
Date Month/day/year
Site ID Site location identifier number
Start_Time (CDT) Start time of sampling in Central Daylight Time (CDT)
Stop_Time (CDT) Stop time (CDT); stop time is not included in ON_GVSM_Raw.txt; most sampling was 5-10 minutes in duration
Latitude WGS84 Latitude of sample site in decimal degrees, World Geodetic System 1984 (WGS84)
Longitude WGS84 Longitude of sample site in decimal degrees, WGS84
UTM_Easting WGS84 Easting of sample site in meters, WGS84, Zone 14
UTM_Northing WGS84 Northing of sample site in meters, WGS84, Zone 14
TP-mV A Impedance probe (ThetaProbe) millivolt reading in V, position A
TP-mV B ThetaProbe millivolt reading in V, position B
TP-mV C ThetaProbe millivolt reading in V, position C
TP-VSM_gc A ThetaProbe volumetric soil moisture (VSM) in m3/m3 from general calibration, position A
TP-VSM_gc B ThetaProbe VSM in m3/m3 from general calibration, position B
TP-VSM_gc C ThetaProbe VSM in m3/m3 from general calibration, position C
TP-VSM_scc A ThetaProbe VSM in m3/m3 from site-specific calibration, position A
TP-VSM_scc B ThetaProbe VSM in m3/m3 from site-specific calibration, position B
TP-VSM_scc C ThetaProbe VSM in m3/m3 from site-specific calibration, position C
Can_ID 0-3 cm Soil can identifier for 0-3 cm sample
Can_Wgt 0-3 cm Soil can weight in grams for 0-3 cm sample
Wet_Wgt 0-3 cm Wet weight in grams of soil for 0-3 cm sample
Dry_Wgt 0-3 cm Dry weight in grams of soil for 0-3 cm sample
GSM 0-3 cm Gravimetric soil moisture in g/g for 0-3 cm sample
Bulk_Density 0-3 cm Bulk density in g/cm3 for 0-3 cm sample
VSM 0-3 cm Volumetric soil moisture (VSM) in m3/m3 for 0-3 cm sample
Can_ID 3-6 cm Soil can identifier for 3-6 cm sample
Can_Wgt 3-6 cm Soil can weight in grams for 3-6 cm sample
Wet_Wgt 3-6 cm Wet weight of soil in grams for 3-6 cm sample
Dry_Wgt 3-6 cm Dry weight of soil in grams for 3-6 cm sample
GSM 3-6 cm Gravimetric soil moisture in g/g for 3-6 cm sample
Bulk_Density 3-6 cm Bulk density in g/cm3 for 3-6 cm sample
VSM 3-6 cm VSM in m3/m3 for 3-6 cm sample
Total VSM 0-6 cm Total VSM in m3/m3 for 0-6 cm
Scoop? Was a scoop tool used for this sample? If so, an average bulk density was used for the calculation of VSM.

The summary soil moisture data files, ON_GVSM_Sum.txt and OS_GVSM_Sum.txt, contain the following information (ON_GVSM_Sum.txt does not contain a stop time column).

Column Heading Description
Date Month/day/year
Site ID Site location identifier number
Start Time (CDT) Start time of sampling in Central Daylight Time (CDT)
Stop_Time (CDT) Stop time (CDT); stop time is not included in ON_GVSM_Sum.txt; most sampling was 5-10 minutes in duration
Latitude WGS84 Latitude of sample site in decimal degrees, World Geodetic System 1984 (WGS84)
Longitude WGS84 Longitude of sample site in decimal degrees, WGS84
UTM_Easting WGS84 Easting of sample site in meters, WGS84, Zone 14
UTM_Northing WGS84 Northing of sample site in meters, WGS84, Zone 14
TP-mV average Average impedance probe (ThetaProbe) millivolt reading in V
TP-mV stdev Standard deviation of ThetaProbe millivolt reading in V
TP-VSM_gc average Average ThetaProbe volumetric soil moisture (VSM) in m3/m3 from general calibration
TP-VSM_gc stdev Standard deviation of ThetaProbe VSM in m3/m3 from general calibration
TP-VSM_ssc average Average ThetaProbe VSM in m3/m3 from site-specific calibration
TP-VSM_scc stdev Standard deviation of ThetaProbe VSM in m3/m3 from site-specific calibration

The raw temperature data files ON_Temp_Raw.txt and OS_Temp_Raw.txt contain the following information (some column headings may differ slightly between the two files).

Column Heading Description
Date Month/day/year
Site ID Site location identifier number
Start Time CDT Start time of sampling in Central Daylight Time (CDT); ON_Temp_Sum.txt has "Local Time CDT"
Stop Time CDT Stop time of sampling in CDT; stop time is not included in ON_Temp_Sum.txt; most sampling was 5-10 minutes in duration
Latitude WGS84 Latitude of sample site in decimal degrees, World Geodetic System 1984 (WGS84)
Longitude WGS84 Longitude of sample site in decimal degrees, WGS84
UTM_Easting WGS84 Easting of sample site in meters, WGS84, Zone 14
UTM_Northing WGS84 Northing of sample site in meters, WGS84, Zone 14
Apogee IRT Infrared thermometer reading in degrees Celsius
Temp 1 cm Soil temperature probe reading in degrees Celsius at 1 cm
Temp 5 cm Soil temperature probe reading in degrees Celsius at 5 cm
Temp 10 cm Soil temperature probe reading in degrees Celsius at 10 cm

File Naming Convention

Files are named Od_param_type.txt, where

d
=
either N for Northern or S for Southern
param
=
either GVSM for soil moisture data or Temp for temperature data
type
=
either Raw for raw data or Sum for summary data

File Size

The data files range in size from approximately 35 KB to 126 KB.

Spatial Coverage

Southernmost Latitude: 34.49º N
Northernmost Latitude: 36.80º N
Westernmost Longitude: 98.29º W
Easternmost Longitude: 97.97º W

Temporal Coverage

Data were collected from 2 July 2003 to 17 July 2003.

Temporal Resolution

All data were collected daily at multiple sites. The sampling was typically conducted between 11:30 A.M. and 3:00 P.M. local time.

Parameter or Variable

Parameter Description

Parameters in this data set are: gravimetric soil moisture (GSM), volumetric soil moisture (VSM), bulk density (BD), and surface and soil temperature. The following table describes the units of measurement and sources of each parameter.

Parameter Units of Measurement Source/Sensor(s)
Gravimetric soil moisture Grams of water per grams of dry soil (g/g) Manual soil collection
Volumetric soil moisture Cubic meters of water per cubic meters of dry soil (m3/m3) Manual soil collection and impedance probes (ThetaProbes)
Bulk density Grams per cubic centimeter (g/cm3) Manual soil collection
Surface and soil temperature Degrees Celsius Infrared thermometers, temperature probes

Sample Data Record

The following sample from ON_GVSM_Sum.txt illustrates the data. The first four data rows are shown; only the first four columns and last four columns of each row are shown (the column headings in the sample indicate the included columns).

Date
Site ID
Start Time
Latitude
... 
TP-VSM_gc
TP-VSM_gc
TP-VSM_ssc
TP-VSM_ssc
CDT
WGS84
... 
average
stdev
average
stdev
7/2/2003
ON01
14:01
36.8036
... 
0.148
0.012
0.113
0.01
7/3/2003
ON01
11:01
36.8036
... 
0.106
0.013
0.074
0.012
7/4/2003
ON01
11:20
36.8036
... 
0.076
0.005
0.045
0.004
7/5/2003
ON01
11:20
36.8036
... 
0.069
0.019
0.039
0.018

Error Sources

Impedance Probes

Impedance probes (ThetaProbes) were used for volumetric soil moisture sampling. For various reasons, including extremely dry conditions, severe weather restrictions, miscommunication among personnel, and cultivation, some sites were not sampled on particular days. Occasionally, a ThetaProbe rod was broken because of very hard and dry soil conditions. When possible, the broken rod was replaced. When it was not possible to replace the rod, a new instrument was used.

Bulk Density

The coring tool used extracts a known volume of soil that was used to compute a bulk density. There is potential for compaction with this tool, resulting in higher bulk densities when compared to other methods. Occasionally, a scoop tool was used to retrieve a soil sample when soil was excessively dry. This scoop tool does not allow for a bulk density estimate; therefore, the average bulk density from all coring tool samples was used for computation of volumetric soil moisture for scoop samples. Also, the bulk density of the 0-3 cm sample was considered less reliable than the 3-6 cm sample; therefore, the 3-6 cm bulk density was used for all calculations.

3. Data Access and Tools

Data Access

Data are available via FTP.

Volume

The total volume is approximately 415 KB.

Related Data Collections

See related information on the NSIDC Soil Moisture Experiment (SMEX) Web site.

4. Data Acquisition and Processing

Theory of Measurements

This Oklahoma regional soil moisture site is within a single AMSR-E/Aqua passive microwave footprint (approximately 50 km), and measurements were taken around the time of the Aqua satellite overpass (approximately 13:30 local time). The goal of soil moisture sampling in these regional sites is to provide a reliable estimate of volumetric soil moisture and variance within the AMSR-E footprint.

Sampling

Sampling was performed on sites approximately a quarter section (0.8 km by 0.8 km) in size. In Oklahoma South (OS), a grid of 51 individual sites was sampled, covering approximately 50 km by 100 km (4 by 13 sites; site # 15 was not sampled). In Oklahoma North (ON), a grid of 36 individual sites was sampled, covering approximately 50 km by 75 km (3 by 12 sites). One location in each of the sites was sampled. The sampling was conducted between 11:30 and 15:00 local time.

Volunteers used a coring tool to extract a single soil moisture sample of the 0-3 cm and 3-6 cm soil layers. The coring tool extracts a known volume of soil which is used to compute a bulk density. Occasionally, a scoop tool was used to retrieve a soil sample when soil was excessively dry.

For volumetric soil moisture sampling, the primary measurement was the 0-6 cm dielectric constant at a single location in each site using impedance probes (ThetaProbes). The dielectric constant was converted to volumetric soil moisture using a calibration equation.

Computing Bulk Density and Soil Moisture

In the laboratory, samples were weighed, dried, and then weighed again. Gravimetric soil moisture (GSM) was computed as:

GSM = (wet weight - dry weight) / dry weight

The bulk density (BD) was determined by dividing the dry soil mass by the volume of the sample (3 cm) cylinder.

Volumetric soil moisture (VSM) was computed as:

VSM = GSM * BD

Note: For the 0-3 cm samples, the 3-6 cm bulk density was used due to the unreliability of the 0-3 cm samples. For samples collected with the scoop tool, the average bulk density from all coring tool samples was used for computation of volumetric soil moisture.

Sensor or Instrument Description

Manual Soil Collection

Samples were collected manually in the field using coring tools, rectangular shaped scoop tools, and soil cans.

Infrared Thermometer (Pyrometer) and Temperature Probe

The surface temperature was sampled using handheld infrared thermometers, which are OMEGA OS643-LS Infrared Pyrometers. The instrument has an emissivity of 0.95, accuracy of ± 3%, and temperature range of 0 to 260°C (32 to 500°F). Refer to OMEGA Engineering for more information. Soil temperature was obtained using a temperature probe inserted to depths of 1 cm, 5 cm, and 10 cm. Several different temperature probes were used, but all have a metal rod, plastic top, and digital readout.

OS643-LS Infrared Pyrometer
Temperature Probe

Impedance Probes (ThetaProbes)

Investigators used impedance probes to measure surface volumetric soil moisture. The probes were Type ML2 ThetaProbe manually-operated impedance instruments manufactured by Delta-T Devices, Ltd. The ThetaProbes have four separate 6 cm stainless steel rods that were inserted vertically into the soil. Each instrument was connected to a handheld reader that delivered the electrical pulse, detected the return signal, and converted the period to voltage between 0 and about 1 V.


ML2 ThetaProbe

The software provided by the probe manufacturer calibrates the ThetaProbes by calculating an estimate of volumetric soil moisture according to the following equation:

Theta =
1.07 + 6.4V - 6.4V2 + 4.7V3 - a0

a1

where a0 and a1 are 1.6 and 8.4, respectively. These estimates are provided in the data files.

Researchers also performed site-specific calibration for each field of sampling. Probe voltage readings from a row sampling point were compared to the volumetric soil moisture measured at the same point. A regression relationship was developed and new volumetric soil moisture values were estimated.

5. References and Related Publications

Please see the SMEX03 Web site for more information, and the NSIDC SMEX Web site to access data.

Cosh, M. H., T. J. Jackson, R. Bindlish, J. S. Famiglietti, and D. Ryu, A comparison of soil moisture impedance probe calibration techniques, submitted to Soil and Tillage Research, 2003.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

AMSR-E Advanced Microwave Scanning Radiometer - Earth Observing System
ASCII American Standard Code for Information Interchange
BD Bulk Density
CDT Central Daylight Time
FTP File Transfer Protocol
GC General Calibration
GSM Gravimetric Soil Moisture
IRT Infrared thermometer
NASA National Aeronautics and Space Administration
NSIDC National Snow and Ice Data Center
OK Oklahoma
SMEX Soil Moisture Experiment
SMEX03 Soil Moisture Experiment 2003
SSC Site-Specific Calibration
TP ThetaProbe
URL Uniform Resource Locator
USDA-ARS United States Department of Agriculture - Agricultural Research Service
UTM Universal Transverse Mercator
VSM Volumetric Soil Moisture
WGS84 World Geodetic System 1984

Document Creation Date

July 2006

Document URL

http://nsidc.org/data/docs/daac/nsidc0289_smex03_reg_gsm_ok.gd.html