SMEX03 ThetaProbe Soil Moisture Data: Alabama

Summary

This data set includes soil moisture data measured with Delta-T Devices’ ThetaProbe ML2 sensors for the Soil Moisture Experiment 2003 (SMEX03), conducted from 22 June to 02 July 2003 in northern Alabama and southern Tennessee, USA.  The output parameters include voltage readings from the probe and volumetric soil moisture calculated from the measured voltage.  Data were collected at crop, pasture, and forested sites.  Data are provided in either a Microsoft Excel spreadsheet file or in three 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.

Crosson, William, and Laymon, Charles. 2003. SMEX03 ThetaProbe Soil Moisture Data: Alabama. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Overview Table

Category	Description
Data format	Microsoft Excel file ASCII text files
Spatial coverage and resolution	Southernmost Latitude: 34.68° N Northernmost Latitude: 35.16° N Westernmost Longitude: 87.08° W Easternmost Longitude: 85.78° W
Temporal coverage and resolution	22 June 2003 to 02 July 2003 Daily measurements
File naming convention	ThetaProbe_AL.xls All_Samples_AL.txt Mean_VWC_AL.txt Mean_mV_AL.txt
File size	Range from 5 KB - 390 KB
Parameter(s)	Voltage Volumetric soil water content
Procedures for obtaining data	Available via FTP

Table of Contents

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

1. Contacts and Acknowledgments

Investigator(s)

William Crosson
Universities Space Research Association
Global Hydrology and Climate Center
NASA Marshall Space Flight Center
Huntsville, Alabama 35812 USA

Charles Laymon
Universities Space Research Association
Global Hydrology and Climate Center
NASA Marshall Space Flight Center
Huntsville, Alabama 35812 USA

Technical Contact

Acknowledgements

The investigators would like to acknowledge the following people:

Frank Archer, Karnita Golson, and Linda Cornett of Alabama A&M University and the many Alabama A&M students, faculty, and staff members who collected and processed the field data. 

Ashutosh Limaye of the Universities Space Research Association, Global Hydrology and Climate Center, NASA Marshall Space Flight Center, Huntsville, Alabama USA.

2. Detailed Data Description

Format

Data are provided as a Microsoft Excel file with three worksheets or as three separate text files.  Each tab-delimited text file contains the same data as each individual worksheet of the Excel file.

The following tables detail the column headings for each of the three worksheets in the Excel file and the three text files. Refer to Table 1 for details on All_Samples_AL, Table 2 for details on Mean_mV_AL, and Table 3 and 4 for details on Mean_VWC_AL.

All_Samples_AL contains all voltage and volumetric water content (VWC) data collected:

Table 1: Column headings for All_Samples_AL

Column Heading	Description
Date	Date readings made (MM/DD/YY)
DOY	Day of Year
Site	Alabama site # (1-59)
Lat	Latitude of site (decimal degrees)
Long	Longitude of site (decimal degrees)
Sample	Sample # (1-3) at site
Time	Time of day in the format HH:MM
mV	ThetaProbe voltage reading (millivolts)
VWC	Volumetric water content (%) calculated using the generalized ThetaProbe calibration equation

Mean_mV_AL contains means of the 3 voltage readings taken at each site on each day:

Table 2: Column headings for Mean_mV_AL

Column Heading	Description
Date	Date reading was made
1, 2, 3, ..., 59	Mean ThetaProbe reading (millivolts) for Alabama site 1, 2, 3, ..., 59
All Site Mean	Mean of ThetaProbe readings (millivolts) for all sites for the day

Mean_VWC_AL contains two different tables. The top table contains means of the 3 VWC measurements at each site on each day:

Table 3: Column headings for the top half of Mean_VWC_AL

Column Heading	Description
Date	Date readings made
1, 2, 3, ..., 59	Mean VWC values for AL sites 1, 2, 3, ..., 59
All Site Mean	Mean of VWC values across sites for the day
# Sites Reporting	Number of sites (of 59 total) reporting for the day
Minimum	Minimum VWC across sites for the day
Maximum	Maximum VWC across sites for the day
Corn Mean, Soybeans Mean, etc.	Mean VWC for each landcover class for the day
Alpha Mean, Bravo Mean, etc.	Mean VWC for each sampling team for the day
Limestone Mean, Madison Mean, etc.	Mean VWC for each county for the day
EASE 1 Mean, EASE 2 Mean, etc.	Mean VWC for each EASE grid cell (1-10) for the day

The bottom table describes the areas sampled during this investigation. The column on the left hand side of the table lists the dates of each day of the investigation (6/22/03-7/2/03) in month/day/year format (mm/dd/yy). The rows along the top are described in the following table:

Table 4: Row headings for the bottom half of Mean_VWC_AL

Row Heading	Description
Landcover	Type of landcover investigated at the site (soybean, pasture, grassland, etc.,).
County	The county in which the investigation took place.
Team	Name of the team that visited the site.
EASE Grid Cell	EASE-Grid cell number (1-10).

For each site and date, a value of 1 means that the site was sampled on that day, and a value of 0 means that the site was not sampled that day.

File and Directory Structure

  All files are located within the same directory on the FTP site.

File Naming Convention

The Excel file is named ThetaProbe_AL.xls and contains three worksheets. The text files have the same names as the individual worksheets of the Excel file, with the suffix .txt and are named as follows: All_Samples_AL.txt, Mean_VWC_AL.txt, and Mean_mV_AL.txt.

File Size

The Excel file size is approximately 390 KB, and the text file sizes range from 5 to 91 KB.

Spatial Coverage

Southernmost Latitude: 34.68° N
Northernmost Latitude: 35.16° N
Westernmost Longitude: 87.08° W
Easternmost Longitude: 85.78° W

Spatial Resolution

Three readings were taken at random locations within an approximate 1 m diameter circle at each site.

Temporal Coverage

Data was acquired from 22 June 2003 to 02 July 2003.

Temporal Resolution

ThetaProbe readings were taken daily between 11:00 a.m. and 3:00 p.m. central daylight time (CDT).

Parameter or Variable

Parameter Description

The parameters in this data set include raw voltage readings in millivolts (mV) and volumetric soil moisture (%).

Sample Data Record

The following is a sample of the first five rows from the data file All_Samples_AL.txt:

Date	DOY	Site	Lat	Long	Sample	Time	mV	VWC
6/22/03	173	1	35.1114	-87.0021	1	12:55		22
6/22/03	173	1	35.1114	-87.0021	2	12:56		23.4
6/22/03	173	1	35.1114	-87.0021	3	12:57		23.3
6/22/03	173	2	35.1254	-86.8723	1	11:24		37.6
6/22/03	173	2	35.1254	-86.8723	2	11:25		28.5

The following is a sample of the first five rows from the data file Mean_mV_AL.txt (only the first 6 and the last 5 columns are shown):

Date	Site -->	1	2	3	4	...	56	57	58	59	All Site Mean
6/22/03						...	486.33	866.00	881.33	661.50	600.83
6/23/03		381.67		825.00	410.67	...	387.67	893.00	833.00	677.67	548.80
6/24/03		249.33	224.33	798.00	491.33	...	362.00	811.00	882.00	608.00	519.54
6/25/03		251.67	280.00	638.33	267.67	...	423.00	821.67	814.00	591.33	476.01
6/26/03		399.33	437.00	615.33	302.00	...	376.67	715.00	804.00	455.67	484.46

The following is a sample of the first five rows of the top table from the data file Mean_VWC_AL.txt (not all columns are shown):

Date	Site -->	1	2	3	...	58	59	All Site Mean	# Sites Reporting	...	EASE 10 Mean
6/22/03		22.9	32.7	34.2	...	39.9	27.4	25.4	50	...	30.7
6/23/03		14.8		36.3	...	36.9	27.8	22.3	49	...	29.1
6/24/03		8.6	7.5	34.6	...	37.0	24.6	20.9	50	...	27.5
6/25/03		8.7	10.0	25.9	...	35.7	23.8	18.9	43	...	26.3
6/26/03		15.0	17.0	24.8	...	35.1	17.8	19.2	44	...	24.3

Error Sources

Some data values are missing due to equipment failure or transportation problems to the test sites. Depending on the type of equipment failure, data were sometimes able to be recovered. When the data logger had a downloading error, VWC was recovered from the field books; but the voltage readings were not recorded. In the case of a total logger failure or transportation issues, no data were obtained for that day. Data that were not collected or were lost are flagged as -9999, and data that were determined to be in error are flagged as -999.

VWC values reported here were calculated using the default calibration equation for mineral soils. See the ThetaProbe ML2x Principles of Operation and Applications document (PDF, 286 KB) for a discussion of calibration issues. In post-experiment analysis, it was found that for 12 sites at which reliable bulk density measurements were available, VWC values obtained using this calibration compared well with values obtained concurrently from gravimetric sampling. Therefore, the default calibration equation was applied for all sites.

3. Data Access

Data Access

Data are available via FTP.

Volume

The total data volume is approximately 540 KB.

Related Data Collections

• AMSR-E Soil Moisture Validation Data: Data sets from SMEX campaigns available at NSIDC.
• AMSR-E/Aqua Data at NSIDC: AMSR-E standard products available at NSIDC.

4. Data Acquisition and Processing

Theory of Measurements

Volumetric soil water content is expressed as a ratio of the volume of water present in a soil sample to the total volume of the sample and is a dimensionless parameter. The ThetaProbe measures this by responding to changes in the dielectric constant of the soil. For more information on the theory behind these measurements, please see the ThetaProbe ML2x principles of operation and applications document (PDF, 286 KB).

Sensor or Instrument Description

Investigators used ML2 ThetaProbes to measure surface (0-6 cm) volumetric soil moisture. The probes are manually-operated impedance instruments manufactured by Delta-T Devices, Ltd.

Data Acquisition Methods

The ML2 ThetaProbes have four separate 6 cm stainless steel rods which are inserted vertically into the soil. Each instrument was connected to a handheld data logger, which delivers the electrical pulse, detects the return signal, and converts the period to voltage between 0 V and about 1 V. For AMSR-E validation purposes, soil moisture was sampled each day between 11:00 a.m. and 3:00 p.m. CDT.

Derivation Techniques and Algorithms

The software provided by the ThetaProbe manufacturer calibrates the probes to soil moisture, calculating an estimate of volumetric water content according to Equation 1:

VWC =	1.07 + 6.4V - 6.4V2 + 4.7V3 - a0
		(Equation 1)
	a1

where the coefficients a₀ and a₁ are 1.6 and 8.4 for mineral soils, respectively.

The resulting VWC estimates are provided in the data files. Three readings were taken at random locations within an approximate 1 m diameter circle at each site. Field averages of voltage and derived VWC were calculated by taking the arithmetic mean of the three values.

5. References and Related Publications

Miller, J. D. and G. J. Gaskin. ThetaProbe ML2x Principles of Operation and Applications. Macaulay Land Use Research Institute. http://www.macaulay.ac.uk/MRCS/pdf/tprobe.pdf, 286 KB.

The following Web site contains related information:

• USDA Soil Moisture Experiments 2003: The United States Department of Agriculture's SMEX03 Web site provides in depth information on the science mission and goal of the SMEX project.

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
CDT	Central Daylight Time
FTP	File Transfer Protocol
GSFC	Goddard Space Flight Center
NASA	National Aeronautics and Space Administration
NSIDC	National Snow and Ice Data Center
PDF	Portable Document Format
URL	Uniform Resource Locator
USDA	United States Department of Agriculture
VWC	Volumetric Water Content

Document Creation Date

September 2007

Document URL

http://nsidc.org/data/docs/daac/nsidc0306_smex03_thetaprobe_al.gd.html