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SMEX02 Iowa Regional Ground Soil Moisture Data

Summary

The parameters for this data set include gravimetric and volumetric soil moisture, bulk density, and soil temperature. This data set is part of the Soil Moisture Experiment 2002 (SMEX02). The SMEX studies are designed to evaluate, among other things, the accuracy of AMSR-E soil moisture data.

This study was conducted during June and July 2002 in the area around Ames, Iowa, USA. Data were collected in crop fields using a variety of methods and sensors: manual soil samples for gravimetric and bulk density data, infrared pyrometers for soil moisture, and hydra probes for soil moisture and temperature.

Data are provided in 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

Jackson, T., M. Cosh, and J. Famiglietti. 2003. SMEX02 Iowa Regional Ground Soil Moisture Data. [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 files.
Spatial coverage Southernmost Latitude: 41.7º N
Northernmost Latitude: 42.7º N
Westernmost Longitude: 93.8º W
Easternmost Longitude: 93.2º W
Temporal coverage 25 June 2002 to 12 July 2002
File naming convention "IA" designates the Iowa regional site. Raw data files in each directory contain the word RAW, summary data files contain the word SUM.
File size 14 KB to 157 KB
Parameter(s) Gravimetric and volumetric soil moisture, bulk density, soil and surface 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) Name and Title

Thomas J. Jackson, Hydrologist, Mike Cosh, General Physical Scientist, USDA ARS Hydrology Lab; and Jay Famiglietti, Associate Professor, Earth System Science, University of California.

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.

2. Detailed Data Description

Format

ASCII tab-delimited text files. The following tables detail the column headings for each data file.

"IA_GSM_Raw_V2.txt" - Gravimetric Soil Moisture Raw Data Columns

Column Heading Description
Date month/day/year
DOY Numerical day of the year
IA Site Site location identification number
CanID Can identification number
Depth Depth (cm)
WetWgt Wet weight (g)
DryWgt Dry weight (g)
GSM Gravimetric soil moisture
in grams of water per grams of dry soil
CanWgt (g)in grams
Bulk_Density Bulk density (g/cm3)
VSM Volumetric soil moisture (m3/m3)

"IA_TEMP_RAW.txt" - Soil and Surface Temperature Raw Data Columns

Column Heading Description
Date Month/day/year
DOY Numerical day of the year (Julian date)
IA Site Site location identification number
Latitude Latitude in decimal degrees
Longitude Longitude in decimal degrees
Easting UTM Easting in meters (Zone 15)
Northing UTM Northing in meters (Zone 15)
Time Local daylight saving time
surface Surface temperature in °C
1cm Soil temperature at 1 cm depth in °C
5 cm Soil temperature at 5 cm depth in °C
10cm Soil temperature at 10 cm depth in °C

"IA_TP_RAW.txt" and "IA_TP_SUM.txt" - Theta Probe Raw and Summary Data Columns

Column Heading Description
Date Month/day/year
DOY Numerical day of the year (Julian date)
SiteID Site location identification number
Latitude Latitude in decimal degrees
Longitude Longitude in decimal degrees
Easting UTM Easting in meters (Zone 15)
Northing UTM Northing in meters (Zone 15)
Row Location in tilled row:
0=in row; for example, in line with planted corn or soybeans
0.5=midpoint between two rows
0.25=midpoint between 0 and 0.5 locations
mV Theta probe output in volts (V)
Stdev
(in IA_TP_SUM.txt only)
Standard deviation of voltage output in V
VSM_gc Volumetric soil moisture content from generalized calibration of voltage data
Stdev
(in IA_TP_SUM.txt only)
Standard deviation of VSM_gc
VSM_ssc Volumetric soil moisture content from specific calibration of voltage data
Stdev
(in IA_TP_SUM.txt only)
Standard deviation of VSM_ssc

File and Directory Structure

Directory structure is set up according to the type of data:

File Naming Convention

Most file names contain the letters IA to designate the Iowa regional soil moisture site. Raw data files in each directory contain the word RAW, summary data files contain the word SUM. Some files also contain a version number (such as V2 for version 2) if the file has been revised.

File Size

File sizes range from 14 KB to 157 KB.

Spatial Coverage

Southernmost Latitude: 41.7º N
Northernmost Latitude: 42.7º N
Westernmost Longitude: 93.8º W
Easternmost Longitude: 93.2º W

Temporal Coverage

25 June 2002 to 12 July 2002

Temporal Resolution

All data were collected daily at multiple sites.

Parameter or Variable

Parameter Description

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

Parameter Unit of Measurement Sensor
Gravimetric soil moisture grams of water per grams of dry soil manual soil collection
Volumetric soil moisture grams of water per grams of dry soil manual soil collection
Bulk density grams per cubic centimeter (g/cm3) manual soil collection
Surface and soil temperature degrees Celsius OS643-LS Infrared Pyrometers
Surface and soil temperature (volumetric) degrees Celsius Theta Probes
Soil Moisture Water Fraction Volume (m3/m3) Theta Probes

Sample Data Record

The following is a sample from the gravimetric data file "IA_GSM_Raw_V2.txt":

Screen capture of a portion of a data file

The following sample is taken from the soil and surface temperature average data file "IA_TEMP_RAW.txt":

Screen capture of a portion of a data file

Error Sources

Theta Probe

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 probe 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 theta probe was used.

3. Data Access and Tools

Data Access

Data are available via FTP.

Related Data Collections

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

4. Data Acquisition and Processing

Theory of Measurements

The regional soil moisture site is within a single satellite passive microwave footprint (approximately 50 km), and measurements were taken around the time of the Aqua AMSR overpass (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.

Section Sampling

Sampling was performed on sites approximately a quarter section (0.8 km by 0.8 km) in size. A grid of 47 individual sites was sampled, covering approximately 50 km by 100 km (4 by 12 sites). One location in each of the 47 sites was sampled. The sampling was conducted between 12:00 and 15:00 local time.

Gravimetric and Volumetric Sampling

The primary measurement was the 0-6 cm dielectric constant at a single location in each site using Theta Probes. The dielectric constant is converted to volumetric soil moisture using a calibration equation. Volunteers used a coring tool to extract a single soil moisture sample of the 0-1 cm and 1-6 cm soil layers.

Computing Volumetric Soil Moisture and Bulk Density

Samplers used the following steps to compute volumetric soil moisture and bulk density:

Sensor or Instrument Description

Gravimetric and Soil Moisture

Samples were collected manually. In the laboratory they were weighed, dried, then weighed again.

Theta Probes

Investigators also used theta probes to measure surface volumetric soil moisture. The probes were Type ML2 manually-operated impedance instruments manufactured by Delta-T Devices, Ltd. The theta probes have 4 separate 6-cm stainless steel rods that were inserted vertically into the soil. Each instrument was connected to a handheld reader, which delivers the electrical pulse, detects the return signal, and converts the period to voltage between 0 and about 1 V.

The software provided by the probe manufacturer calibrates the theta probes 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 soil specific calibration for each field of sampling. Theta 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 VSM values were estimated.

5. References and Related Publications

Please see the SMEX02 site for more information, and the NSIDC SMEX site to access data.

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
FTP File Transfer Protocol
GSM Gravimetric Soil Moisture
NASA National Aeronautics and Space Administration
NSIDC National Snow and Ice Data Center
SMEX Soil Moisture Experiment
SMEX02 Soil Moisture Experiment 2002
TP ThetaProbe
URL Uniform Resource Locator
UTM Universal Transverse Mercator
VSM Volumetric Soil Moisture

Document Creation Date

July 2003

Document URL

http://nsidc.org/data/docs/daac/nsidc0182_smex_regional_sm.gd.html