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. Boulder, CO: National Snow and Ice Data Center. Digital media.
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:
- gravimetric
- soil_and_surface_temp
- theta_probe
- theta_probe_avg
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":
The following sample is taken from the soil and surface temperature average data file "IA_TEMP_RAW.txt":
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:
- Compute the gravimetric soil moisture (GSM) and dry mass
- Divide the dry mass of the soil by the volume of the cylinder (1 or 5 cm) to obtain the bulk density (BD)
- Compute volumetric soil moisture (VSM):
VSM=GSM*BD
Note: for the 0-1 cm samples, the 1-6 cm BD was used because of the unreliability of the 0-1 cm samples
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
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