Data Set ID:

SMEX02 Watershed Vitel Network Soil Moisture Data, Walnut Creek, Iowa, Version 1

This data set contains several parameters measured for the Soil Moisture Experiment 2002 (SMEX02).

This is the most recent version of these data.

Data Format(s):
  • ASCII Text
Spatial Coverage:
N: 42, 
S: 41.9, 
E: -93.4, 
W: -93.8
Temporal Coverage:
  • 23 June 2002 to 12 July 2002
Temporal ResolutionNot specifiedMetadata XML:View Metadata Record
Data Contributor(s):Thomas Jackson, Michael Cosh

Geographic Coverage

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As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Jackson, T. and M. Cosh. 2003. SMEX02 Watershed Vitel Network Soil Moisture Data, Walnut Creek, Iowa, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: [Date Accessed].

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Detailed Data Description


Data consist of tab-delimited ASCII text files. The following table describes the column headings for each data file.

Heading Description
Year Four-digit year
DOY Numerical day of year
Hour Local daylight savings time
Site Watershed site number (corresponds to the data file name):
   3 =   WC03    41.9802° N   93.7399° W
   6 =   WC06    41.9330° N   93.7534° W
  13 =  WC13    41.9521° N   93.6877° W
  14 =  WC14    41.9460° N   93.6961° W
151 = WC151    41.9378° N   93.6632° W
152 = WC152    41.9378° N   93.6647° W
161 = WC161    41.9342° N   93.6627° W
162 = WC162    41.9355° N   93.6641° W
  23 =  WC23    41.9925° N   93.5351° W
  24 =  WC24    41.9928° N   93.5288° W
  25 =  WC25    41.9423° N   93.5394° W
  33 =  WC33    41.9755° N   93.6444° W
V1 Voltage 1
V2 Voltage 2
V3 Voltage 3
V4 Voltage 4
Soil Soil type (1 = Sand; 2 = Silt; 3 = Clay)
Ercent Real dielectric constant
Eicent Imaginary dielectric constant
Temp °C
Ercent25 Real dielectric constant, temperature corrected to 25° C
Eicent25 Imaginary dielectric constant, temperature corrected to 25° C
Waterwfv Water fraction volume in m3/m3 (volumetric soil moisture)
NACLburden Soil salinity in g NaCl/liter
Concent Soil conductivity in S/m
concent25 Temperature-corrected (25 C) soil conductivity in S/m
watercond Temperature-corrected (25 C) soil water conductivity in S/m
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File and Directory Structure

There are 12 data files (ASCII text files), one for each probe. All files are located in the tower_vitel_sensors directory.

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File Naming Convention

File names are of the form "WCxxx_HP_VSM.txt," where:

  • WC signifies the Walnut Creek watershed site
  • xxx is a number that corresponds to the flux tower location near which the probe was installed
  • HP signifies Hyrda Probe
  • VSM signifies volumetric soil moisture
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File Size

File sizes range from 25 KB to 145 KB.

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Spatial Coverage

Data were collected at sites within approximately:

Southernmost Latitude: 41.9° N
Northernmost Latitude: 42.0° N
Westernmost Longitude: -93.8° W
Easternmost Longitude: -93.4° W

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Temporal Coverage

Data were collected from 23 June 2002 to 12 July 2002.

Temporal Resolution

Data were collected hourly.

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Parameter or Variable

Parameter Description

Parameters in this data set include the following:

Parameter Unit of Measurement
Soil temperature °C
Soil moisture Water fraction volume (m3/m3)
Soil salinity g NaCl/liter
Soil conductivity MKS unit S/m

Sample Data Record

The following is a sample from file "WC23_HP_VSM.txt." Only the header and the first four data rows are shown; only the first 10 of the 19 columns are shown.

Year DOY Hour Site V1 V2 V3 V4 Soil Ercent
2002 177 1000 23 1.9783 1.538 1.2116 0.6332 2 8.8
2002 177 1100 23 1.9745 1.5322 1.2018 0.56798 2 8.82
2002 177 1200 23 1.9718 1.5279 1.1944 0.53559 2 8.83
2002 177 1300 23 1.9711 1.5262 1.1908 0.50254 2 8.84
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Software and Tools

No special tools are required to view these data.

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Data Acquisition and Processing

A network of Vitel Hydra Probes was installed near most of the flux towers that were established for the SMEX02 experiment. A total of 12 probes were installed in mid-June 2002 throughout the Walnut Creek watershed area at a depth of 5 cm. The probes recorded data on an hourly basis.

flux tower
Flux Tower

Derivation Techniques and Algorithms

Processing Steps

The Vitel Type A Hydra Probe (HP) soil moisture probe determines soil moisture and salinity by making a high frequency (50-MHz) complex dielectric constant measurement, which simultaneously resolves the capacitive and conductive parts of a soil's electrical response. The capacitive part of the response is most indicative of soil moisture, while the conductive part reflects mostly soil salinity. Temperature is determined from a calibrated thermistor incorporated into the probe head.

The HP has three main structural components: a multiconductor cable, a probe head, and sensing tines. The probes were installed horizontally in the soil, with the center tine at a depth of 5 cm.

The HP measured the real and imaginary dielectric constants. Because both the real and imaginary dielectric constants vary somewhat with temperature, a temperature correction using the measured soil temperature is applied to produce temperature-corrected values for the real and imaginary dielectric constant. The temperature correction amounts to calculating what the dielectric constants should be at 25° C.

The output data from an HP consists of a time stamp and four voltages (V1-V4), which are converted to estimate the soil moisture and soil temperature through a program provided by Stevens-Vitel. The program requires the four voltages and a soil classification (sand=1, silt=2, and clay=3). For this region, silt was used as the appropriate soil type for each of the HP installations. Preliminary calibration of the probes indicated that with the exception of the WC33 site, the probes measured volumetric soil moisture accurately for the installation location. Therefore, the quality control of these data was limited to removing samples for which the program returned erroneous data because of corrupted voltages. These voltages may be a result of several things, including faulty installation, lightning strikes, and rodent impact. Erroneous samples were removed, so the data are not continuous for every HP.

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Sensor or Instrument Description

Measurements were taken using Vitel Type A Hydra Probes (HP). This version is compatible with Campbell CR-10 data loggers; the temperature output voltage never exceeds 2.5 V. Refer to the manufacturer, Stevens-Vitel for more information.

Vitel hydra probe
Vitel Hydra Probe

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References and Related Publications

Contacts and Acknowledgments

Tom Jackson 
Hydrology and Remote Sensing Laboratory
US Department of Agriculture (USDA) - Agricultural Research Service (ARS)
Beltsville, MD

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

Lynn McKee
Hydrology and Remote Sensing Laboratory
US Department of Agriculture (USDA) - Agricultural Research Service (ARS)
Beltsville, MD


Many graduate students and volunteers collected the field data

Document Information


20 August 2005

No technical references available for this data set.

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