Data Set ID:

SMEX02 Soil Moisture and Temperature Profiles, Walnut Creek, Iowa, Version 1

This data set contains rainfall, soil moisture, and soil temperature data collected for the Soil Moisture Experiment 2002 (SMEX02).

This is the most recent version of these data.

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Data Format(s):
  • ASCII Text
  • Microsoft Excel
Spatial Coverage:
N: 41.992, 
S: 41.934, 
E: -93.529, 
W: -93.664
Temporal Coverage:
  • 23 June 2002 to 23 July 2002
Temporal ResolutionNot specifiedMetadata XML:View Metadata Record
Data Contributor(s):Charles Laymon, William Crosson

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.

Crosson, W. and C. Laymon. 2003. SMEX02 Soil Moisture and Temperature Profiles, 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 are provided in a Microsoft Excel file with multiple sheets and as text files for each field. The text files contain the same data as the Excel file. In the text files, data are in tab-delimited format.

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File Naming Convention

The Excel file is named "soil_profile_data.xls" and contains worksheets labeled by field number. The text files are named "soil_profile_fieldxx.txt," where xx corresponds to the field ID number.

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File Size

The Excel file is 4.87 MB; text files range from 140 KB to 605 KB.

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Total volume of all data files is 6.13 MB.

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Spatial Coverage

Southernmost Latitude: 41.934
Northernmost Latitude: 41.992
Westernmost Longitude: -93.664
Easternmost Longitude: -93.529

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Temporal Coverage

Measurements for fields 15 and 16 were taken daily between 23 June 2002 and 23 July 2002. Measurements for fields 23 and 24 were taken daily between 24 June 2002 and 23 July 2002.

Temporal Resolution

Measurements were recorded at 10-second intervals and averaged over 15-minute output intervals. The data files contain only the 15-minute averages.

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Parameter or Variable

Parameter Description

Parameters in this data set are soil moisture, soil temperature, and rainfall.

Parameter Range

The following table details the column headings for the data files. Where the column is a parameter, the description includes the sensor and unit of measurement.

Column Heading Description
DOY Numerical day of the year (Julian date)
Time Time in the form HHMM (Central Daylight Time)
Elapsed Time Elapsed time in hours since DOY 174, hour 0
Field ID The number of the field: 15, 16, 23, or 24 (also indicated by the worksheet or file name)
Batt_V The voltage of the battery supplying power to the data system (nominally 12 V)
Rain Rainfall amount (in mm) from tipping bucket gauge
WCR Per (pit ID, depth) Period (msec) measured by the WCR in pit A or B, at depths of 2, 5, 10, 15, 20, and 30 cm
VWC Per (pit ID, depth) Volumetric water content (VWC) (percentage) measured in pit A or B, at depths of 2, 5, 10, 15, 20, and 30 cm
Soil T (pit ID, depth) Soil temperature (in °C) measured in pit A or B, at depths of 2, 5, 10, 15, 20, and 30 cm
Mux Temp Multiplexer temperature (in °C)
TCAV Average temperature (in °C) of upper 5 cm soil layer

Sample Data Record

The following sample shows a small number of the actual columns in the data tables. The sample contains one column of each kind of measurement that exists in the data. The actual data tables contain measurements for multiple sites, where the sample below shows data for only one site, including the multiplexer reading and the average soil temperature for the upper 5 cm.

DOY Time Elapsed Time Field ID Rain WCR Per A2 VWC A2 Soil T A2 Mux Temp TCAV
All Times CDT mm msec % C C C
175 1415 38.25 23 0 0.965 9.6 36.52 38.01 27.48
175 1430 38.5 23 0 0.964 9.5 36.82 37.98 29.52
175 1445 38.75 23 0 0.964 9.49 37.03 38.16 32.84
175 1500 39 23 0 0.963 9.4 37.18 37.05 33.66
175 1515 39.25 23 0 0.962 9.32 37.39 36.36 33.87
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Software and Tools

No special tools are required to view the text files. A spreadsheet program, text viewer, or Web browser will display the Microsoft Excel file.

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Data Acquisition and Processing

The data collection stations were located at four fields in the Walnut Creek watershed. Fields 15 and 24 were corn fields; 16 and 23 were soybean fields. The schematic below illustrates how the pits were arranged at the study fields. At each field, two pits were dug within 5 m of each other to a depth of 30 cm. For corn fields, pit A was dug between the rows perpendicular to rows. Pit B was adjacent to a row, 15 cm from the stems of the plants. For soybean fields, the two profiles were established with no regard to row structure.

In each pit, soil moisture and temperature measurements were made at six depths: 2, 5, 10, 15, 20, and 30 cm. Soil moisture was measured at each depth using a WCR, a device based on time domain reflectometry. WCRs were installed with both rods in the horizontal plane. The WCR rods in pit A were installed parallel to and equidistant from the rows. The rods in pit B were installed perpendicular to and under the crop row. Soil temperature was measured at the six depths in pit A using soil temperature probes (STPs). In addition, the mean 0-5 cm temperature was measured in pit A using 4-sensor averaging thermocouple (TCAV) probes installed at 1, 2, 3, and 4 cm depths. A tipping bucket rain gauge having 0.1 mm resolution was deployed above the vegetation at each plot.

Schematic drawing of the pit configurations

Schematic drawing of the pit configurations, courtesy of Alabama A&M University

Derivation Techniques and Algorithms

Processing Steps

Conversion of WCR periods to volumetric water content (VWC) is a two-step process. The period is corrected for temperature effects, and then the temperature-corrected period is used to obtain VWC.

Temperature-corrected period (tcor) is calculated from measured period (tmeas) and soil temperature (T) at the same depth using:

tcor = tmeas - (T-22)[tmeas(1 - c1) - co]/15

where co = 0.80 and c1 = 0.1824.

Error Sources

Missing and erroneous data were identified based on field notes and post-experiment examination of output. Missing periods of brief duration (approximately one hour or less) were filled using linear interpolation. The following tables indicate dates and times of missing or bad data and what action was used to correct them.

Site 15

Data begins 174/1715, ends 193/1115

Sensor Date/time Explanation Data Correction
WCR Per B15 180/1115 Checking sensor wiring Interpolated
WCR Per B20 180/1100-1115 Checking sensor wiring Interpolated
WCR Per B20 183/1000 Connecting storage module Interpolated
Soil T A5 Before 183/1015 Bad sensor Set to missing value
All WCRs 187/1230-1630 Intermittent battery problem Interpolated periods

Site 16

Data begins 174/1615, ends 193/1145

Sensor Date/time Explanation Data Correction
Soil T A2 All Sensor improperly deployed Set to missing value
Rain 191/1945-2030 Unknown Set to Field 15 value

Field 23

Data begins 175/1415, ends 204/1345

Sensor Date/time Explanation Data Correction
WCR B10, B15 All All Data indicates sensors were too shallow None; data are suspect

Field 24

Data begins 175/1515, ends 204/1345. No missing or bad data.

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Sensor or Instrument Description

The WCRs were calibrated post-experiment using data from soil cores taken daily at each field. This was done to improve the fit (relative to the default Campbell equation) between WCR gravimetric water content (GWC) estimates and GWC measured at each site. VWC is computed from the calibrated WCRs. Temperature-corrected periods for the WCRs were compared at each site with GWC values from the sliced cores for the corresponding depths, 2 cm and 5 cm. WCR measurements at these depths were compared with sliced core data for 0-4 cm and 4-6 cm layers. Soil core data were not available for greater depths. The following table summarizes the linear regression analysis for each field and sensor. Columns labeled A sites, B sites, and All sites contain coefficients derived by pooling the indicated data. The A2 and B2 columns refer to the 2 cm measurements for each site. The A5 and B5 columns refer to the 5 cm measurements for each site. The linear regression equations shown in the tables below were applied to each sensor as follows.

Note: Values in italics were not applied in computing the final calibrated VWC values.

Field 15

Sensor A2 B2 A5 B5 A sites B sites All sites
Correlation 0.94 0.97 0.90 0.96 0.89 0.94 0.70
Slope 62.45 84.76 48.70 62.75 52.80 72.43 35.58
Intercept -58.24 -69.90 -45.02 -50.22 -48.81 -58.95 -25.75

Field 16

Sensor A2 B2 A5 B5 A sites B sites All sites
Correlation 0.90 0.93 0.81 0.89 0.85 0.81 0.81
Slope 120.86 94.79 76.73 56.95 99.54 60.96 71.80
Intercept -12.12 -80.37 -66.37 -47.43 -90.0 -49.44 -60.94

Field 23

Sensor A2 B2 A5 B5 A sites B sites All sites
Correlation 0.87 0.93 0.83 0.87 0.48 0.78 0.55
Slope 75.07 81.17 86.09 69.60 21.26 53.87 27.21
Intercept -63.99 -68.99 -93.54 -63.45 -12.51 -44.37 -18.03

Field 24

Sensor A2 B2 A5 B5 A sites B sites All sites
Correlation 0.90 0.91 0.81 0.92 0.65 0.76 0.63
Slope 72.33 51.37 53.03 52.72 32.35 32.97 26.09
Intercept -68.44 -38.14 -54.28 -47.34 -24.93 -21.62 -15.63

For sensors at 2 cm and 5 cm, the equation for the specific sensor was applied. For sensors at depths of 10, 15, 20 and 30 cm, the equation for A sites or B sites was applied. The exception was at Field 16 for the A sensors. The large slope and intercept values in the A2 and A sites equations resulted in some negative moisture contents. Therefore, the All sites equation was applied for all A sensors at Field 16. For other sites, the All sites equations were not used, but are shown here for completeness.

The regression model is of the form
GWC = Slope* tcor + Intercept
For example, for Field 15, sensor A2, if tcor = 1.0 msec, then:
GWC = 62.45*1.0 - 58.24 = 4.21 (%)

GWC was then converted to VWC using the following bulk densities for each field:

Field Bulk density (g/cm^3)
15 1.20
16 1.15
23 1.25
24 1.15
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References and Related Publications

Contacts and Acknowledgments

William Crosson, meteorologist, and Charles Laymon, Hydrologist, Earth System Science Program/National Space Science and Technology Center/Global Hydrology and Climate Center, Huntsville, AL, USA.


Ashutosh Limaye and Frank Archer, Universities Space Research Association/National Space Science and Technology Center/Marshall Space Flight Center (USRA/NSSTC/MSFC), Huntsville, AL, USA

Document Information


August 2003

No technical references available for this data set.

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