SMEX02 Soil Moisture and Temperature Profiles, Walnut Creek, Iowa

Summary

This data set contains rainfall, soil moisture, and soil temperature data collected for the Soil Moisture Experiment 2002 (SMEX02). The parameters measured are soil moisture, soil temperature, and rainfall. Soil moisture and temperature were measured at various depths: 2, 5, 10, 15, 20 and 30 cm.

Data were collected in four fields, each containing two measurement sites called "pit A" and "pit B." The study was conducted in the Walnut Creek watershed in south central Iowa, USA, during June and July 2002. Water content reflectometers (WCRs) measured soil moisture. Soil temperature probes (STPs) and averaging thermocouple (TCAV) probes measured soil temperature. A tipping bucket rain gauge measured rainfall.

Data are provided in ASCII text files and a Microsoft Excel file, 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

Crosson, W. and C. Laymon. 2003. SMEX02 Soil Moisture and Temperature Profiles, Walnut Creek, Iowa. Boulder, CO: National Snow and Ice Data Center. Digital media.

Overview Table

Category	Description
Data format	Data are provided in a Microsoft Excel file with multiple sheets and as text files.
Spatial coverage	Southernmost Latitude: 41.934 Northernmost Latitude: 41.992 Westernmost Longitude: -93.664 Easternmost Longitude: -93.529
Temporal coverage and resolution	23 June 2002 to 23 July 2002. Measurements were taken daily at 10-second intervals and averaged over 15-minute output intervals.
File size	Excel file is 5 MB; text files range from 140 KB to 605 KB.
Parameter(s)	Soil moisture, soil temperature, and rainfall.
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

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.

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

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

2. Detailed Data Description

Format

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.

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.

File Size

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

Spatial Coverage

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

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.

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

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.

3. Data Access and Tools

Data Access

Access the files via FTP.

Volume

Total volume of all data files is 6.13 MB.

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.

Related Data Collections

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

4. 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.

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* t_cor + Intercept
For example, for Field 15, sensor A2, if t_cor = 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

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 (t_cor) is calculated from measured period (t_meas) and soil temperature (T) at the same depth using:

t_cor = t_meas - (T-22)[t_meas(1 - c₁) - c_o]/15

where c_o = 0.80 and c₁ = 0.1824.

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.

FTP	File Transfer Protocol
GWC	Gravimetric Water Content
SMEX	Soil Moisture Experiment
STP	Soil Temperature Probes
TCAV	Averaging Soil Thermocouple Probe
VWC	Volumetric Water Content
WCR	Water Content Reflectometer

Document Creation Date

August 2003

Document URL

http://nsidc.org/data/docs/daac/nsidc0185_smex_soil_profiles.gd.html