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SMEX02 Watershed Soil Moisture Data, Walnut Creek, Iowa

Summary

This data set combines data for several parameters measured for the Soil Moisture Experiment 2002 (SMEX02). The parameters include bulk density, gravimetric and volumetric soil moisture, and surface and subsurface soil temperature. This study was conducted during June and July 2002 in the Walnut Creek watershed in south-central Iowa, USA. Data were collected in crop fields, using a variety of methods and sensors (manual soil samples for bulk density and gravimetric soil moisture data, infrared pyrometers for surface and subsurface soil temperature, and impedance probes for volumetric soil moisture). 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

We kindly request that you cite the use of this data set in a publication using the following citation example. For more information, see our Use and Copyright Web page.

Jackson, T. 2003. SMEX02 Watershed Soil Moisture Data, Walnut Creek, Iowa. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.

Overview Table

Category Description
Data format ASCII tab-delimited text
Spatial coverage Southernmost Latitude: 41.7° N
Northernmost Latitude: 42.7° N
Westernmost Longitude: 93.8° W
Easternmost Longitude: 93.2° W
Temporal coverage and resolution Data were collected daily from 23 June 2002 to 12 July 2002.
File naming convention Most file names contain "WC" to designate the Walnut Creek watershed site; raw data file names contain "raw"; summary data file names contain "sum"; other strings, such as "bd", "gsm", "temp", or "tp", are used in the file name to indicate different types of data. File names may be uppercase or mixed upper- and lowercase.
File size File sizes range from approximately 3 KB to 1 MB.
Parameter(s) Bulk density, gravimetric soil moisture, surface and subsurface soil temperature, volumetric soil moisture
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

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

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

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

Many graduate students and volunteers collected the field data.

2. Detailed Data Description

Format

Data consist of ASCII tab-delimited text files.

File and Directory Structure

Data files are in subdirectories according to the data type and collection method. The subdirectories are described in the following table.

Subdirectory Name Parameter in Data Files
bulk_density Bulk density, manually collected soil samples; raw and summary data
gravimetric Gravimetric soil moisture, manually collected soil samples
soil_and_surface_temp Surface and subsurface soil temperature, measured with infrared pyrometers; raw data
soil_and_surface_temp_avg Surface and subsurface soil temperature, measured with infrared pyrometers; summary data
theta_probe Surface volumetric soil moisture measured with impedance probes (ThetaProbes); raw data
theta_probe_avg Surface volumetric soil moisture measured with impedance probes (ThetaProbes); summary data

File Naming Convention

All data files have the file name extension ".txt". File names may be uppercase or mixed upper- and lowercase. Most data file names contain the letters "WC" to designate the Walnut Creek watershed site. When applicable for a given parameter, raw data file names contain the letters "raw" and summary data file names contain the letters "sum". Bulk density file names contain the letters "BD". Gravimetric soil moisture data file names contain the letters "GSM" (file SOILCAN.txt is the soil can weight file). Temperature data file names contain the letters "TEMP". Volumetric soil moisture (ThetaProbe) files contain the letters "TP".

File Size

Text file sizes range from approximately 3 KB to 1 MB.

Spatial Coverage

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

Temporal Coverage and Resolution

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

Parameter or Variable

Parameter Description

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

Parameter Unit of Measurement Source/Sensor
Bulk density g/cm3 Manual soil collection
Gravimetric soil moisture grams of water per grams of dry soil Manual soil collection
Surface and subsurface soil temperature °C Infrared pyrometers and temperature probes
Volumetric soil moisture water fraction volume (m3/m3) Impedance probes (ThetaProbes)

The tab-delimited ASCII text files contain data in row and column format. In each file, header rows indicate the contents of each column, and the headers are followed by data rows. The following sections describe the contents of the various data files. For each file, the column headings are listed, exactly as they appear in the data files, along with a brief description of each columns' contents. Field coordinates are given in the WGS84 Datum. Any suspect or missing data in the files are flagged with a "null" value such as -99 or -999.

Bulk Density

The raw bulk density data file, "RawDataSMEX02BD.txt", contains the following information.

Column Heading Description
Site ID Site name
Date (M/D/Y) Month/day/year
Row Spacing

Location in crop row: "In", "Between", or "none", indicating
in a row, between rows, or no rows (such as broadcast soybeans)

Site Loca Specific sampling location in site, either a, b, c, or d
Veg Crop Type of crop
Start Volume Starting volume in ml
Final Volume Final volume in ml
Total Volume Total volume in ml
Bag Tare Wt Weight of empty sample bag in g
Wet Weight Weight of wet soil in g
Dry Weight Weight of dry soil in g
% H2O Gravimetric soil moisture
BD g/cm 3 Bulk density (g/cm3)
(no heading given) Notes, if any

The summary file, "SumDataSMEX02BD.txt", contains the following information (the means of all samples at each site).

Column Heading Description
Site ID Site name
Date (m/d/y) Month/day/year
Row Spacing Location in crop row: "IN", "BE", or "AL", indicating
in a row, between rows, or all the same
Veg Crop Type of crop
BD g/cm 3 Bulk density (g/cm3)
(no heading given) Notes, if any
Gravimetric Soil Moisture

The raw gravimetric soil moisture data file, "WC_GSM_RAW.txt", contains the following information.

Column Heading Description
Date Sample date as month/day/year
DOY Numerical day of the year
SiteID Site identifier (WCnn, where nn is site number)
Sample Measurement location number in each site
Lat Latitude of the site in decimal degrees
Lon Longitude of the site in decimal degrees
UTM Easting in m Universal Transverse Mercator (UTM) Easting in meters
UTM Northing in m UTM Northing in meters
CanID ID that distinguishes between soil cans of different weights
Depth Code indicating depth; 0 = 0-1 cm, 1 = 1-6 cm
WetWgt in g Initial wet soil weight in grams
DryWgt in g Dry soil weight in grams
GSM in g/g Gravimetric soil moisture in grams of water per grams of soil
Notes Notes about the data, if any

The summary gravimetric soil moisture data file, "WC_GSM_SUM.txt", contains the following information.

Column Heading Description
Date Sample date as month/day/year
DOY Numerical day of the year
SiteID Site identifier (WCnn, where nn is site number)
Lat Centroid Latitude of the site centroid in decimal degrees
Lon Centroid Longitude of the site centroid in decimal degrees
UTM Easting in m Centroid UTM Easting in meters of site centroid
UTM Northing in m Centroid UTM Northing in meters of site centroid
#samples Number of samples per site per day
start time (CDT) Start time of sampling in local time
stop time (CDT) Stop time of sampling in local time
average (0-1cm) in grams Mean GSM of 0-1 cm depth
stdev (0-1cm) in grams Standard deviation of GSM of 0-1 cm depth
average (1-6cm) in grams Mean GSM of 1-6 cm depth
stdev (1-6cm) in grams Standard deviation of GSM of 1-6 cm depth
average (0-6 cm) in grams Mean GSM of 0-6 cm depth
stdev (0-6cm) in grams Standard deviation of GSM of 0-6 cm depth
Notes Notes about the data, if any

The "SOILCAN.txt" file contains can number and the weight, in grams, of the various soil cans used for sample collection. When soil can weights are not available, the average weight of all the cans should be used, because they are all of the same type.

Temperature

The raw temperature file, "WC_TEMP_RAW.txt", contains the following information.

Column Heading Description
Date Date in month/day/year format
DOY Numerical day of the year
Site ID Site identifier (WCnn, where nn is site number)
Sample Measurement location code in each site
latitude Latitude in decimal degrees
longtide Longitude in decimal degrees
Easting UTM Easting in meters (Zone 15)
Northing UTM Northing in meters (Zone 15)
LST Land surface temperature in °C
1 cm Soil temperature at 1 cm depth in °C
5 cm Soil temperature at 5 cm depth in °C
10 cm Soil temperature at 10 cm depth in °C

The summary temperature file, "WC_TEMP_SUM.txt", contains the means and standard deviation by field (site) and day, including the following information.

Column Heading Description
Date Date in month/day/year format
DOY Numerical day of the year
Site ID Site identifier (WCnn, where nn is site number)
latitude Latitude in decimal degrees
longitude Longitude in decimal degrees
Easting UTM Easting of centroid of sampling sites in meters
northing UTM Northing of centroid of sampling sites in meters
number points Number of samples per site per day
start tme Start time of sampling in local time
stop time Stop time of sampling in local time
average LST Mean land surface temperature
stdev LST Standard deviation of land surface temperature
average 1 cm Mean soil temperature at 1 cm depth
stdev 1 cm Standard deviation of soil temperature at 1 cm depth
average 5 cm Mean soil temperature at 5 cm depth
stdev 5 cm Standard deviation of soil temperature at 5 cm depth
average 10 cm Mean soil temperature at 10 cm depth
stdev 10 cm Standard deviation of soil temperature at 10 cm depth
Volumetric Soil Moisture (ThetaProbe)

The raw volumetric soil moisture (ThetaProbe) data file, "WC_TP_RAW.txt", contains the following information.

Column Heading Description
Date Date as month/day/year
DOY Numerical day of the year
SiteID Site identifier (WCnn, where nn is site number)
SampleNo Measurement location number in each site
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 of planted crops
0.25 = midway between the 0 and 0.5 locations
V ThetaProbe voltage output in volts
VSM_gc Volumetric soil moisture content from generalized calibration of voltage data
VSM_ssc Volumetric soil moisture content from specific calibration of voltage data

The summary volumetric soil moisture (ThetaProbe) data file, "WC_TP_SUM.txt", contains the following information.

Column Heading Description
Date Date as month/day/year
DOY Numerical day of the year
SiteID Site identifier (WCnn, where nn is site 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)
#ofsamples Number of samples taken at site
V Average ThetaProbe voltage output in volts
Stdev Standard deviation of ThetaProbe voltage output in volts
VSM_gc Volumetric soil moisture content from generalized calibration of voltage data
Stdev Standard deviation of VSM_gc
VSM_ssc Volumetric soil moisture content from specific calibration of voltage data
Stdev Standard deviation of VSM_ssc

Sample Data Records

The following sample is from the raw gravimetric soil moisture data file "WC_GSM_Raw.txt". Not all columns are shown.

Date DOY ... Sample ... CanID Depth WetWgt
in g
DryWgt
in g
GSM
in g/g
6/25/2002 176 ... 2 ... 129 0 83.2 79.1 0.107
6/25/2002 176 ... 2 ... 130 1 137.2 124.8 0.147
6/25/2002 176 ... 6 ... 131 0 84.3 81 0.083
6/25/2002 176 ... 6 ... 132 1 136.6 118.8 0.227
6/25/2002 176 ... 9 ... 133 0 79.5 75.8 0.106
6/25/2002 176 ... 9 ... 134 1 148 133.3 0.159
6/25/2002 176 ... 13 ... 135 0 63.2 61 0.109
6/25/2002 176 ... 13 ... 136 1 170.6 144.6 0.252

The following sample is from the average temperature data file "WC_TEMP_SUM.txt." Not all columns are shown.

Date ... Site ID ... number
points
... average
LST
... stdev
1 cm
... average
10 cm
stdev
10 cm
6/25/2002 ... WC01 ... 4 ... 37.25 ... 3.153 ... 25.9 1.134
6/25/2002 ... WC03 ... 5 ... 38 ... 3.564 ... 27.9 1.342
6/25/2002 ... WC04 ... 4 ... 44.5 ... 4.125 ... 26.225 1.664
6/25/2002 ... WC05 ... 4 ... 31.5 ... 1.746 ... 24 1.92
6/25/2002 ... WC06 ... 5 ... 30 ... 3.009 ... 25.12 0.638
6/25/2002 ... WC08 ... 4 ... 32.25 ... 1.352 ... 23.875 0.206

Error Sources

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 ThetaProbe 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 ThetaProbe was used.

3. Data Access and Tools

Data Access

Data are available via FTP.

Software and Tools

No special tools are required to view these data.

Related Data Collections

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

4. Data Acquisition and Processing

Sensor or Instrument Description

Manual Soil Collection (Bulk Density and Gravimetric Soil Moisture)

Soil samples were manually collected in the field using spatulas, scoop tools, and soil cans. These samples were used for bulk density and gravimetric soil moisture measurements. Soil samples were weighed with balances and dried in ovens.

Thermometers (Temperature)

The surface temperature was sampled using handheld infrared thermometers, and soil temperature was obtained using a digital thermometer probe inserted to depths of 1 cm, 5 cm and 10 cm. The handheld infrared thermometers were OMEGA OS643-LS Infrared Pyrometers. The instrument has an emissivity of 0.95, accuracy of ± 3%, and temperature range of 0 to 260°C (32 to 500°F). Refer to OMEGA Engineering for more information.


OS643-LS Infrared Pyrometer

ThetaProbes (Volumetric Soil Moisture)

Volumetric soil water content is the ratio between the volume of water present in the soil and the total volume of the sample. Investigators used impedance probes to measure surface volumetric soil moisture, by determination of the apparent dielectric constant.

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


ML2 ThetaProbe and Reader

The software provided by the probe manufacturer calibrates the ThetaProbes 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 the constants a0 and a1 depend on soil type and for this experiment are 1.6 and 8.4, respectively. These estimates are provided in the data files.

Researchers also performed soil-specific calibration for each sampling field. ThetaProbe voltage readings from the quarter row sampling point were compared to the volumetric soil moisture measured at the same point. A regression relationship was developed and new volumetric soil moisture values were estimated.

Data Acquisition Methods

Section Sampling

Sampling was performed on sites approximately one quarter section (0.8 km by 0.8 km) in size. As shown in the following illustration, 14 points were sampled in a field for a variety of variables. The site sampling generally took place each morning between approximately 8:00 a.m. and 11:30 a.m., local time. Some samples (such as gravimetric soil moisture and temperature), were taken at only four points in a site, marked "All" in the illustration. If the sampled field had a flux tower, additional sampling was done at the base of each tower.

Chart of row sampling layout

Section Sampling Layout

The sampling site numbers are included in the gravimetric soil moisture raw data file ("WC_GSM_Raw.txt") and both (ThetaProbe) volumetric soil moisture data files ("WC_TP_RAW.txt" and "WC_TP_SUM.txt").

Bulk Density Sampling

Bulk density sampling was conducted at all sites. In general, four samples per site were taken. Efforts were made to co-locate the bulk density samples with the gravimetric soil moisture sampling points. In cases where these points could not be easily identified, approximations of location were made. After selecting a location, vegetation was removed and a volume of surface soil was extracted to a depth of 5 cm. Where row direction could be determined, two samples were taken directly in the row of plants (row spacing noted as "In" in the raw data file ) and two samples were taken halfway between the plant rows (row spacing noted as "Between"). In the case of broadcast soybeans, "none" is noted as the row spacing.

Temperature Sampling

Surface and subsurface soil temperature sampling was conducted at the four points selected for gravimetric soil moisture (GSM) sampling in each section (marked "All" in the Section Sampling Layout illustration). The surface temperature was sampled using handheld infrared thermometers, with one averaged surface thermal infrared temperature made per point. Soil temperature was obtained using a digital temperature probe inserted to depths of 1 cm, 5 cm and 10 cm.

Gravimetric Soil Moisture Sampling

Gravimetric soil moisture samples were taken at only four points, marked "All" in the Section Sampling Layout illustration. Researchers took 6 cm deep samples at each point using a scoop tool. Each sample was was then cut into a 0 cm to 1 cm depth section and a 1 cm to 6 cm depth section.

Volumetric Soil Moisture (Impedance ThetaProbe) Sampling

Investigators used manually inserted impedance probes (ThetaProbes) and a handheld reader to measure surface volumetric soil moisture. The probe was inserted vertically into the soil (0-6 cm) at three points along the crop row. The observed soil moisture and voltage was recorded along with the date, time, field number, site number, and other relevant comments.

Processing Steps

Bulk Density Processing

Using the volume of soil extracted and the dry weight of the soil extracted, the bulk density was computed using the following steps:

It is also possible to compute the soil moisture of the sample used for bulk density using the following formula:

Gravimetric soil moisture = (wet weight - dry weight) / dry weight

When summarizing the data, if the in-plant row measurements (noted as "IN") were greater than 0.10 different than the halfway-between-plant row measurements (noted as "BE"), both values are given for the field. If this difference was less than 0.10, it is assumed that there was no difference between the bulk density samples, and a single value (noted as "AL") is reported for all of the field.

Gravimetric Soil Moisture Processing

Researchers used a balance to weigh the wet soil obtained in the field, heated the soil in an oven to dry it, then weighed the dry soil. The soil moisture of the sample can be calculated using the following formula:

Gravimetric soil moisture = (wet weight - dry weight) / dry weight

Investigators used soil bulk density to convert the gravimetric samples to volumetric soil moisture using a volume extraction technique. Four samples per site were taken. Where they could determine row direction, two samples (labeled "In") were taken directly in the row of plants, and two samples (labeled "Between") were taken halfway between the plant rows. In the case of broadcast soybeans, "none" denotes the row spacing.

Investigators attempted to co-locate the bulk density samples with the gravimetric soil moisture sampling points. In cases where these points could not be easily identified, they used approximate locations.

Volumetric Soil Moisture (ThetaProbe) Processing

Watershed surface soil moisture was sampled each morning during the experiment. ThetaProbe 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 volumetric soil moisture values were estimated.

Field averages were calculated using a simple averaging scheme that counted the quarter row reading twice for a total of four data points per sampling site. A field average was computed by first computing the sampling site average and then computing the average among the (approximately) 14 sampling sites. Finally, standard deviations were calculated.

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
DOY Day of Year
FTP File Transfer Protocol
GSM Gravimetric Soil Moisture
LST Land Surface Temperature
NSIDC National Snow and Ice Data Center
SMEX Soil Moisture Experiment
UTM Universal Transverse Mercator
WC Walnut Creek

Document Creation Date

2 July 2003

Document Revision Date

20 August 2005

Document URL

http://nsidc.org/data/docs/daac/nsidc0143_smex_WC_watershed.gd.html