Impedance Probe Measurements of Surface Soil Moisture Near the ARM CART Central Facility (CF)
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The Data
These data result from daily measurements of surface (0-6 cm) volumetric soil moisture using a manually inserted probe and hand-held reader (see below). Samples were collected from several different fields near the ARM CART Central Facility on regular grids at several different spatial scales. Depending on the particular data set (scale), the sampling frequency is hourly, daily, or occasional.
The Files
Surface soil moisture data from the Central Facility are organized into files grouped by spatial scale. Data from field CF04, the site of continuous intensive investigation, are contained in a text file (CF04.txt). The data for the full section, i.e., contiguous fields CF03, CF04, CF05, and CF06, collected along transects spaced 200 m apart, are contained in a separate text file (CF03-CF06.txt). Each of these data files comprises a spreadsheet in a Microsoft Excel workbook file (CF_ThetaProbe.xls). The files and their contents are described in the following tables.
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Three sites within CF04 were selected for higher resolution nested-scale grid sampling (see below). Data for each day that sampling took place at each site are contained in a separate text file. These text files also make up a separate spreadsheet in a Microsoft Excel workbook.
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Data collected on a grid coincident with the radiometer footprints on CF01 and CF02 are provided in two ASCII files (CF01footprint.txt and CF02footprint.txt). These data are also provided as spreadsheets in an Excel workbook (SLMRFootprint.xls).
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The Science
Surface volumetric soil moisture was measured with a ThetaProbes, Type ML1. The ThetaProbe is a manually-operated impedance instrument manufactured by Delta-T Devices, Ltd., Cambridge, England and distributed in the U.S. by Dynamax, Inc., Houston, TX. The ThetaProbe has four 6 cm stainless steel rods (Figure 1) and was inserted vertically into the soil. The instrument was connected to a hand-held reader (Figure 2), which delivers the electrical pulse, detects the return signal and converts the period to voltage between 0 and about 1 V for dry to saturated conditions. In the field, observed voltages were recorded on Sharp Wizard pocket organizers, which were downloaded to a PC each evening.
Prior to the SGP97 field experiment, a plan was developed to evaluate the subpixel-scale spatial variability of surface soil moisture in different settings within the experiment domain. Six sites were chosen for this purpose, one of which was located near the DOE Atmospheric Radiation Measurement Cloud Atmosphere Radiation Testbed (ARM CART) Central Facility. A 7 x 7 sampling grid with 100 m spacing between points was imposed on CF04, a quarter section size field, 800 x 800 m (Figure 3). The perimeter sampling sites were all 100 m from the edge of the field. Sample sites were identified using differentially-corrected GPS equipment with accuracy of several meters and a Universal Transverse Mercator coordinate system. Each site was marked with a flag so that it could be easily relocated.
Additional transects were located on the fields contiguous with CF04, namely CF03, CF05, and CF06 (Figure 4). These transects were spaced 200 m apart with sampling sites spaced 100 apart along the transect (Figure 5). These transects were extended to within 10 m of the edges of the fields and additional transects were located near the boundary between CF04 and CF05 and between CF03 and CF06. This provided nearly complete coverage of the full section at 100 m spacing in the north-south direction and 200 m spacing in the east-west direction. Three additional points (50, 51, 52) were added to the transects in CF04 to extend them to the north edge of the field.
Soil moisture of the entire full section CF03, CF04, CF05, and CF06 was sampled on five occasions, June 27, July 3, 7, 11 and 14. On July 11, fields CF03, CF04 and CF06 were sampled, but field CF05 was not. Instead, it was sampled on July 12. These data have been combined with the data for July 11.
Three sites within CF04 were identified for higher resolution sampling. Sampling grids of four different scales were centered over sites 8, 33 and 38 within CF04. Points of the 6 x 6 grids were spaced 10, 2, 1, and 0.1 m apart. Each point and scale was measured three times under different moisture conditions. Because of the time required, it was difficult to complete all of the measurements in a single day. Therefore, either data are missing from a particular scale at one or more points, or the sampling was completed the following day. The row and column designation is from the top-left (NW) corner. Row designations increase from north to south and column designations increase from west to east.
In addition to the once-a-day GSM samples near the SLMR footprint on CF01 and CF02, several ThetaProbe measurements were also collected each. Measurements were made from June 19 (day 170) to July 15 (day 196) on CF01 and to July 16 (day 197) on CF02. There was a period between July 8 to 10 during which measurements were suspended on CF01 because the soil was too hard and there was a high risk of breaking ThetaProbe rods. Measurements resumed on July 11 after some rain. These measurements were collected on a grid coincident with, but slightly larger (~6 x 6 m) than, the radiometer footprint. Initially, measurements were made on a 3 x 3 grid on both CF01and CF02, but after June 21, only a 2 x 3 grid was used on CF01. Values reported in the data set are the mean of the six or nine measurements.
Surface soil moisture on field CF04 was sampled each afternoon (~13:00-16:00) between June 21 (day 172) to July 14 (day 195) with the exception of July 1 (day 182). Personnel equipped with a ThetaProbe, Reader and Wizard walked to each sample site identified with a flag. The probe was inserted vertically into the soil (0-6 cm) within 50 cm radius of the flag. Only one measurement was made. The observed voltage was recorded in the Wizard along with the date, time, field number, site number, and other relevant comments.
For various reasons, such as flooding, severe weather restrictions, miscommunication among personnel, and eventually cultivation etc., some sites were not sampled on particular days. For example, some sites were within patches of two meter high weeds that made it difficult to find the exact site each time, particularly when new personnel were involved who were unfamiliar with the location of the site. Also, some sites could not be reached after the heaviest rain event because some flags were submerged in deep water and could not be located or reached.
On a few occasions under very hard soil conditions, a probe rod was broken. When possible, the broken rod was replaced and the ThetaProbe was put back in service. When it was not possible to replace the rod, a new ThetaProbe was put into service. In all, a total of six probes were used in the vicinity of the Central Facility during the experiment.
Cores were "manufactured" with soil from CF04, CF01 and CF02. Oven dry soil was disaggregated and placed in four-inch diameter PVC cores, 10 cm long, with a filter paper and wire mesh over the bottom. The cores were packed as they were filled to obtained the mean bulk density of the field (determined from 21 cores collected from CF04 and four samples by L. McKee for CF01 and CF02). Four replicate cores were used for CF04 and two for CF01 and CF02. After saturating the samples from the bottom, the voltage was measured and cores were weighed to determine water content. Samples were dried slowly, initially by air drying at ambient room temperature and then in an oven at 30-35°C. After each drying period, the cores were removed from the oven and allowed to cool overnight before measuring the voltage and weight so as to eliminate any possible temperature effect on the probe measurements. Voltage for each sample was measured in triplicate at each moisture level and averaged. Cubic polynomial regression equations ( R = 0.995 or better) were used to define the relationship between voltage and volumetric water content.
Descriptive statistics were calculated for each field for each day. Values of volumetric water content that exceeded plus or minus two standard deviations of the mean were flagged for investigation. Analysis of the "outliers" involved identifying the trends observed at a particular site where the outlier occurs. In some cases, that particular site was frequently the wettest of driest in the field. In most cases, the outliers were deemed to be accurate and no further action was taken. In a few cases, the anomalies could not be resolved and were deleted.
Data Access and Contacts
FTP SiteThe Theta Probe(CF) data is available in the following GES DISC ftp site:
Theta Probe (CF) data ftp site
Points of Contact
The Principal Investigator for the impedance data from the Central Facility area is
- Charles Laymon
- Global Hydrology and Climate Center
- 977 Explorer Blvd.
- Huntsville, AL 35806
- E-mail: charles.laymon@msfc.nasa.gov
- Voice: 256-922-5885
- Fax: 205-922-5723
- For more information regarding GES DISC data, contact:
- Hydrology Data Support Team
- Goddard Earth Sciences
Data and Information Services Center (GES DISC)
Code 610.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
E-mail: hydrology-disc@listserv.gsfc.nasa.gov
Voice: 301-614-5165
Fax: 301-614-5268
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Last update:Thu Jan 28 09:34:12 EST 1999 Page Info: Hydrology Data Support Team -- hydrology-disc@listserv.gsfc.nasa.gov Website Curator: Anthony Drake NASA official: Steve Kempler, DAAC Manager -- Steven.J.Kempler@nasa.gov |