Impedance Probe Measurements of Surface Soil Moisture in Little Washita (lw)
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 in the Little Washita watershed near Chickasha, OK. Depending on the particular data set (scale), the sampling frequency is hourly, daily, or occasional.
The Files
Surface soil moisture data from Little Washita are organized into separate files. Data from fields lw03, lw13, and lw21, the sites of daily intensive investigation, are contained in the text files (lw03.txt, lw13.txt, and lw21.txt). Each of these data files comprises a spreadsheet in a Microsoft Excel workbook file (lw_ThetaProbe.xls). The files and their contents are described in the following tables.
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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 by hand and inputted 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, three of which were located in the Little Washita watershed near Chickasha, OK. A 7 x 7 sampling grid with 100 m spacing between points was imposed on all three fields which were a quarter section size field, approximately 1 x 1 km (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. Each site was marked with a flag so that it could be easily relocated.
Measurements were made from June 19 (day 170) to July 16 (day 197) on lw03 and lw13 and from June 22 (day 173) to July 16 (day 197) on lw21. On several days, no samples were taken on lw21 because of agricultural work or because of rain on the field. On fields lw03 and lw13, samples were not taken on a few days because of rain or because the soil was too hard and there was a high risk of breaking ThetaProbe rods.
Surface soil moisture on fields lw03, lw13, and lw21 was sampled each afternoon (~13:00-16:00) between June 19 and July 16 with a few exceptions personnel equipped with a ThetaProbe and voltage reader 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. The observed voltage was recorded along with the date, time, field number, site number, and other relevant comments.
For various reasons, such as extremely dry conditions, severe weather restrictions, miscommunication among personnel, and eventually cultivation etc., some sites were not sampled on particular days.
On occasion, a probe rod was broken because of very hard and dry soil conditions. 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.
Site specific calibration by SGP97 researchers has produced results similar to the average mineral soil calibration curve presented by Gaskin and Miller (1996). Therefore, the Gaskin and Miller curve was used to produce the volumetric water content data for these files.
Because of the extremely dry conditions in the Little Washita area for most of the SGP97 experiment, a few measurements produced values of 0.00 when voltage was converted to volumetric soil moisture content. Because these values do represent the extremely dry conditions in parts of the field, they were left in the data set.
Data Access and Contacts
The Theta Probe(CF) data is in the following GES DISC ftp site:
Points of Contact
The Principal Investigator for the impedance data from the Little Washita area is
- Jay Famiglietti
- Department of Geological Science
- The University of Texas at Austin
- Austin, TX 78712
- Email: jfamiglt@maestro.geo.utexas.edu
- 512-471-3824(voice)
- 512-471-9425(Fax)
- For more information about 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
References
Gaskin, G.J. and Miller, J.D., 1996. Measurement of soil water content using a simplified impedance measuring technique. Journal of Agricultural Engineering Resources, 63: 153-160.
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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 |