Impedance Probe Measurements of
Surface Soil Moisture in
El Reno (ER)
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 around the USDA ARS Grazing Lands Research Laboratory in El Reno on 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 El Reno are organized into files grouped by spatial scale. Data from fields ER05 and ER13, the sites of daily intensive investigation, are contained in the text files (ER05.txt and ER13.txt). Each of these data files comprises a spreadsheet in a Microsoft Excel workbook file (ER_ThetaProbe.xls). The files and their contents are described in the following tables.
|
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, two of which were located in El Reno near the USDA ARS Grazing Lands Research Laboratory. A 7 x 7 sampling grid with 100 m spacing between points was imposed on ER05, a quarter section size field, 800 x 800 m. A 27-point rectangular grid with 100 meter spacing was laid out on ER13 (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.
Data were collected for 28 days during June and July of 1997. Data collection began on June 19 and continued nearly every day during the experiment.
Surface soil moisture on field ER05 and ER13 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 and 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 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.
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.
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.
Data Access and Contacts
The Theta Probe data is available in the following GES DISC ftp site:
- Points of Contact
The Principal Investigator for the impedance data from the El Reno area is
- Paul Houser
- Hydrological Sciences Branch / Data Assimilation Office
- NASA Goddard Space Flight Center; Code 974
- Greenbelt, Maryland 20771
- E-mail: houser@horton.gsfc.nasa.gov
- Voice: 301-286-7702
- Fax: 301-286-1758
- For more information regarding GES DISC data, contact:
- Hydrology Data Support Team
- Goddatd 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
 |
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 |