The dielectric constant of soil is a potentially sensitive indicator of soil moisture. Since, the magnetic permeability of all naturally occurring soils is near that of free space, dielectric measurements serve to fully characterize the electromagnetic response of soils. Many of the indirect methods of soil moisture measurement permit frequent or continuous measurements in the same place with only small expenditure of time. Thus, changes in water content with time can be approximated. The soil impedance is sensitive to the moisture content of the soil and can be used to calculate the volumetric water content of the soil. Soil impedence techniques using probes have been demonstrated to show small-scale diurnal variations that would be completely missed by small-scale spatial variations in the gravimetric sampling scheme. Furthermore, the basically non-destructive nature of the fixed probes minimize the impact of the sampling technique on the dynamic behavior of the region under study.
Soil Impedence Data (FIFE).
(Soil Impedance Measurements of Soil Moisture.)
This data set contains soil moisture data collected using soil impedence techniques. Soil impedance information was used to calculate the volumetric water content of the soil. This data were collected at five locations throughout the FIFE study area during the 1987 Intensive Field Campaigns (IFC).
The primary scientific objective of this research was to characterize the soil moisture distribution in the FIFE study area. The surface soil moisture was measured by impedance probes and conventional techniques at a number of strategic locations.
Soil temperature, and complex impedance of the soil.
Daily measurements of the soil dielectric properties at 5 and 10 cm depths were obtained at five locations throughout the FIFE study area during the 1987 Intensive Field Campaigns (IFC). Calculated soil volumetric water contents were compared with gravimetric soil moisture measurements collected at the same locations by the FIFE staff science team. In addition to the five fixed locations, measurements were made throughout each of the IFCs along three transects underlying airborne Pushbroom Microwave Radiometer (PBMR) flights. The impedance measurements were compared with the results of gravimetric sampling done in support of these flights. Examination of the data revealed that the impedance probe is a more consistent source of time series information than traditional measurements, and is potentially more closely linked to the physical parameters which are both remotely sensible and required for surface energy/mass exchange determination.
Dr. Stephen G. Ungar
NASA Goddard Space Flight Center
Characterization of Soil Moisture Distribution and Its Influence on Remote Sensing Observations.
Dr. Stephen G. Ungar
NASA Goddard Sp. Fl. Ctr.
The Soil Impedance Measurements of Soil Moisture data were collected by Stephen G. Ungar from NASA Goddard Space Flight Center; Robert Layman, Jeffrey E. Campbell and John Walsh from Dartmouth College; and Harlan J. McKim from the U.S. Army Cold Regions Research and Engineering Laboratory.
One effort in evaluating and using remotely sensed data has been that of determining the correlation between in situ measurements and remotely sensed measurements and quantifying the added information value of the remotely sensed data. Moisture in the upper layers of the soil profile is an important portion of the total water balance of the earth-atmosphere system. Research has shown that remote sensing observations are sensitive to variations in soil moisture. In interpreting and applying remotely sensed moisture data to provide ancillary data that complements hydrological observations, one must be conscious of the conceptual framework on which the interpretations are based.
The need for indirect methods for obtaining water content or indices of water content is evident when the time and labor involved in gravimetric sampling are considered, especially when these measurements are used to validate remotely sensed soil moisture estimates (frequently involving a large number of observations). In addition to requiring a waiting time for oven-drying, gravimetric determinations are destructive, and therefore each sample must be taken at a different place in the soil system under study. Destructive sampling may disturb an experiment and may increase the possibility that a change in water content with position in a sampling area may be interpreted falsely as a change in water content with time at a particular location.
The dielectric constant of common soil mineralogical materials ranges from about 2 to 14 while the dielectric constant of water is approximately 80. Hence, the dielectric constant of soil is a potentially sensitive indicator of soil moisture. Since, the magnetic permeability of all naturally occurring soils is near that of free space, dielectric measurements serve to fully characterize the electromagnetic response of soils.
Many of the indirect methods of soil moisture measurement permit frequent or continuous measurements in the same place and, after equipment is installed, with only small expenditure of time. Thus, if a suitable calibration curve is available, changes in water content with time can be approximated. The Radio Frequency Soil Moisture Probe works by measuring the complex electrical impedance of the soil. The soil impedance is sensitive to the moisture content of the soil and can be used to calculate the volumetric water content of the soil.
The actual probe is a coaxial arrangement of seven tines, a center tine surrounded by six tines. The probe which is inserted in the soil is the bottom element of a voltage divider, the upper element is a 510 Ohm resistor. Electrically the probe in the soil appears as a capacitor with a shunt resistor. The capacitive reactance is a function of the probe geometry and the real part of the soil (+water) dielectric constant. The shunt resistance is the parallel sum of the imaginary part (lossy) of the soil (+water) dielectric constant and the finite resistivity of the soil. The system is energized with a 10 MHz voltage source. The voltage drop and voltage phase shift across the 510 Ohm resistor are measured with a vector voltmeter.
To make in situ soil moisture measurements for comparison with data derived from remotely sensed data.
Soil impedance and soil temperature.
The Radio Frequency Soil Moisture Probe (RFSMP) measures the complex electrical impedance of the soil at a frequency of 10 MHz. The soil impedance is sensitive to the moisture content of the soil and thus can be used to calculate the volumetric water content of the soil. A coaxial probe measures the dielectric permittivity. The probe is essentially a coaxial transmission line with six outer tines held at ground potential and a shorter inner tine (insulated from the electrical guard ring and the outer tines by Teflon insert). A voltage can then be applied to the inner tine via the BNC connector. This probe design results in a well-defined electrical-field volume with nearly no field leakage outside of the probe.
A voltage signal produced by a Network Analyzer is sent via the coaxial cable through a Directional Bridge into the probe, which is placed in a medium. The reflected signal is isolated by the directional coupler and introduced into the network analyzer, where the complex ratio of the reflected voltage to the incident voltage is measured. The data are then transferred to a computer for reduction and storage.
The probe is a coaxial arrangement of seven tines, a center tine surrounded by six tines. The dielectric constant is measured within cylindrical volume defined by these tines. The dimensions are given in the Calibration Specifications Section. Measurements were made with probes implanted horizontally at 5 cm and 10 cm as described in the Data Acquistion Section.
Integrated values for the top 7 cm were also obtained along transects as indicated in the Spatial Coverage Section.
Hanover, NH 03755.
Pertinent probe parameters. _____________________________________________________ Probe specifications Value _____________________________________________________ Inner tine length 5.92 cm Probe length 6.35 cm Electrical length 6.10 cm Tine diameter 0.47 cm Probe radius 1.14 cm Sensing-volume capacitance 2.38 pF Stray capacitance 2.38 pF
Measurement of the dielectric response of several liquids with known dielectric constants, shows a typical agreement between measured and reported values of better than 1% over the entire frequency range (Campbell 1990).
The probes were calibrated by the manufacturer.
Electronics were calibrated at least once per series of measurements (i.e., once per transect and at least once at each fixed site).
The uncertainty in bulk density estimates from the FIS data base contributed significant noise when attempting to calibrate the impedance probes against the volumetric estimates based on the bulk density corrected gravimetric readings.
For the 1987 FIFE experiment a total of 20 probes were installed at 5 sites, 4 at each site. Two at a depth of 5 cm and two at 10 cm. The probes were set horizontally with the center line at the indicated depth. In addition to the set probes, spot readings were made along the flight lines of airborne active and passive radar systems. The technique employed for dielectric measurements along the flight lines was to insert the probe vertically, producing an integrated value for the volumetric moisture content of the top 7 cm of soil. The procedure was to insert a probe in the soil, read it, remove it and then repeat the process at the next point along the flight line. The distance between points was approximately 100 yards.
The FIFE study area with areal extent of 15 km by 15 km, is located south of the Tuttle Reservoir and Kansas River, and about 10 km from Manhattan, Kansas, USA. The upper left corner of the area has UTM coordinates of 4,334,000 Northing and 705,000 Easting in UTM zone 14.
Measurements were made at five fixed sites in 1987, and 3 fixed sites in 1989. In addition to the five fixed locations in 1987, measurements were made throughout each of the 1987 IFCs along three transects underlying airborne Pushbroom Microwave Radiometer (PBMR) flights and were compared with the results of gravimetric sampling done in support of these flights. These 3 transects traversed watersheds 1D and 2D, each running from east to west and spaced approximately 250 m apart. Sampling points were selected at approximately 50-m intervals along each of the transects. The sampling locations are listed below:
SITEGRID STN NORTHING EASTING LATITUDE LONGITUDE ELEV -------- --- -------- ------- -------- --------- ---- 2133-IPU 906 4329726 711604 39 05 34 -96 33 12 443 2731-IPU 1 4328625 711102 39 04 59 -96 33 34 446 2840-IPU 883 4328485 712935 39 04 53 -96 32 18 2940-IPU 884 4328177 712943 39 04 43 -96 32 18 3040-IPU 885 4327961 712949 39 04 36 -96 32 18 3140-IPU 886 4327899 712951 39 04 34 -96 32 18 4268-IPU 32 4325626 718579 39 03 15 -96 28 27 445 4439-IPU 11 4325219 712795 39 03 07 -96 32 27 445 4439-IPU 916 4325193 712773 39 03 06 -96 32 28 443 4609-IPU 17 4324766 706700 39 02 58 -96 36 41 398 8639-IPU 21 4316771 712827 38 58 33 -96 32 36 440 8739-IPU 926 4316699 712845 38 58 31 -96 32 35 442 SITEGRID SLOPE ASPECT -------- ----- ------ 2133-IPU 1 TOP 2731-IPU 2840-IPU 2940-IPU 3040-IPU 3140-IPU 4268-IPU 4439-IPU 4439-IPU 2 N 4609-IPU 8639-IPU 8739-IPU 1 TOP
At all sites the probes were located inside the electric fences with the exception of site 17 (sitegrid = 4609-IPW). At site 17 the probes were 50 yards south of the fenced area adjacent to a neutron bore hole.
Sampling volume consisted of cylinders defined by probe dimensions (radius 1.14 cm, effective length 6.1 cm). This is comparable to the sample size obtained with gravimetric borings.
Soil samples were collected from late spring through the fall of 1987, during the Intensive Field Campaigns (IFC) of approximately 2 weeks each, and during the late summer IFC in 1989 (July 26 - August 11).
IFC# Dates ----- ------------------- IFC-1 05/26/87 - 06/06/87 IFC-2 06/25/87 - 07/11/87 IFC-3 08/06/87 - 08/21/87 IFC-4 10/05/87 - 10/16/87 IFC-5 07/24/89 - 08/12/89
Readings were obtained once per day along the transects during active PBMR flights, and twice per day at fixed stations during IFCs 1 through 4. Samples were acquired once per day at all 3 sites during IFC-5.
The SQL definition for this table is found in the SOIL_IMP.TDF file located on FIFE CD-ROM Volume 1.
Parameter/Variable Description Range Units Source
SITEGRID_ID This is a FIS grid location code. FIS Site grid codes (SSEE-III) give the south (SS) and east (EE) cell number in a 100 x 100 array of 200 m square cells. The last 3 characters (III) are an instrument identifier.
STATION_ID The three-digit FIFE site 1, FIS identification number for the site 11, where the data were collected. 17, 21, 32, 883-886, 916, 926
OBS_DATE The date on which the data were min = 24-MAY-87, GSFC collected. max = 10-AUG-89
OBS_TIME The time of the observation. min = 1140, [GMT] GSFC max = 2540
SAMPLE_DISTANCE The approximate distance (in min = 1, [meters] GSFC meters) along the transect to the max = 2871 west of Route 177 (N/S road).
PROBE_DEPTH The depth that the probe is set. min = 3, [cm] SCALE max = 20 MEASURE
SOIL_TEMP The soil temperature. min = 5.5, [degrees THERMOMETER max = 37.7 Celsius]
TOTAL_IMPEDANCE The total impedance of the soil min = 0, [ohms] IMPEDANCE (vector sum of the capacitive max = 5494.79 PROBE reactance and resistance).
FIFE_DATA_CRTFCN_CODE * The FIFE Certification Code for CPI=checked FIS the data, in the following format: by principal CPI (Certified by PI), CPI-??? investigator, (CPI - questionable data). CPI-MRG=merged data
LAST_REVISION_DATE data, in the format (DD-MMM-YY). max = 22-JUN-90
Decode the FIFE_DATA_CRTFCN_CODE field as follows:
The primary certification codes are: EXM Example or Test data (not for release). PRE Preliminary (unchecked, use at your own risk). CPI Checked by Principal Investigator (reviewed for quality). CGR Checked by a group and reconciled (data comparisons and cross-checks). The certification code modifiers are: PRE-NFP Preliminary - Not for publication, at the request of investigator. CPI-MRG PAMS data that are "merged" from two separate receiving stations to eliminate transmission errors. CPI-??? Investigator thinks data item may be questionable.
SITEGRID_ID STATION_ID OBS_DATE OBS_TIME SAMPLE_DISTANCE PROBE_DEPTH ----------- ---------- --------- -------- --------------- ----------- 4609-IPU 17 24-MAY-87 2003 10 4609-IPU 17 24-MAY-87 2005 5 8639-IPU 21 24-MAY-87 1808 10 8639-IPU 21 24-MAY-87 1808 5 SOIL_TEMP TOTAL_IMPEDANCE FIFE_DATA_CRTFCN_CODE LAST_REVISION_DATE ---------- --------------- --------------------- ------------------ 25.400 368.750 PRE 08-DEC-88 26.100 237.440 PRE 08-DEC-88 22.500 329.690 PRE 08-DEC-88 22.700 495.960 PRE 08-DEC-88
Soil samples were collected from late spring through the fall of 1987 for approximately 2 weeks each and during the late summer IFC in 1989. Measurements were made at five fixed sites in 1987, and 3 fixed sites in 1989. In addition to the five fixed locations in 1987, measurements were made throughout each of the 1987 IFCs along three transects underlying airborne Pushbroom Microwave Radiometer (PBMR) flights. Sampling points were selected at approximately 50 meter intervals along each of the transects.
A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.
The CD-ROM file format consists of numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe. There are no spaces between the fields. Each file begins with five header records. Header records contain the following information: Record 1 Name of this file, its table name, number of records in this file, path and name of the document that describes the data in this file, and name of principal investigator for these data. Record 2 Path and filename of the previous data set, and path and filename of the next data set. (Path and filenames for files that contain another set of data taken at the same site on the same day.) Record 3 Path and filename of the previous site, and path and filename of the next site. (Path and filenames for files of the same data set taken on the same day for the previous and next sites (sequentially numbered by SITEGRID_ID)). Record 4 Path and filename of the previous date, and path and filename of the next date. (Path and filenames for files of the same data set taken at the same site for the previous and next date.) Record 5 Column names for the data within the file, delimited by commas. Record 6 Data records begin.
Each field represents one of the attributes listed in the chart in the Data Characteristics Section and described in detail in the TDF file. These fields are in the same order as in the chart.
The volumetric water content of the soil is a function of the capacitive reactance of the probe (real part of the soil plus water dielectric constant). To calculate the volumetric water content of the soil it is necessary to know the dependence of capacitive reactance on soil moisture content and other soil parameters. It is possible to do a crude calculation by assuming the capacitive reactance to be a linear function of soil moisture content and estimating the soil parameters.
The data were recorded and delivered to the information system in ASCII format.
The real part (resistance) and the imaginary part (capacitive reactance) can be calculated using the following expressions.Where:
Capacitive reactance of the probe.
As currently designed, the soil impedance probes are subjected to sampling errors similar to those of gravimetric measurement when used along transects. However, probes are a superior alternative to traditional gravimetric techniques at fixed locations, where they may be placed early in the season allowing vegetation to recover adequately. In these situations the probes have been demonstrated to show small-scale diurnal variations that would be completely missed by small-scale spatial variations in the gravimetric sampling scheme. Furthermore, the basically non-destructive nature of the fixed probes minimize the impact of the sampling technique on the dynamic behavior of the region under study.
Impedance probe correlations based upon gravimetric field averages were considerably higher than correlations attempted with the individual measurements identified as closest to the probe. This is primarily caused by the considerable small-scale variation inherent in gravimetric sampling.
See the Data Characteristics Section.
Measured dielectric response of several liquids with known dielectric constants, shows a typical agreement between measured and reported values of better than 1% over the frequency range of interest.
The soil impedance probes used for the FIFE were subject to sampling errors similar to those of gravimetric measurement when used along transects. However, probes are a superior alternative to traditional gravimetric techniques at fixed locations, where they may be placed early in the season allowing vegetation to recover adequately. In these situations the probes have been demonstrated to show small-scale diurnal variations that would be completely missed by small-scale spatial variations in the gravimetric sampling scheme. Furthermore, The basically non-destructive nature of the fixed probes minimize the impact of sampling technique on the dynamic behavior of the region under study.
The data verification performed by the ORNL DAAC deals with the quality of the data format, media, and readability. The ORNL DAAC does not make an assessment of the quality of the data itself except during the course of performing other QA procedures as described below.
The FIFE data were transferred to the ORNL DAAC via CD-ROM. These CD-ROMs are distributed by the ORNL DAAC unmodified as a set or in individual volumes, as requested. In addition, the DAAC has incorporated each of the 98 FIFE tabular datasets from the CD-ROMs into its online data holdings. Incorporation of these data involved the following steps:
Each distinct type of data (i.e. "data set" on the CD-ROM), is accompanied by a documentation file (i.e., .doc file) and a data format/structure definition file (i.e., .tdf file). The data format files on the CD-ROM are Oracle SQL commands (e.g., "create table") that can be used to set up a relational database table structure. This file provides column/variable names, character/numeric type, length, and format, and labels/comments. These SQL commands were converted to SAS code and were used to create SAS data sets and subsequently to input data files directly from the CD-ROM into a SAS dataset. During this process, file names and directory paths were captured and metadata was extracted to the extent possible electronically. No files were found to be corrupted or unreadable during the conversion process.
Additional Quality Assurance procedures were performed as follows:
As errors are discovered in the online tabular data by investigators, users, or DAAC staff, corrections are made in cooperation with the principal investigators. These corrections are then distributed to users. CD-ROM data are corrected when re-mastering occurs for replenishment of CD-ROM stock.
All the fixed probes functioned for the entire exercise except for the probe implanted at 5 cm in site 32 (sitegrid = 4268-IPU) which was defective after June 1, 1987. There are a few other random readings that are invalid which will be apparent when the data is analyzed and can be treated individually.
This data set could be used in conjunction with other soil moisture data to validate the soil moisture values predicted from the airborne remote sensing measurements during FIFE. It could be used with caution in similar prairie landscapes to compare remote sensing derived soil moisture and field measured soil moisture.
This data set could be used in conjunction with other soil moisture data to validate the soil moisture values predicted from the airborne remote sensing measurements during FIFE.
The FIFE field campaigns were held in 1987 and 1989 and there are no plans for new data collection. Field work continues near the FIFE site at the Long-Term Ecological Research (LTER) Network Konza research site (i.e., LTER continues to monitor the site). The FIFE investigators are continuing to analyze and model the data from the field campaigns to produce new data products.
Software to access the data set is available on the all volumes of the FIFE CD-ROM set. For a detailed description of the available software see the Software Description Document.
ORNL DAAC User Services
Oak Ridge National Laboratory
Telephone: (865) 241-3952
FAX: (865) 574-4665
ORNL Distributed Active Archive Center
Oak Ridge National Laboratory
Telephone: (865) 241-3952
FAX: (865) 574-4665
Users may place requests by telephone, electronic mail, or FAX. Data is also available via the World Wide Web at http://daac.ornl.gov.
FIFE data are available from the ORNL DAAC. Please contact the ORNL DAAC User Services Office for the most current information about these data.
The Soil Impedance Measurements of Soil Moisture data are available on FIFE CD-ROM Volume 1. The CD-ROM file name is as follows:
Where yy is the last two digits of the year (e.g. Y87 = 1987) and mm is the month of the year (e.g., M12 = December). Note: capital letters indicate fixed values that appear on the CD-ROM exactly as shown here, lower case indicates characters (values) that change for each path and file.
The format used for the filenames is: ydddMULT.sfx, y is the last digit of the year (e.g. 7 = 1987, and 9 = 1989), and ddd is the day of the year (e.g., 061 = sixty-first day in the year). The filename extension (.sfx), identifies the data set content for the file (see the Data Characteristics Section) and is equal to .SIM for this data set.
Campbell, J.E. 1990. Dielectric properties and influence of conductivity in soils at one to fifty megahertz. Soil Sci. Soc. Am. J. 54:332-341.
Gardner, W.H. 1986. Water content. p.635-662. In: A. Klute (ed.) Methods of Soil Analysis. Part 1. Physical and mineralogical methods. 2nd ed. Agronomy Monogr. 9. ASA and SSSA. Madison, WI.
Davis, J.L., and A.P. Annan. 1977. Electromagnetic detection of soil moisture: Progress report I. Can. J. Remote Sens. 3:76-86.
Engman, E.T., W. Kustas, T.J. Schmugge, and J.R. Wang. 1987. Relationship among the remotely sensed soil moisture, streamflow, and evapotranspiration. AGU Fall Meeting. San Francisco.
Engman, E.T., G. Angus, and W. Kustas. 1989. Relationships between the hydrologic balance of a small watershed and remotely sensed soil moisture. Remote Sensing and Large Scale Processes. IAHS Publ. No. 186. Proc. IAHS. 3rd Int. Asso. Baltimore, MD.
Geiger, F.E., and D. Williams. 1972. Dielectric constants of soils at microwave frequencies. NASA Tech. Rep. TMS-65987. Washington, DC.
Gogineni, S. 1990. Radar measurements of soil moisture over the Konza prairie. AMS Symposium on the First ISLSCP Field Expt. Anaheim, CA.
Paquet, J. 1964. Etude dielectrique des materiaux humides. Onde Electr. 44:940-950.
Peck, E.L., T.R. Carroll, and D.M. Lipinski. 1990. Airborne gamma radiation soil moisture measurements over short flight lines. AMS Symposium on the First ISLSCP Field Expt. Anaheim, CA.
Scott, J.H., R.D. Carroll, and D.R. Cunningham. 1967. Dielectric constant and electrical conductivity of moist rock from laboratory measurements. J. Geophys. Res. 72:5101-5110.
Wang, J.R., J.C. Shiue, E.T. Engman, and T.J. Schmugge. 1988. The soil moisture variations of two small watersheds in Konza prairie as estimated from the L-band radiometric measurements. Geophys. Res. Lett. (in review).
Wang, J.R., J.C. Shiue, E.T. Engman, and T.J. Schmugge. 1988. The soil moisture mapping with L-band pushbroom microwave radiometer in FIFE. AGU EOS (in review).
Wang, J.R., and J.C. Shieu. 1989. Remote sensing of soil moisture variations with PBMR. AGU EOS. 70 (issue 15). No. 347.
The Collected Data of the First ISLSCP Field Experiment is archived at the EOS Distributed Active Archive Center (DAAC) at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee (see Data Center Indentification). Documentation about using the archive and/or online access to the data at the ORNL DAAC is not available at this revision.
A general glossary for the DAAC is located at Glossary.
A general list of acronyms for the DAAC is available at Acronyms.
May 6, 1994 (citation revised on October 14, 2002).
Warning: This document has not been checked for technical or editorial accuracy by the FIFE Information Scientist. There may be inconsistencies with other documents, technical or editorial errors that were inadvertently introduced when the document was compiled or references to preliminary data that were not included on the final CD-ROM.
Previous versions of this document have been reviewed by the Principal Investigator, the person who transmitted the data to FIS, a FIS staff member, or a FIFE scientist generally familiar with the data.
February 18, 1996.
Ungar, S. G. 1994. Soil Impedance Data (FIFE). Data set. Available on-line [http://www.daac.ornl.gov] from Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A. doi:10.3334/ORNLDAAC/108. Also published in D. E. Strebel, D. R. Landis, K. F. Huemmrich, and B. W. Meeson (eds.), Collected Data of the First ISLSCP Field Experiment, Vol. 1: Surface Observations and Non-Image Data Sets. CD-ROM. National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland, U.S.A. (available from http://www.daac.ornl.gov).