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ARS Home » Pacific West Area » Pendleton, Oregon » Columbia Plateau Conservation Research Center » Research » Publications at this Location » Publication #168863
Title: SPATIALLY CHARACTERIZING APPARENT ELECTRICAL CONDUCTIVITY AND WATER CONTENT OF SURFACE SOILS WITH TIME DOMAIN REFLECTOMETRY

Author
item WRAITH, J - MONTANA STATE UNIV
item Robinson, David
item JONES, S - UTAH STATE UNIV
item Long, Daniel

Submitted to: Computers and Electronics in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2005
Publication Date: 3/1/2005
Citation: Wraith, J.M., Robinson, D.A., Jones, S.B., Long, D.S. 2005. Spatially characterizing apparent electrical conductivity and water content of surface soils with time domain reflectometry. Computers and Electronics in Agriculture.

Interpretive Summary: By measuring the time of radio signal propagation, time domain reflectometry (TDR) has the ability to measure both water content and apparent electrical conductivity of soil. This paper provides an introduction to the theory of the TDR method and reviews practical applications of TDR for spatially characterizing water content, electrical conductivity, and other water-related attributes in soils. We also discuss handheld- and vehicle-based measurement methods with application to spatial characterization of soil water in a hilly agricultural field, and monitoring water content and nitrate concentrations at three depths under peppermint production. The results indicate that TDR is a potentially useful tool for precision agriculture, and that fixed TDR arrays could serve as real-time monitoring systems for water and fertilizer salts in soil profiles. The primary limitation of the TDR method for spatially characterizing soil water and electrical conductivity is the cable length limitation of about 20-30 m. Mobile platforms are of high interest, and prototype designs have been reported in the literature. Truly 'on-the-fly' TDR measurements for large-scale applications may be feasible in the near future.

Technical Abstract: Unlike other measurement methods discussed in this special issue, time domain reflectometry (TDR) has the ability to measure both water content and apparent electrical conductivity (EC_a) of soils. From simultaneous knowledge of water content and EC_a, the soil solution electrical conductivity (EC_s) and even the concentration of specific ionic constituents such as NO_3-nitrogen may be estimated through soil-specific calibration. This paper provides an introduction, some theoretical background, and a practical review of the TDR method for spatially characterizing water content, EC_a, and EC_s, along with suggestions for inferring matric potential from dielectric measurements, are addressed. We discuss point, handheld, and vehicle-based measurement methods. Applications of TDR to spatially characterize water content in a hilly agricultural field using TDR and gravimetric methods, and to monitor water content and nitrate concentrations at three depths under peppermint production, are presented. A pickup-mounted TDR measured water content at 100X100-m grid spacing in two wheat fields in north-central Montana. Soil water contents, as well as NO_3-N, grain yield, and grain protein increased from upper to lower slopes. Soil water content early in the growing seasons appeared critical to final yields in this rainfed system. An array of fixed TDR probes was monitored over two growing seasons under peppermint in northwest Montana, to estimate water content, EC_S and NO_3-N every 6 h at 12 field locations. Although the field soils appeared uniform, measured spatial patterns of water content, EC_s, and NO_3-N were highly space- and time-variant. These results indicate that TDR is a potentially useful tool for precision agriculture, and that fixed TDR arrays could serve as real-time monitoring systems for water and fertilizer salts in soil profiles. The primary limitation of the TDR method for spatially characterizing water content and EC at soil management scales using fixed arrays is the cable length limitation of about 20-30 m. Mobile platforms are of high interest, and prototype designs have been reported in the literature. Truly 'on-the-fly' TDR measurements of large-scale applications may be feasible in the near future.