A Variable-Volume TDR Probe for Measuring Water Content in Large Soil Volumes

concluded that coaxial probe design was superior to two-wire probes with a balancing transformer. Management of favorable conditions for plant growth within parTopp and Davis (1985) extended measurement capatially wet soil volumes under drip irrigation presents a measurement bilities from a small volume (point) to multiple depths challenge. Typically, soil water status is determined from a single enabling construction of a vertical water content profile. point measurement or several point measurements within the wet volume requiring various assumptions for quantitative interpretation They designed a probe with step changes in rod diameter of total water available for irrigation decisions. We propose a new causing a step change in probe impedance and consecoaxial time domain reflectometry (TDR) probe capable of measuring quently a characteristic reflection marking the discontithe total soil water storage within well-defined soil volumes (referred nuity location on the waveform. Travel times between to as the variable volume coaxial probe, VVCP). The measurement reflections with known physical distances were used to volume is contained between an array of stainless steel conductors determine water contents in the regions between disconembedded in the soil and arranged in a coaxial configuration with tinuities. Hook et al. (1992) designed a probe for profilvariable spacing. The average water content in the spacing between ing soil water with depth using two metal strips mounted the inner and outer circular arrays of conductors is determined from on a plastic core where shorting diodes defined the travel time analysis of VVCP waveforms. Laboratory and field tests various segments along the probe. Redman and DeRyck of the VVCP established that waveforms and travel time analyses (1994) measured the soil water content profile with resulted in dielectric permittivities and water contents similar to those obtained with standard three-rod TDR probes. For uniform distribudepth using a probe comprised of a polyvinyl chloride tion of soil water VVCP measurements were independent of conduc(PVC) access tube with two metal rods attached to its tor spacing or soil volume. Variable volume coaxial probe-determined exterior. Another profiling probe was designed by Ferré water balance within a partially wet soil volume was in excellent et al. (1998) using two short rods placed within two agreement with the amount of water applied from a point source parallel access tubes. Small diameter wires lead to the indicating accurate and integrative volumetric measurement. top of the measurement interval, limiting the sensitivity of the probe to the medium above the region of interest. Extension of TDR measurement capabilities to enA estimation of average water content of able water content sensing within large and well-defined wetted soil volumes is important for evaluation soil volumes are desirable for management of drip irriand management of drip irrigation systems. Measuregation systems due to the nonuniform wetting and highly ments of the extent of wetted soil volumes are often dynamic changes in soil water status within active plant made by either exhaustive soil sampling or deployment rooting zones. Coelho and Or (1997) discussed aspects of a dense bank of sensors to monitor the spatial distriof wetted volume sensing as compared with point meabution of soil water content (Coelho and Or, 1997). surements and highlighted their importance for proper Among presently available soil water monitoring techmanagement and establishment of automatic irrigation niques, TDR offers the most accurate and repeatable threshold values under nonuniform wetting conditions field method. Time domain reflectometry is capable of induced by drip irrigation. The primary objective of this simultaneous measurement of water content and electristudy was to develop and test a new TDR probe design cal conductivity within the same soil volume. The termed the variable volume coaxial probe (VVCP) for sensing water content in large and partially wet soil method capitalizes on the large disparity between the volumes. dielectric permittivities of water (εw 81) and other soil constituents such as air (εa 1) and soil solid particles (εs 3–5). Consequently, measured bulk dielectric THEORY permittivity (εb) is dominated by the water phase. The We designed a new TDR probe for measurement of average TDR method determines soil εb from the travel time of volumetric water content (or water storage) in a large soil a step electromagnetic pulse along a buried waveguide volume confined within a coaxial transmission line formed by (TDR probe). Conventional TDR probes are comprised concentric arrangement of VVCP conductors (stainless steel of two or three parallel metal rods completely embedrods). The use of transmission lines for dielectric measurements is well-established (Bussey, 1980) using a variety of ded in the medium of interest (Topp et al., 1982). Zegewaveguides ranging from simple parallel wires to fully coaxial lin et al. (1989) evaluated various probe designs and designs (Zegelin et al., 1989). The coaxial design has the advantage that the electrical field is confined between the inner and outer conductors, and it propagates in a transverse electromagC.F. Souza and E.E. Matsura, Faculdade de Engenharia Agrı́cola, netic (TEM) mode, which is the simplest form of guided wave Univ. Estadual de Campinas, Campinas, SP, Brazil; D. Or, Univ. of Connecticut, Storrs, CT. Received 5 Feb. 2002. *Corresponding author propagation. Consequently, the measurement is confined to ([email protected]). Abbreviations: EC, electrical conductivity; TDR, time domain reflecPublished in Soil Sci. Soc. Am. J. 68:25–31 (2004).  Soil Science Society of America tometry; TEM, transverse electromagnetic; VVCP, variable volume coaxial probe. 677 S. Segoe Rd., Madison, WI 53711 USA