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Time Domain Reflectometry Response to Lateral Soil Water Content Heterogeneities

Water Resources Center, Desert Research Inst., 2505 Chandler Ave., Suite 1, Las Vegas, NV 89120

S. D. Cloud

Dept. of Physics, Univ. of Nevada, 4505 Maryland Parkway, Las Vegas, NV 89154. Research supported by the Desert Research Inst.

ABSTRACT

Water-content heterogeneities can occur between the parallel rods of the time domain reflectometry (TDR) probe if TDR is used in cracked soils or in laboratory studies with probes oriented in a variety of directions. The accuracy of water-content measurements using TDR under such heterogeneous conditions was investigated in a series of laboratory experiments. Two cases were studied: (i) one TDR rod in dry soil and the other in wet soil; and (ii) an artificial soil crack filled with water or air between the two TDR rods. In Case 1, TDR readings were heavily biased toward the water content of the dry soil. This effect occurs because the electromagnetic wave (EMW) reaches the end of the waveguide quicker in dry than in wet material. Because the time delay of the earliest reflection signal is read from the TDR, the travel time of the dry material is measured. In Case 2, the air-filled crack between the waveguide rods had only a small effect on measured water content when the soil was dry; however, water-content measurements were reduced by as much as 46% in wetter soils. The effect of water-filled cracks was small in both dry and wet soils. The effect of soil cracks is explained by a static capacitance model. Results from the experiments do not contradict previous work that demonstrated that TDR measures the average water content of heterogeneous layers oriented perpendicular to the TDR probe axis. In the two cases examined here, knowledge of the orientation of soil water heterogeneities relative to the TDR waveguide rods is necessary for correct data interpretation.

^* Corresponding author.

Received for publication December 4, 1990.

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