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High-Resolution Measurement of Root Water Uptake Using Automated Time-Domain Reflectometry

Dep. of Plant and Soil Science, Montana State Univ., Bozeman, MT 59717

J. M. Baker

USDA-ARS Soil and Water Management Unit, 1991 Upper Buford Circle, St. Paul, MN 55108

*Corresponding author.

ABSTRACT

Methods for monitoring changes in soil water content with high resolution in both space and time are needed to help quantify patterns of soil-water uptake by plant roots. We designed and tested an automated time-domain reflectometry (TDR) system for precise, continuous measurement of such changes. A two-dimensional root-zone enclosure was constructed from stainless steel wire cloth, with a parallel pair waveguide centered within each of 10 rectangular compartments. The wire cloth confined the electromagnetic pulse from the TDR without restricting root growth or water movement. Digitized waveforms from the analog TDR unit were captured, analyzed, and converted to volumetric water content by a program written for a datalogger. A sorghum (Sorghum bicolor [L.] Moench) plant was grown for 84 d, during which soil water content and changes in weight were monitored at 15-min intervals. Standard deviations in water-content measurement were estimated to be between 0.0008 and 0.001 m³ m^–3. Water-balance computations of evapotranspiration agreed well with measurements by a precision electronic balance, with a standard deviation of 0.0012 m³ m^–3 during a 6-d period. The wire mesh supported the root system, which had grown through it, even after soil was removed by rinsing with water. This will facilitate determination of root densities corresponding to measured changes in water content. The high degree of spatial and temporal resolution afforded by the system make it a valuable tool for studying the dynamics of water flow in the root zone, and information provided by this method may allow resolution of some issues that have remained controversial due to lack of measurement ability.

Contribution of the USDA-ARS and Dep. of Soil Science, Univ. of Minnesota. Minnesota Agric. Exp. Stn. publ. no. 18543.

Received for publication November 2, 1990.

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