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Soil Physics Note

A Small-Scale Matric Potential Sensor Based on Time Domain Reflectometry

^a Dep. of Water Resources Engineering, Lund Univ., Box 118, SE-221 00 Lund, Sweden
^b Land Resources and Environmental Sciences Dep., Montana State Univ., P.O. Box 173120, Bozeman, MT 59717-3120, USA
^c Engineering Geology, Lund Univ., Box 118, SE-221 00 Lund, Sweden

^* Corresponding author (magnus.persson{at}tvrl.lth.se)

Measurements of soil matric potential () are needed in many soil science applications. In the present study, a small matric potential sensor having a length of 30 mm and a diameter of 9.6 mm was developed. The sensor consists of two coils made of lacquer-coated copper wires embedded in gypsum. The dielectric constant of the gypsum (K_gypsum) was measured with time domain reflectometry (TDR). The gypsum probes were tested in a sandy loam soil using a pressure extractor to obtain the relationship between K_gypsum and , allowing inference of the soil in equilibrium with the sensor. It was shown that the gypsum probe could give estimates with a root mean square error (RMSE) of around 50 kPa in the range of –20 to –1500 kPa. Most of the sensitivity of the sensor was in the –50- to –1000-kPa range.

Abbreviations: RMSE, root mean squared error • TDR, time domain reflectometry

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