Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 6 5 2002 10.5194/hess-6-949-2002 http://www.hydrol-earth-syst-sci.net/6/949/2002/ http://www.hydrol-earth-syst-sci.net/6/949/2002/hess-6-949-2002.html http://www.hydrol-earth-syst-sci.net/6/949/2002/hess-6-949-2002.pdf 949 955 0000-00-00 Technical Note:<br>Practical considerations on the use of down-sized time-domain reflectometry (TDR) probes M. A. Mojid Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh – 2202, Bangladesh E-mail: m.mojid@fz-juelich.de Nine time-domain reflectometry (TDR) probes, 2 to 10 cm long, were evaluated by comparing their measurement accuracy of TDR-pulse travel time in a sand and sandy loam soil, and electrical conductivity in NaCl solutions. TDR probes &lt;2.5 cm in length generated trough-haped TDR waveforms with rounded corners at the points of the pulse reflection from the probe ends. The sharpness of the pulse reflection on the waveforms increased with both the increasing probe length and soil-water content. The transition time for the propagation of TDR pulse at the probe entrance increased as the soil dried up. The increased transition time caused a rightward movement of the first peak of the waveform at the probe entrance. Because of such peak movement, TDR-support software algorithm determined travel path of TDR pulse through the probe that was smaller than the actual travel path. TDR-measured pulse travel time <i>t</i>_TDR varied erratically with the predicted pulse travel time <i>t</i>_g (from volumetric soil-water content) for the probes &lt;2.5 cm in length. But, for all probes ³2.5 cm in length, <i>t</i>_TDR varied linearly with <i>t</i>_g and followed the 1:1 line. TDR could not measure <i>t</i>_TDR &lt;300 ps accurately. A minimum probe length <i>L</i>_min and the lowest allowable soil-water content <i>q</i>_min that the probe can accurately measure govern this lowest pulse travel time <i>t</i>_min. The mean absolute deviation between <i>t</i>_TDR and <i>t</i>_g was 77 ps for the 2.3 cm long probe and 1.39 ps for all probes ≥2.5 cm in length. All probes ≥2.5 cm in length measured electrical conductivity of salt solutions s_TDRthat compared well with the electrical conductivity measured by a conductivity meter s_m. The length of the probes did not exert any noticeable influence on the accuracy of electrical conductivity measurement.</p> <p style="line-height: 20px;"><b>Keywords: </b>TDR probe, pulse travel time, dielectric constant, electrical conductivity</p>