Prototype time-domain reflectometry probes for measurement of moisture content near the soil surface for applications to “on-the-move” measurements

New time-domain reflectometry (TDR) probes were developed for estimating moisture conditions near the soil surface. Two types of probes, knife-shaped (K) and surface (S) types, were devised, and their performance was investigated in comparison with conventional rod-type probes. The K-type probe consists of two parallel, knife-shaped electrodes that can move in the shallow soil-layer without drawing out. The S-type probe looks like a sled with two parallel edges (electrodes), and can slide smoothly over the soil surface. This probe uses only the lower half of the electromagnetic field around the electrodes, but the estimates of moisture contents were comparable to those by conventional rod-type probes. The new probes would expand the applications of TDR principle especially to “non-penetrating” or “on-the-move” measurements of moisture conditions near the soil surface.