A simple noncontact soil moisture probe for weather and climate applications

The measurement of soil moisture is important for many practical applications. We describe the theoretical design of a simple, noncontact, electromagnetic probe that complements many existing soil moisture measurement techniques. The approach uses a low-frequency (i.e., 50–150 MHz) antenna operating in proximity of the soil. The presence of the soil affects the antenna input impedance, which in turn depends on the distance between the soil and antenna and the complex dielectric constant of the soil. The latter strongly depends on the soil wetness, which suggests that bulk soil moisture integrated over a depth of roughly 1 m can be inferred from antenna impedance measurements. This is in contrast with many current higher-frequency techniques that penetrate only a few centimeters into the soil and provide only near-surface values of soil wetness. Our work suggests that under ideal conditions bulk soil moisture can be mapped with an accuracy on the order of 1% over horizontal scales spanning a few tens of meters to a few kilometers using simple low-frequency antennas.