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. Plain Language Summary Soil moisture information to depths of 1 m is critical for a variety of geophysical applications, including the accurate forecasting of precipitation, river flow levels and floods, and drought prediction and monitoring. While there are a large number of remote sensing techniques available that measure surface moisture values, relatively few if any can access deeper so‐called root‐zone depths. We present theoretical results for a noncontact technique that uses a simple, thin‐wire, loop antenna operating at 50–150 MHz and a height of approximately 1 m above the soil surface to infer bulk soil moisture properties integrated over a depth of roughly 1 m with an accuracy on the order of 1% under ideal conditions. In principle, the technique can be applied using a moving vehicle or drone to map soil moisture over horizontal length scales that bridge the gap between in‐situ measurements and regional‐scale satellite retrievals. Key Points Soil moisture is a critical quantity relevant for many geophysical applications A wide variety of techniques can be used to infer soil moisture, each with its own set of advantages and disadvantages A simple, noncontact method using a low‐frequency antenna operating near the soil surface can be used to map bulk soil moisture