Bilinear Analysis for Soil Moisture Determination Using an Open-Ended Dielectric Probe

Soil water content is a crucial variable in hydrological modeling, agricultural water management, and most studies dealing with soil. Open-ended probes have been widely used in physics and chemistry for permittivity measurements in liquids and biological tissue. However, due to their small sampling volume and associated complex signal analysis, these probes have not been extensively used in soil measurement but it has excellent potential for on-the-go subsurface measurements Jones et al., 2006. The large contrast in dielectric permittivity between water (80) and other soil constituents (minerals = 5-10, air = 1) make electromagnetic-based methods ideal for water content determination. While low frequency EM measurements are often plagued with conductivity and Maxwell Wagner effects associated with ions, measurement frequencies above about 5 GHz enter the realm of water relaxation, which reduces the water-solid-air dielectric contrast. Obtaining measurements at higher frequencies generally requires more expensive instrumentation, though costs are being reduced by high volume chips and circuits from cellular phones and other common commodities. The objectives of this research were to design and model the dielectric response of an open-ended dielectric probe for soil water content measurement. We will target an optimal measurement frequency in the range of 300 to 5000 MHz to overcome problems described above. The dielectric permittivity measurement is converted to volumetric water content using dielectric mixing or empirical relations. The development of a robust, compact and low cost electromagnetic water content sensor will have a wide range of application such as hydrological modeling, agricultural water management, and most studies of the soil. It will provide high measurement accuracy compared with other commercial sensors on the market.