Evaluation of Several Dielectric Mixing Models for Estimating Soil Moisture Content in Sand, Loam and Clay Soils

As part of a NOAA-funded project, studies are being conducted at the University ofPuerto Rico-Mayagez Campus using surface-based ground penetrating radar (GPR) tomeasure soil moisture content. The GPR will eventually be used to verify values of soilmoisture at several locations in Puerto Rico using active radar and passive satellite-basedsensors. As a part of the estimation process, it is necessary to relate moisture content to theGPR-measured dielectric constant. The motivation for this study was the need to select anappropriate dielectric mixing model for the wide range of soils being considered in the study. Animportant requirement of the dielectric mixing model was that it works well with input dataavailable from NRCS Soil Survey Reports (e.g., soil texture, available water capacity, etc). Theadvantage of using this type of data is that it can be readily incorporated into a geographicinformation system (GIS) to be used with the geo-referenced dielectric data of the surface andsatellite-based sensors. This paper provides a review of several dielectric mixing models, and comparesmoisture content estimates for sand, loam and clay soils, based on dielectric dataobtained from a GPR, TDR and Theta Probe. These results are also compared tosoil moisture contents obtained from gravimetric data. Soils were characterized interms of their chemical and physical properties; information needed by several of thedielectric mixing models. In some cases, especially with the loam soil, wide variationsin the dielectric constants and moisture contents were observed.