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 of Puerto Rico-Mayaguez Campus using surface-based ground penetrating radar (GPR) to measure soil moisture content. The GPR will eventually be used to verify values of soil moisture at several locations in Puerto Rico using active radar and passive satellite-based sensors. As a part of the estimation process, it is necessary to relate moisture content to the GPR-measured dielectric constant. The motivation for this study was the need to select an appropriate dielectric mixing model for the wide range of soils being considered in the study. An important requirement of the dielectric mixing model was that it works well with input data available from NRCS Soil Survey Reports (e.g., soil texture, available water capacity, etc). The advantage of using this type of data is that it can be readily incorporated into a geographic information system (GIS) to be used with the geo-referenced dielectric data of the surface and satellite-based sensors. This paper provides a review of several dielectric mixing models, and compares moisture content estimates for sand, loam and clay soils, based on dielectric data obtained from a GPR, TDR and Theta Probe™. These results are also compared to soil moisture contents obtained from gravimetric data. Soils were characterized in terms of their chemical and physical properties; information needed by several of the dielectric mixing models. In some cases, especially with the loam soil, wide variations in the dielectric constants and moisture contents were observed.