Topp equation

The relationship between dielectric permittivity and volumetric water content in mineral soils has been described by Topp et al. (1980) in an empirical fashion using a third-degree polynomial. With θ_v the volumetric water content and K_a the bulk dielectric permittivity of the soil, the equation presented by Topp et al. is

Research has shown that this equation works well in most mineral soils, so a soil specific calibration of the CS650 sensor is usually not necessary. For a soil specific calibration, you can generate an equation relating K_a to θ_v following the methods described in Water content reflectometer user-calibration.