Validation of Dobson's, Schmugge's, and Mironov's Dielectric Models at Four Typical Agrosoils of Krasnoyarsk and Krasnodar krais

In this article, in a wide frequency range from 300 MHz to 2 GHz at a temperature of $20^{\circ}\mathrm{C}$, the validation of Dobson's, Schmugge's, and Mironov's (with one, two, and three soil water relaxations) dielectric models was carried out using four samples of typical agrosoils taken from the arable layer. Variations in the clay fraction, dry bulk density, and organic matter content of soil samples ranged from 32% to 56%, from 1.1 g/cm
³
to 1.7 g/cm
³
, up to 6.9%, respectively. It is shown that the Mironov's model with two relaxations (M2) has the best accuracy in describing the measured relative complex permittivity (RCP) spectra for all soils in average. The coefficient of determination (R
²
) and root-mean square error (RMSE) appeared to be equal of $R_{\varepsilon'}^{2}$=0.907 (RMSE$_{\varepsilon'}$=0.83) for real and $R_{\varepsilon'}^{2}$=0.960(RMSE$_{\varepsilon^{\prime \prime}}$=0.89) for imaginary parts of RCP, when comparing the measured and calculated RCPs. Error of soil moisture retrieval, based on M2 dielectric model and the observations of reflection coefficient at nadir was estimated. For four agricultural soils, it is shown that the retrieved volumetric soil moisture values with the determination coefficient $\mathrm{R}^{2}$=0.975 and RMSE =2% coincide with the true values of soil moisture. It has also been found that small inaccuracies (several tens of percent) in determining the dry bulk density and clay content of soils have practically no effect on the error in retrieving the volumetric soil moisture.