Optimum Frequency Range for Remote Sensing of Soil Moisture with Various Texture, Density and Organic Matter Content

In this article, the frequency range, within which the dependence of reflectivity on soil moisture is invariant respect to changes in soils texture, dry bulk density and organic matter content, has been established. The analysis is based on dielectric measurements of 16 natural mineral nonsaline soils in the frequency range from 45-100 MHz to 2 GHz. The moisture of soils varied from air dry to the field capacity. The clay, silt and sand content in the soil samples are ranging from 0% to 76%, from 1% to 93%, and from 0 to 100%, respectively. The dry bulk density and organic matter content in soil samples are ranging from 0.7 g/cm
³
to 1.8g/cm
³
, and from 0.6% to 69%, respectively. As a result, it is shown, that the variations in texture, dry bulk density, and organic matter content of soils have the least effect in the frequency range from 125 MHz to 820 MHz (with minimum at a frequency of 350 MHz) on the dispersion of the dependence of refractive index on volumetric moisture. As an example, the case of smooth bare soil surface remote sensing at a frequency of 435 MHz is considered. It is shown that for the entire set of soils, a single calibration curve for conversion of reflectivity to volumetric soil moisture is sufficient to retrieve soil moisture with a root-mean square error of 2.1% and a determination coefficient of 0.981. The established frequency range can be recommended for microwave remote sensing methods of soil moisture measurement, not depended on soils texture.